Título Conceptual Preliminary Optical Design

INSTITUTO DE ASTRONOMÍA

UNIVERSIDAD NACIONAL AUTÓNOMA DE MÉXICO

Espectrógrafo óptico de mediana y baja dispersión para el Observatorio de San Pedro Mártir

Fecha: 21/04/05 Código: ESOPO-CPOD-A-EO1 No. de páginas: 66 Versión: 1

Título

Conceptual Preliminary Optical Design

ESOPO

CONCEPTUAL PRELIMINARY OPTICAL DESIGN

CÓDIGO: ESOPO-OP-A-CPOD1 VERSIÓN: 1

PÁGINA: 2 DE 66

APPROVAL CONTROL

Prepared by:

J. Jesús González

Optics Head: ESOPO Project

Francisco Cobos

Optical Designer

Revised by: Carlos Tejada

Optical Designer

Approved by:

Alejandro Farah

Project Manager

Authorized by:

Rafael Costero

PI ESOPO Project

Juan Echevarría

Responsible for ESOPO Project

ESOPO



PÁGINA: 3 DE 66

DOCUMENT CHANGE RECORD

Issue Date Section Page Change Description

0 26/12/02 First draft version 1

28/05/03

01 - 43

OP002-01 “Optical Preliminary

Concept” Document Revised Version submitted

to ESOPO SAC 2 Revised version to

ESOPO SAC

ESOPO



PÁGINA: 4 DE 66

Lista de abreviaciones

ESOPO Espectrógrafo óptico de mediana y baja dispersión para el Observatorio de San Pedro Mártir

DM Dichroic mirror

FM Folder mirror

FOV Field of View

AR Anti-Reflection Coatings

SPM San Pedro Mártir

OAN Observatorio Astronómico Nacional

SAC Scientific Advisory Committee

ESOPO



PÁGINA: 5 DE 66

TABLE OF CONTENTS

APPROVAL CONTROL............................................................................................................ 2 DOCUMENT CHANGE RECORD .......................................................................................... 3 OPTICAL PRELIMINARY DESIGN ...................................................................................... 8 1. INTRODUCTION ........................................................................................................... 8 1.1 USE OF MUST/SHOULD ........................................................................................................ 8 2. HIGH LEVEL REQUIREMENTS (HLR).................................................................... 8 2.1 SPECTRAL RANGE: .............................................................................................................. 8 2.2 F/RATIO CONFIGURATION: ................................................................................................. 8 2.3 HIGH EFFICIENCY:.............................................................................................................. 8 2.4 RESOLUTION:...................................................................................................................... 9 2.5 OPERATION MODES: ........................................................................................................... 9 2.6 MAXIMUM REAL RESOLUTION:............................................................................................ 9 2.7 SPECTRAL RESOLUTION SAMPLING: .................................................................................... 9 2.8 PUPIL DIAMETER:............................................................................................................... 9 2.9 GRATINGS:.......................................................................................................................... 9 2.10 FIELD: ................................................................................................................................ 9 2.11 DETECTOR SCALE: .............................................................................................................. 9 2.12 DETECTOR NOISE AND LINEARITY:....................................................................................... 9 2.13 SLIT WIDTH:........................................................................................................................ 9 2.14 CONFIGURATION CHANGES:.............................................................................................. 10 2.15 REPETITIVE PERFORMANCE AND STABILITY: ...................................................................... 10 2.16 DESIGN, OPERATION AND SURVIVAL AMBIENT REQUIREMENTS: ......................................... 10 3. THE CONCEPTUAL OPTICAL DESIGN ................................................................ 11 3.1 WAVELENGTH COVERAGE .............................................................................................. 11 3.2 PUPIL DIAMETER ............................................................................................................. 12 3.3 COLLIMATION.................................................................................................................. 13

3.3.1 Collimator Focal Distance.................................................................................... 13 3.3.2 Collimator Concept ............................................................................................... 13 3.3.3 Refractive Collimator System.............................................................................. 16

ESOPO



PÁGINA: 6 DE 66

3.4 DICHROIC BEAMSPLITTER: POSITION AND INCLINATION ANGLE ................................... 17 3.4.1 Dichroic Spectral Response ................................................................................ 18

3.5 PLANE REFLECTION GRATING MOUNT ........................................................................... 21 3.5.1 Camera-Collimation angle ................................................................................... 22 3.5.2 Selection of a Mount type for the non-Littrow reflecting gratings .................. 22

3.5.2.1 Efficiencies Comparison of Grating Mounts........................... 25 3.6 GRATING COATINGS ........................................................................................................ 28 3.7 A SAMPLE OF GRATINGS FOR ESOPO............................................................................. 29 3.8 CCD DETECTORS............................................................................................................. 31

3.8.1 Other Potential Coatings...................................................................................... 33 3.9 ESOPO CAMERAS ........................................................................................................... 36 3.10 ESOPO IMAGE QUALITY................................................................................................. 37 3.11 OPTICS: TRANSMISSION AND AR EFFICIENCIES.............................................................. 41 3.12 INSTRUMENT (SINGLE ARM OPERATION): OPTICS + GRATING

+ DETECTOR + FOLDER MIRROR ..................................................................................... 41 3.13 INSTRUMENT (DUAL ARM OPERATION): OPTICS + GRATING

+ DETECTOR + DICHROIC ................................................................................................ 42 3.14 ACTUAL EFFICIENCY: INSTRUMENT ON TELESCOPE & COMPARISON

WITH BRERA B&CH......................................................................................................... 42 4. DESCRIPCIÓN DEL PROCESO DE DISEÑO ÓPTICO ........................................ 44 4.1 GENERAL ......................................................................................................................... 44 4.2 DISEÑO ÓPTICO ............................................................................................................... 45

4.2.1 Particular: Colimadores ......................................................................................... 49 4.2.1.1 COLIMADOR (BRAZO AZUL) ................................................. 49 4.2.1.2 COLIMADOR (BRAZO ROJO).................................................. 49

4.2.2 Particular: Cámaras ............................................................................................... 50 4.2.2.1 CÁMARA (BRAZO AZUL)........................................................ 50 4.2.2.2 CÁMARA (BRAZO ROJO) ........................................................ 50

SCHOTT .............................................................................................................. 50 5. CONCEPT DIRECTRICES TOWARDS A FINAL DESING.................................. 52 6. OPTICAL ELEMENTS: DIMENSIONS & WEIGHTS ........................................... 58 7. BLANK GLASSES, PROPERTIES AND DIMENSIONS: ESTIMATED COST BUDGET .................................................................................................................................... 59 8. REQUIREMENT COMPLIANCE MATRIX ............................................................ 61

ESOPO



PÁGINA: 7 DE 66

9. OBJETIVO .................................................................................................................... 65 10. INTRODUCCIÓN......................................................................................................... 65 10.1 ANTECEDENTES.......................................................................................................... 65

ESOPO



PÁGINA: 8 DE 66

OPTICAL PRELIMINARY DESIGN

1. INTRODUCTION The ESOPO High Level Requirements and Specifications are shown in the ESOPO-CI-A-REAN1 document, which is based on the ESOPO Scientific Goals, shown in the ESOPO-CI-A-OCEAN1 document. These documents determine the ESOPO spectrograph optical design.

1.1 Use of must/should

“Must” is used for requirements, whereas “should” is reserved for guidelines. Requirements are mandatory and guidelines are not mandatory, although their fulfillment should be strongly pursued. The word “guidelines” appears explicitly in the paragraph title, when necessary. Guidelines are always expressed in terms of “should” statements.

2. HIGH LEVEL REQUIREMENTS (HLR) The optical concept has to comply with the high level requirements of ESOPO; a full list of the instrument HLR (a translation of Section 6 in the ESOPO-CI-A-REAN1 document) is here summarized:

2.1 Spectral Range:

ESOPO is an optical low-dispersion & long-slit spectrograph, which must be optimized for the 350-900 nm spectral range (not necessarily integrated field required).

2.2 F/Ratio Configuration:

The spectrograph must be coupled to the f/7.5 Cassegrain focus configuration (with its own guider) of the SPM 2.1 m Telescope.

2.3 High efficiency:

This is a highest priority requirement. The minimum efficiency value for each wavelength (without telescope but including detector) must be:

• at 3500 Å > 15%,

• at 4500 Å > 35%,

• at 5500 Å > 36%,

• at 7500 Å > 40% and

• at 9000 Å > 15%

(Guideline goal: without grating or CCD: >80% from 3500-9000 Å).

ESOPO



PÁGINA: 9 DE 66

2.4 Resolution:

The whole spectral range must be observable at once, at least at the minimum resolution (R~2000 or higher).

2.5 Operation Modes:

Operation must allow the use of either arm (blue or red) alone -without dichroic- or both arms simultaneously.

2.6 Maximum Real Resolution:

A maximum actual FHWM resolution must be achieved with not greater than a 1200 l/mm grating, with a maximum cross section of 154 x 206mm, and a nominal slit width of 0.8’’. Maximum variation of actual resolution along the slit must be less than 10% (guideline goal: <5%).

2.7 Spectral Resolution Sampling:

From more than 2 up to 4.0 pixels per FWHM (0.8” slit).

2.8 Pupil Diameter:

Pupil size must be limited by the size of commercial (off-shelf) gratings.

2.9 Gratings:

They must be interchangeable. At least two gratings must be usable without need of opening the instrument (guideline goal: 3 gratings per arm).

2.10 Field:

The field around target must be visible and it must be possible to place target on the slit before and during exposures. The long slit length must cover a minimum 8’ field size, with a 10’ maximum guideline field goal size.

2.11 Detector scale:

Detector plate-scale: ≤ 0.5 ’’/pixel (a guideline goal, since spectroscopic sampling has priority over spatial sampling).

2.12 Detector noise and linearity:

Reading data at the detector must have a read out noise: < 8e- (guideline goal: < 5e-). The detector response must be linear up to a 250 signal-to-noise (full-well capacity > 62,500 e-/pixel).

2.13 Slit width:

Minimum slit width ≤ to the spectrograph diffraction limit. Maximum slit width ≥ 9” (guideline goal: 10”).

ESOPO



PÁGINA: 10 DE 66

2.14 Configuration changes:

They must be in comparison arcs ≤ 1 min; other components (grating, slit, etc.): ≤ 3-5 min.

2.15 Repetitive performance and stability:

They must be such that:

a. Resolution should not degrade by more than 2.5% over 30 min

exposures, b. Resolution constant within 5% after changing and coming back to a given

configuration (guideline goal: 3%), c. Relative calibrations at beginning/end of night (related to spectral

dispersion and resolution, plate scale and responses along slit and spectral directions) must apply to all within-night data with a confidence level better than 10% (in relative functional shape, but not in an absolute sense or zero point) under the nocturnal temperature changes (+/-6˚C), flexures or other derivatives (guideline goal: 5%).

2.16 Design, operation and survival ambient requirements:

They must be considered as:

a. Optimization temperature and atmospheric pressure: T=3˚C; P=562

mmHg. b. Minimum range of operation (fulfilling requirements): from -10˚C to 16˚C

and 545-570 mmHg. c. Minimum survival range: from -16˚C to 34˚C, and from 500 to 1100

mmHg.

ESOPO



PÁGINA: 11 DE 66

3. THE CONCEPTUAL OPTICAL DESIGN

The ESOPO optical design must consider three main components:

1) A collimating system that yields a highly corrected parallel-ray beam of a

convenient diameter (pupil size large enough to provide the required maximum resolution).

2) A concept for the diffracting elements. 3) A camera to give the proper sampling and wavelength coverage with a specific

detector.

The whole system can be considered a focal reducer (with an intermediate collimated section) that matches the f/7.5 configuration of the 2.1 meter OAN telescope at SPM (In fact, since the primary mirror has been diaphragmed, from its original 2108 mm value to 2000 mm, the f/7.5 configuration has actually an F/7.905 value) .

This section in the document gives a brief introduction of the proposed optical concept for ESOPO; explaining how the design has been conceived, based on the purpose of satisfying the previously stated HLR for the instrument. A more detailed description of the actual ESOPO optical design is shown in this document next section.

3.1 Wavelength Coverage

In order to cover the whole spectral range (3500-9000 Angstroms) at once, during an observation, the concept considers a two arm system separated by a dichroic mirror. The “blue” arm is to be optimized for covering the 350 to 700 nm octave and the “red” arm for the complementary octave from 450 to 900 nm. The beam splitter (dichroic mirror in this design) must provide a wide enough overlap (at least 500 A full-width at halved top efficiency).

The two-arm system solution provides some important advantages, related with the efficiency of each arm, mainly by means of the CCD detector characteristics selection, which makes feasible to satisfy the highest priority efficiency requirements. The choice of glasses and AR coatings, in each arm optical elements, drives in the same direction.

ESOPO



PÁGINA: 12 DE 66

dichroic5750

3500 90006000

3500 9000

5500

7000

Blue optimization

4500

Red optimization

bluearm

redarm

dichroic5750

3500 90006000

3500 9000

5500

7000

Blue optimization

4500

Red optimization

dichroic5750

3500 9000dichroic

5750

3500 900060006000

3500 90003500 9000

55005500

7000

Blue optimization

70007000

Blue optimization

4500

Red optimization

45004500

Red optimization

bluearm

redarm

bluearm

redarm

Figure 1. The whole spectral range will be covered with a two-arm system.

3.2 Pupil Diameter

The pupil diameter determines the resolutions that can be achieved with a given grating and slit width. Since its size is given by the collimator focal length, the pupil aperture also determines the scale length (or size) of the instrument. The larger the pupil, the higher the resolution, but the longer (heavier and more expensive) the instrument becomes. In particular, for a slit width of θs radians on the sky, the resolution of a Littrow-mounted grating at blaze incidence θb is given by:

s

b

DdR

θθ

λλ tan2

=∂

≡

Where d is the depth of the grating collimated beam (or pupil diameter) and D is the diameter of the telescope’s stop aperture (2000 mm in our case). ESOPO must provide an actual resolution R=5000 with a grating width not larger than 206 mm under a slit aperture of 0.8” ( see HLR 2.6), a 100mm pupil (R=25783.1 tan θb) seems a reasonable choice to achieve this requirement with a reasonable error budget (table 1).

ESOPO



PÁGINA: 13 DE 66

Table 1. A 100 mm pupil delivers enough resolution and construction budget.

Expected Resolution with a 100mm-pupil (1200 ll/mm Littrow-mounted grating)

1st Order Blaze Wavelength

Blaze Angle

Θb

Diffraction Resolution

(θs≡λ/D)

Slit-limited Resolution (θs

=0.8”)

Degradation budget

350 12.12° 122737 5538 11%

400 13.89° 123613 6374 27%

500 17.46° 125794 8108 62%

600 21.10° 128624 9949 99%

750 26.74° 134374 12992 160%

850 30.66° 139507 15287 206%

3.3 Collimation

3.3.1 Collimator Focal Distance Since the collimator must match the telescope’s F/#, the effective focal length of the collimating optical system is determined by the desirable pupil diameter. Given the dimensions of commercial gratings and the required resolutions, we have chosen in this concept a 100 mm diameter pupil. Table 1 above shows that this pupil delivers enough budgets to design and build ESOPO, implying Fcoll ≈ 790 mm. Since the total track from the telescope’s focal plane (slit position) to the pupil (position of diffracting elements) is of the order of twice the Fcoll value, the dimension scale of the ESOPO spectrograph is expected then to be around 1600 mm.

3.3.2 Collimator Concept A very important measure of a collimator performance is its degree of collimation. For a given point in the sky, rays (principal and marginal) arrive parallel to each other at the telescope aperture (telescope pupil, usually the primary mirror)) with the same incidence angle (as incoming from the same field point) and converging close to a point at the telescope focal surface; then from this surface the now diverging ray-cone is intercepted by the collimator and the rays -at different heights- leave parallel to each other in a direction determined by the field and the telescope-collimator relative amplification. In principle, if all these rays from a field are exactly parallel (i.e., perfect collimation), they will all be equally diffracted by the dispersive element (interference filter, prism, etalon, grating, etc). But, in general, even for perfect telescope/collimators, the principal and marginal pupil rays leave the collimator with slightly different angles, and the resolution that a grating can deliver is never actually achieved. The 2.1m telescope design limits the collimation degree to no better than 0.0226 minutes of arc (RMS non-parallelism of pupil rays for all fields within a 5’ radius). This is not bad, but for some gratings (even at low-resolutions) a fraction of an arc-minute difference in incidence angle can translate

ESOPO



PÁGINA: 14 DE 66

into a considerable difference in wavelength (line broadening) or effective resolution degradation.

The plausible collimator concepts considered here are:

a. A single-surface (conic) reflective collimator, which requires a deviation between

incoming and reflected beams. The deviation can be accomplished with a tilt, an off-axis displacement of the conic, centring the field of view away from the optical axis, or via a combination of the three options. This simple solution provides an achromatic collimated beam, but its collimation degree is far from perfect and introduces severe higher-order aberrations (coma-like), requiring typically faster or more difficult cameras.

b. A multi-lens refractive (all spheres) collimator, that requires no deviation of the beam but involves multiple optical surfaces, absorption and reflection losses, and introduces chromatic effects. Important advantages of a refractive collimator, though, are its high collimation degree without adding up severe high-order aberrations (other than a significant spherical aberration). A better collimated and lower-order aberrated pupil yields higher resolutions, and together with a slower/simpler camera can produce better performance than a system with a single conic (off-axis, tilted or field-decentred) reflective collimator.

The following table summarizes some of the main properties, advantages and disadvantages of both concepts as applied to ESOPO.

ESOPO



PÁGINA: 15 DE 66

• Table 2. Collimator Concepts Compared

Collimator Concept

Collimation a)

[´] Image size b)

[µm]

Fcoll c)

[mm]Fcam

d)

[mm] Aberrations and Notes

Idealized e):

Paraxial 0.023 1.94 789.8 347.4 Keeps telescope aberrations unaltered

On-axis parabola

0.093 5.76 790.6 347.7

On-axis best conic f)

0.050 3.43 791.0 347.9

8o separation: g)

Off-axis parabola

0.650 40.17 786.8 347.6

Tilted parabola 0.900 35.13 789.2 347.6

Decentred FOV 0.400 30.20 784.3 347.7

Best conic 0.350 25.40 787.8 347.8

Keeps Spherical, increases Coma (significantly) and Field Distortion, reduces Astigmatism and Field Curvature. Best conic produces higher Spherical aberration and Field Distortion. System may need more that 8o separation for ray and mechanical clearance. So, effects can be much more severe.

Refractive:

Doublet + Triplet (all spherics)

0.037 3.83 791.4 348.1

Increases mostly Spherical Aberration Field Curvature and Distortion, introduces chromatic effects. Little Coma & no Astigmatism added.

a) RMS non-parallelism of principal and .3, .5 and .8 marginal rays, for fields within a 5’ FOV b) Spot-radius on the detector produced by a paraxial camera (no aberrations added nor

corrected) c) Focal distance for a mean (and chromatic) 100 mm pupil d) Focal distance for detector scale of 0.4”/pix after aberrations e) These systems are just for reference: Paraxial is theoretical and the rest provide no beam

clearances f) This tends to prefer more elliptical and spherical than parabolic or hyperbolic conics g) ESOPO may need as much of 12o deviation angles for optical and mechanical clearances, de-

collimation, image size and aberrations might be significantly higher.

ESOPO



PÁGINA: 16 DE 66

3.3.3 Refractive Collimator System

The collimation degree of the refractive collimator is much superior, allowing the actual resolution delivered by the instrument to be as close as possible to the theoretical expectations from the slit/pupil/grating combinations (e.g. table 1) and not limited by the spectrograph. The reflecting systems produce no chromatic and only small spherical aberrations, but produce large amounts of coma, severely limiting minimum image sizes that simple and fast cameras can deliver. The present design is hence based on a fully refractive (all spherical surface) collimating system, in which its chromatic residual effects are well under control, its total efficiency (with proper anti-reflective coatings) is comparable to the reflective conic, and its spherical aberration can be easily compensated even with a simple camera.

The actual collimator design consists of two systems (each with 5 lenses):

a) A field-lens pre-collimator doublet (close to the slit) practically achromatic from

0.35 to 0.9 µm. So, this optical element is common to both arms. b) A collimating triplet for each arm (one optimized for the 0.35-0.70 µm blue arm,

and the red arm optimized from 0.45µm to 0.9µm) that naturally prefer to be located far from the slit, allowing for ample room to position a dichroic and the folding mirrors.

c) Dual-beam format: the Scientific Advisory Committee recommended that ESOPO should be planned as a dual-beam instrument from the beginning, rather than leaving a second beam as a possible later addition. A dual-beam configuration will increase observing efficiency for many projects by approximately doubling the wavelength range observed at one time. It will also enable optimised glasses, coatings and detectors to be used in each - blue and redarm.

ESOPO



PÁGINA: 17 DE 66

Figure 2 Present Design Blue and Red Arms. Both arms share a common pre-collimating “Field Lens” doublet (Silica and CaF2). For a clear display, the blue-arm is folded before (left) and the red arm after (right) with 45º mirrors placed at arbitrary distances. The blue-arm collimating triplet (Silica, S-FPL53, N-BAK2) as well as the red-arm collimating triplet (S-BAL11, S-FPL53 , S-BAL11) produce 100 mm highly achromatic and collimated pupils, and were optimized for best performance (transmission, image quality and collimation degree) for a 370-700 nm and a 450-900nm ranges respectively. The diameter of the Field Lens is ~ 71 mm; the collimator triplets have an aperture of 118 mm to cover a full 10’ long slit field of view. The effective focal length of the blue collimator is 791.53 mm at 5250 A, while that for the red collimator is 791.35 mm at 6750 A.

3.4 Dichroic Beamsplitter: Position and Inclination Angle

A tilted dichroic mirror separates the red and blue beams. To minimize UV transmission losses, through the dichroic substrate, the blue arm is to be reflected by the dichroic. Consequently, the red beam will be transmitted through the dichroic and folded afterwards with an additional fold mirror. The dichroic can be removed to illuminate the red channel only, or replaced with a fold mirror to illuminate the blue channel only.

Since the dichroic is a flat in a non-collimated light, it introduces a bit -but negligible- of sphericity and astigmatism into the transmitted (red) beam. The dichroic can be removed

ESOPO



PÁGINA: 18 DE 66

and the red channel used alone, or can be switched by a conventional mirror for use of the blue arm alone.

The actual position and deviation angles of the dichroic and folding mirrors are somewhat irrelevant to the Optical Design, and its performance, and hence were selected by the ESOPO Mechanical Team as follows:

1. The Dichroic Mirror (DM) reflects the blue beam with a deviation of 45º (22.5º incidence).

2. The Folder Mirror (FM) reflects the red beam with a deviation -45º (-22.5º

incidence), where the minus sign indicates folding 180º away from the blue beam.

3. The DM is placed between the Field Doublet and the blue-collimator triplet, at a distance such that the blue gratings get as close as possible to the telescope “platina”.

4. The FM is placed after the DM at the minimal distance that warrants enough beam clearance and no vignetting and pushing up the red grating as high as possible (but somewhat lower than the blue pupil).

5. Independently of the final deviation angle chosen (45º in the present concept), the mechanical workgroup stated the convenience and advantages of having the dichroic angle to be equal (but of opposite sign) to the camera-collimator angle (“spectrograph” angle) chosen, as discussed in the following section, under optical criteria.

6. If the Optical Design chooses a different camera-collimator angle, the placement of the DM and FM will be reviewed by the Mechanical Team according to points 2) and 3) as well as considering other criteria, improvements or potential conflicts.

The geometry imposed by the fifth point above places the cameras (and dewars) parallel to the telescope optical axis for optimal minimization of liquid nitrogen spillage. This geometry also minimizes the total diameter of the instrument and the torque around the telescope tub.

3.4.1 Dichroic Spectral Response The dichroic will have a transition from reflecting to transmitting over a 50 nm band centered at approximately 575 nm, allowing overlap between red and blue spectra to simultaneously calibrate both channels.

At the present time we have not investigated providers nor defined detailed requirements for the dichroic, but we do not foresee this element to be on the critical path. Most dual beam spectrographs in operation (LRES on Keck, DBSP on Palomar´s 120”, ISIS on WHT, UVES on VLT, etc), use a dichroic at 45º incidence. The following figures show typical dichroic efficiencies as reported by some of these instruments.

ESOPO



PÁGINA: 19 DE 66

• Figure 3 One of the three dichroics available with the LRES

spectrograph on Keck.

• Figure 4 Reported response of the two dichroic available with the

DBSP on the 120” Palomar telescope.

From the current experience (two figures above) we can expect efficiencies above 90% and cross-over as required. One potential limitation arises from the intermediate-frequency variations, which might limit the flux-calibration when coupled with thermal and pointing flexures.

Dichroic mirrors are designed under the same principles than interference filters and antireflective coatings, depositing a set of different coating layers. They can be designed for any angle of incidence (all transmission, normal incidence included). It is expected that a given quality and performance (smoother response) can be simpler to achieve for incidences closer to normal. Indeed several instruments under construction are adopting smaller deviation angles (e.g. the MODS for the LBT; MIKES for the converted MMT and Magellan; the ATLAS (AAT), spectrographs shown in the following figures).

ESOPO



PÁGINA: 20 DE 66

• Figure 5 The Multi-object Double Spectrograph for the Large

Binocular Telescope (Byard and O’Brien, SPIE 2000)

• Figure 6 The ATLAS spectrograph proposed for the converted MMT

uses a low-angle dichroic (Robertson et al., SPIE 2000).

ESOPO



PÁGINA: 21 DE 66

• Figure 7 Layout of the VPH Spectrograph planned for the Magellan

II telescope, another instrument with a dichroic under less than 450 incidences (Bernstein et al., 2000, SPIE 4008).

In conclusion, the light loss, in either channel due to the dichroic beamsplitter should be expected to be certainly less than 10% in the worst case and ESOPO can demand a goal of less than 5%. We can also expect and demand smoother dichroic responses than shown above for LRIS and ISIS.

3.5 Plane Reflection Grating Mount

Once the pupil size and collimation concept have been selected, we must consider a concept for the diffraction elements and their mount.

The classical diffracting elements for low-resolution spectroscopy are: prisms, reflection/transmission ruled gratings (working on 1st or 2nd order), grisms (transmission gratings mounted on a prism) and low-order “echellete” gratings (in parallel with a suitable cross-dispersing prism/grating or order-selecting filter). Prisms do not easily deliver dispersions higher than a few hundreds; since ESOPO requires reaching a real R of 5000, prisms are not considered. Since no imaging modes are required, there is no need to consider grisms that limit the performance of conventional gratings for the advantage of having a straight (no deviations) light beam to have imaging modes.

The ESOPO concept considers using plane-reflection gratings, mostly because they imply a natural folding of the instrument, are typically more efficient than transmission

ESOPO



PÁGINA: 22 DE 66

grating and do not require choosing different substrates to optimize red or blue performances.

3.5.1 Camera-Collimation angle

In reflection-grating spectrographs with a fixed camera this angle is set depending on the optical apertures and mechanical preferences. Fixing up this angle precludes any grating to be used at the Littrow configuration (parallel incident and diffracted beams), at which most gratings are specified and reach their quoted performances. Grating labs in practice consider as Littrow angles those less than 10o (and actually do not measure their efficiencies strictly at Littrow) but such small “spectrograph” angles are impossible to attain by long-slit spectrographs. Typical B&Ch astronomical spectrographs have camera-collimator angles from 35o to 75o (the Brera’s B&Ch used at the SPM 2.1m telescope is a 64.1o system).

Given the ESOPO pupil (100 mm), its 10’ field of view, and the apertures and clearances of its collimator components, we consider possible to reach spectrograph angles as small as 35o, but have selected 45o as a non compromising solution in this preliminary optical concept.

3.5.2 Selection of a Mount type for the non-Littrow reflecting gratings

The different mount types available for plane (reflective) diffraction gratings produce different effects on pupil size and shape, wavelength coverage and resolution, blaze-wavelength position and actual efficiency curve. In this section we summarize these effects and the reason why we selected a non-classical mount for ESOPO.

Mount types:

• Littrow diffraction, collinear incidence and diffracted beams. Ideal configuration, but

impossible to reach with reflection gratings and a long slit. It is possible in transmission for a fixed grating, unless the camera is allowed to rotate itself or to simulate its rotation with adjustable mirrors.

• (In-plane) Ebert diffraction: for a given camera-collimator angle the grating is mounted with its Normal pointing to the camera (Blaze to Collimator). This produces a magnification of the pupil along the dispersion direction, making the camera optics diameter to be larger than in the Littrow case. This also potentially yields higher resolution and shorter wavelength coverage.

• (In-plane) Non-Ebert diffraction: Normal to Collimator (Blaze to Camera). This is the most typical mount used in optical-astronomical spectrograph. In this case, the anamorphic (off-Littrow) amplification is lower than unity (but as large as in the Ebert case). The camera optics is then not larger in diameter than in Littrow, and the wavelength coverage may be higher, but the actual resolution may decrease and the efficiency losses can be higher (sometimes extreme at high resolutions due to reflection with the internal face of the ruled groves).

ESOPO



PÁGINA: 23 DE 66

• (Off-plane) Conical Diffraction. In this case, the camera-collimation angle is not achieved with a tilt along the dispersion, but the fixed spectrograph angle is achieved tilting the grating normal to the grooves length. This is the typical configuration of X-ray spectrographs (where the large incidence needed for reflection would imply unreasonably high resolutions with in-plane classical mounts).

Given its advantages, ESOPO consider a conical mount to deliver the 45o spectrograph angle. This should not be interpreted as a non-classical solution though, since strictly speaking all fields -but the central one- of a long-slit “classical” spectrograph reach the grating at off-plane incidences (the reason why the spectral lines curve).

The grating equation usually written as λθθ Gmnn rrii =+ sinsin (where ni, nr, θi and θr are the refraction indices and angles for wavelength λ in the incidence and refraction media, respectively; m is the order of diffraction and G is the grating groove density (reciprocal of the groove spacing)), is not general enough to account for how monochromatic slit images of all plane gratings curve with a curvature that depends on wavelength and resolution. In general, the incident and refracted beams from the different fields along the slit do not lie on the plane perpendicular to the grooves. Given their angle of incidence, ε, relative to such plane, the rays along the slit obey the following grating equation:

)sinsin(cos rrii nnGm θθελ += ……….. (1)

If the grating mount is designed for in-plane diffraction (ε ≡ 0) then the central slit field (spectrograph optical axis) is strictly the only field along a long slit with ε = 0. All other slit-fields θl will have a non-zero off-plane incidence given by the amplification factor of the telescope-collimator system ε = θl (Ftel/Fcol) ≠ 0, and strictly suffer off-plane or conical diffraction given by equation (1).

There is no anamorphic amplification in the conical mount (highly variable with R and as large of 70% with a red 1200l/mm grating in an in-plane mount). Instead there is practically constant pupil amplification, not along the slit width (dispersion direction) but along the slit length. For a ε = 22.5o conical incidence this non-anamorphic amplification is simply cos (ε) = 0.92 (or 8.2%) and, unlike the in-plane mounts, practically independent on wavelength and resolution. In principle, this mount should deliver Littrow performance but with a blue shifted blaze function (exactly the same blaze-wavelength shift of the in-plane configurations but keeping efficiency shape).

ESOPO



PÁGINA: 24 DE 66

• Figure 8. In-Plane grating mounts. The Ebert configuration directs the grating normal to the camera (top left panel) i.e., the grating blazed towards the collimator. This mount amplifies the pupil along the wavelength direction (bottom left) relative to the circular Littrow mount (central bottom) and, although it could in principle increase the actual resolution, the camera optics require much higher apertures to minimize vignette losses. The Non-Ebert mount (top right) yields the same spectrograph angle (45º) directing the grating normal to the collimator (blaze to camera) and producing now the same-but-reciprocal anamorphic magnification of the pupil (bottom right) than in the Ebert mount. The actual delivered resolution may be smaller but the wavelength coverage is actually higher than in the Littrow and Ebert mounts. Cameras shown are not the present concept actual ones.

ESOPO



PÁGINA: 25 DE 66

• Figure 9. Conical off-plane mount. The spectrograph angle of 45º is now provided tilting the grating perpendicular to the grooves (the second panel on top is therefore tilted 90º around the camera optical axis). The pupil magnification is not along the dispersion direction like in the in-plane mounts (two panels at bottom left) but along the spatial direction (right bottom panel) relative to the circular pupil of the Littrow mount (third bottom panel). These bottom left diagrams (as in previous figure) are for a high resolution red-blazed 1200 l/mm grating (limiting ESOPO case), the anamorphic amplification on the non-Littrow in-plane mounts depend on wavelength and groove density (resolution) and for the lowest ESOPO resolutions are hardly distinguished from the Littrow case. In contrast, the perpendicular amplification of the conical mount is always present but practically constant for all gratings and wavelengths. The cameras shown are only schematic and are the same in all figures.

To clarify the main doubt that the ESOPO optical team had to finally adopt the Conical mount, a comparison of actual efficiencies of the three potential mounts (Ebert and Non-Ebert in-plane mounts and the off-plane conical mount) was necessary. For this reason we investigated the efficiencies of a given grating under the different sets of mountings, as discussed in the following section.

3.5.2.1 Efficiencies Comparison of Grating Mounts Theoretical efficiencies were computed for a large set of gratings each mounted in different configurations (Littrow and the three possibilities for a 450 camera-angle with in-plane and off-plane mounts). The software used was PCGrate (IIG Inc), and more details are given in appendix C, but the following three graphs presents the case for a

ESOPO



PÁGINA: 26 DE 66

1200 l/mm blue-blazed grating. Differences are even more severe with the red-blazed 1200 l/mm used in previous figures showing mount geometries and pupil amplifications.

Grating 1200 17.5

0

0,2

0,4

0,6

0,8

1

200 400 600 800 1000

Wavelength (nm)

Effic

ienc

y

LittrowEbert TEEbert TMEbertNon EbertConical

Grating 1200 17.5

0

0,2

0,4

0,6

0,8

1

200 400 600 800 1000

Wavelength (nm)

Effic

ienc

y

LittrowNon Ebert TENon Ebert TMEbertNon EbertConical

• Figure 10. Total and parallell and orthogonal polarization efficiencies of in-plane grating mounts. The blaze wavelength is blue shifted, relative to the nominal Littrow value, by a factor equal to the cosine of the spectrograph angle (m = 1). The TM sudden efficiency jumps are the grating anomalies. Although anomalies are less frequent in the Non-Ebert configuration, at extreme incidences (red end) the light towards the camera can be totally blocked by the grooves triangular blaze shape. The total efficiency of unpolarized light is the mean TM and TE polarization efficiencies.

ESOPO



PÁGINA: 27 DE 66

Grating 1200 17.5

0

0,2

0,4

0,6

0,8

1

200 400 600 800 1000

Wavelength (nm)

Effic

ienc

y

LittrowLittrow TELittrow TMEbertNon EbertConical

Grating 1200 17.5

0

0,2

0,4

0,6

0,8

1

200 400 600 800 1000

Wavelength (nm)

Effic

ienc

y

LittrowConic TEConic TMEbertNon EbertConical

• Figure 11. Total and polarization efficiencies of Littrow and off-plane conical grating mounts. As in the non-Littrow in-plane case (previous figure), the blaze wavelength of the conical case (bottom panel) is blue shifted relative to Littrow (top panel) by the same (m-1 times the cosine of the spectrograph angle) but the anomalies and total efficiency Littrow are conserved

ESOPO



PÁGINA: 28 DE 66

Conclusions:

• The conical mount yields the best efficiency, actually identical in shape and

amplitude, than the Littrow case, but blue-shifted (by cos (22.5º)). • The blue shift of blaze wavelength is the same in in-plane and off-plane

mountings for a fixed camera-collimator angle. • The anamorphic amplification of in-plane (off-Littrow) mounts is highly variable

with wavelength and resolution (and potentially higher than 50% in ESOPO). • The Ebert configuration yields potentially higher resolution but a lower efficiency

(polarization dependent), a shorter wavelength coverage and higher potential vignette losses than the Littrow and conical mounts

• The Non-Ebert configuration potentially decreases the actual resolution but increases the wavelength coverage. Its efficiency is typically lower than the Littrow and conical mounts, but depending on the polarization angle can be higher or lower.

• The self-groove losses are clear in the Non-Ebert mount (for polarization perpendicular to the groves) but actually far from the use regime of the ESOPO gratings.

3.6 Grating Coatings

The efficiencies discussed above were estimated assuming normal aluminium coating depositions. Commercial grating providers allow for a variety of reflecting coatings (next figure). It is expected that ESOPO mid and high resolution gratings will be optimized with the best coating depending on their wavelength usage.

ESOPO



PÁGINA: 29 DE 66

• Figure 12 The actual efficiency of reflective diffraction gratings can be boosted choosing the optimal coatings depending on the operational wavelength range.

3.7 A Sample of Gratings for ESOPO

The following table summarizes actual resolution and wavelength coverage that this ESOPO optical concept delivers with a set of ThermoRGL off-shelf gratings. The first column identifies the grating by its ruled density and the blaze angle. The second column lists the Littrow (nominal) blaze wavelength and the corresponding blaze wavelength blue-shifted due to a 450 spectrograph angle (camera-collimator). The remaining columns list the central wavelength selected, the wavelength range covered within 95% of the detector area, the mean inverse dispersion, the actual resolution (the resolution the astronomer actually sees) with a 0.9” slit width, and finally the width of the spectral coverage (which would remain approximately constant when tuning other desired central wavelengths.

ESOPO



PÁGINA: 30 DE 66

• Table 4. A set of commercial gratings for ESOPO

Grating ll/mm

Blaze λ Littrow/45°

λc ∆λ Disp [A/px]

Resolution [A] R

∆λ [A]

300 4.3° 5000/4618 4750 3593-5903 1.29 2.96 1605 2310

200 4.2° 7300/6766 7250 5516-8979 1.93 4.44 1634 3463

385 6.7° 6050/5605 5712 4813-6607 1.00 2.30 2487 1794

600 7.0° 4000/3753 4100 3524-4672 0.64 1.47 2784 1148

600 13.0° 7500/6927 6563 5595-7127 0.63 1.45 4528 1535

900 14.3° 5500/5071 5100 4724-5472 0.42 0.96 5323 748

1200 17.5 5000/4630 4000 3718-4277 0.31 0.72 5573 569

1200 26.7° 7500/6919 6620 6357-6879 0.29 0.67 9885 522

Notes and remarks

• With the dichroic (both arms) the whole wavelength 350-9000nm range can be observed at once with a resolution as high as R≈1600, and a 500nm overlap of the two arms (first two gratings)

• The third grating allows observation from Hβ to Hα ([NII] 6583 included) with either arm, at an interestingly resolution of 2400.

• The forth grating permits observing with the blue arm from [OII]3727 to [Fe III]4368. This grating just fells short of covering up to Hβ.

• High resolution around Hα: The fifth grating covers a region around Hα at high resolution (R≈4500) covering from 5600 A up to [SII] 6725. Then again, the system just fells short of reaching [ArIII]7136 and [OII]7325 with the same set up.

• High Resolution around Hβ: the sixth grating allows observing Hβ, [OIII]4959,5018 and [N I]5199 region at a resolution R≈5300. This is also an interesting region for galactic and extragalactic absorption work that reaches the Mgb and Fe lines up to 5400 A. The Na D-line is not easily reached without losing Hβ.

• High Resolution in the Blue: the seventh grating (tightly) covers from [OII]3727 to [FeV]4227 with a 5500 resolution. It would be highly desirable to be able to cover from [OII]3727 to [OIII]4363 at this resolution.

ESOPO



PÁGINA: 31 DE 66

• Even higher resolution around Hα: The final entry is the nominal red 1200 ll/mm grating used to optimize the design (along with the low resolution gratings and the blue 1200 ll/mm grating). This setup covers from [OI]6364 to [SII]6727 with a the a high actual resolution of 10,000 (double as required).

3.8 CCD Detectors

After an exhaustive search of CCD detectors (Marconi, Lraol, Kodak and SiTe and Thomson-CSF), the ESOPO optics and control teams decided to acquire a pair of E2V Technologies (previously MARCONI) CCD detectors:

• CCD42-90: 2048 x 4608 x 13.5 µm square pixels (blue arm);

• CCD44-82: 2048 x 4096 x 15 µm square pixels (red arm).

These are back-illuminated (high quantum efficiency) detectors, but with an optical variant for different optimal anti-reflective coating (AR coating) as presented in Figure 13. These are low-read-out noise and low-dark-current devices.

Blue CCD: the blue CCD42-90 CCD will cover the ESOPO octave from 3500-7000A, and since it is not easy to reach a total 15% total efficiency at 3500 A, the blue CCD will be Enhanced-Broadband coated. The UV-Enhanced AR coating is discarded since it severely limits the optical response for an impressive gain below 3500A that is actually not required in ESOPO. The actual quantum efficiency (QE) varies from chip to chip, but we have specified a minimum of 60% at 3500 and more than 75% from 4000 to 7000, and expect even better responses. This is then an optimal CCD for the blue arm, allowing great response along the whole blue octave, and a non negligible sensitivity just above 30% up to 9000 A. This CCD could, in principle, be used also with the red octave in ESOPO (4500-9000 A), but not up to specifications.

Red CCD: to reach the required 15% throughput at the high-end at 9000A and to optimally cover the 4500-9000A octave, the red CCD44-82 CCD will be Basic-MidBand coated. This AR coating yields at least 80% efficiencies from 4500 to 7500 and gives the

ESOPO



PÁGINA: 32 DE 66

maximum boost to the chip’s 9000A QE (>40%). This chip will make the optics difficult to reach the required 15% throughput at 9000A (unlike the blue CCD at 3500A).

ESOPO



PÁGINA: 33 DE 66

• Figure 13. The Marconi CCD42-90 for the blue arm (350-700nm) will be coated with the Enhance Broad Band AR coating, while the red arm CCD44-82 (450-900nm) will be served by a different chip but prepared with the Basis MidBand AR coating; in any case, this Marconi device pressures the optical elements to compensate for its not very high 900nm efficiency.

3.8.1 Other Potential Coatings

ESOPO



PÁGINA: 34 DE 66

The MODS (LBT) group reports coatings that are already available for astronomical CCDs. Although these solutions are attractive, they imply dealing with developers other than the CCD provider, potentially making the ESOPO project more complex and expensive.

• Figure 14. For example, a variety of CCD anti-reflection coatings exist for the blue channel CCDs. Some examples are shown with measured quantum efficiency curves for CCDs coated by M. Lesser. Note that all are equivalent at 500 nm.

ESOPO



PÁGINA: 35 DE 66

• Figure 15- Quantum efficiency of high resistivity devices currently being produced by LBNL. If this technology can be applied to large format CCDs and if the “cosmic ray” event detection rate is acceptable, then these sorts of devices offer the possibility of unprecedented quantum efficiency in the red

• Table 4 Comparison of CCD Detectors

Some differences between the Marconi 42-90 blue and 44-82 red CCDs

Quantum Efficiency (typical) 1)

CCD Full-well 1)

[e-/pixel]

Dark Current

[e-/pix/hr] 3500 4000 4500 5500 6500 7500 9000

Blue 100,000 0.000022) 66% 87% 88% 83% 78% 70% 32%

Red 150,000 0.393) 20% 53% 77% 92% 91% 80% 42% 1) At 243 K (-30o C) 2) At 173 K (-100o C) estimated from value at 293Kof 60 e-/pixel/s and the relation D(T) = D(To) (T/To)3 exp[(-9080/T)(1-

T/To)] quoted for the AIMO CCD (excluding white defects). 3) At 173 K (-100o C) estimated from value at 293K of 2000 e-/pixel/s and the relation D(T) = D(To) (T/To)3 exp[(-6400/T)(1-

T/To)] quoted for the NIMO CCD (excluding white defects).

ESOPO



PÁGINA: 36 DE 66

Preventing CCD related emergency situations; the ESOPO project has considered the possibility of purchasing a grade 5 CCD42-90, kept cold in a Dewar (the same type as those to be used with the two scientific quality detectors). In this case, a sacrifice will have to be taken in the cosmetic quality and the quantum efficiency at certain wavelength intervals of the detector, getting in exchange a quite accessible cost (about one fifth of a grade 1 detector cost). This emergency detector will be acquired with a coating that allows a reasonable performance at both, the blue and red spectral ranges. A CCD controller designed and manufactured at the IAUNAM-Ensenada labs, will be attached to this detector.

3.9 ESOPO Cameras

The blue and red cameras have the same number of spherical surfaces, air-glass interfaces, and components (a pair of doublet lenses and four singlets) but strongly differ on the glass selection and actual curvature radii.

The focal length of each camera (about 309.4 mm <blue> and 333.33 mm <red>) were finely tuned such that a 0.9” slit width projects on exactly two pixels along the slit (including aberrations of all optical elements, residual de-collimation, and grating amplifications). This yields a detector scale along the slit length close to 0.45”/pixel, and allows for plenty construction and extra-aberration budgets to reach a sampling of 2.3 pixels per FWHM-resolution element.

• Blue Camera glasses: N-BAK1 (Schott), (2) S-FPL53 (Ohara), BSM51Y (Ohara),

(2) PBL26Y (Ohara), PBL6Y (Ohara) and Silica (Schott, Dewar window). No aspherical surfaces.

• Red Camera glasses: (2) N-SSK5 (Schott), (2) S-FPL53 (Ohara), N-LAK8 (Schott), (2) SF5 (Schott) and Silica (Schott, Dewar window). No aspherical surfaces.

• Surface curvature radii are not particularly difficult. • No lens thickness (central or edge) is particularly fragile • The whole system was optimized in spectroscopic modes (resulting on a highly

apochromatic system) • In both cameras the Dewar window has the same power (concave-plane singlet)

for a straightforward exchange of detector-Dewar systems among arms, but not with other SPM instruments, or –as mentioned above- an emergency Dewar with a grade 5 detector will be used.

The next figure simply summarizes the geometry of both ESOPO cameras, full optical details of the cameras and the rest of the optics are presented in next section.

ESOPO



PÁGINA: 37 DE 66

• Figure 16 Blue (left) and Red (right) ESOPO Cameras. Both systems consist of a pair of doublets and four single lenses (the last one is the Dewar window and is plane at its back surface) but differ on their glass selection for optimal performance within their respective wavelength coverage. The systems do not present any foreseen technical difficulty for fabrication.

3.10 ESOPO Image Quality

The optimal spectroscopic sampling is close to 2 pixels per FWHM resolution element. This sampling is a compromise for the best flux-sampling (>1 pixel/FWHM), a good radial velocity (peak or centroid position) sampling (just above 1 pix/FWHM), a good sampling for line-widths and line-asymmetries (2.3 pix/FWHM) for lines with an internal width close to the chosen resolution. Off course, broader-than-resolution lines are always better oversampled and all the higher moments of the line profile are also measured (enough signal-to-noise provided).

ESOPO



PÁGINA: 38 DE 66

• Figure 17. Encircled energy profiles for the ESOPO blue arm at 700 and 350 nm. The 13.5µm pixels correspond to 0.45”, so the radial scale of the plot correspond to 7.5µm, 0.25”, or 0.555 pixels. In both spectroscopic cases, and for all fields along the whole 10’ slit length, 76% of the energy is contained within a diameter of 10.9µm in the worst case, 0.365” or 0.81 pixels. Each field along the slit is marked with is value in degrees (the +/-5’ fields at the slit edges correspond to 0.083º).

The design philosophy was oriented to achieve a sampling well within the hardest required limit (2.3 pix/FWHM), so the focal lengths of the cameras were such to project the 0.9” slit width on two pixels. Therefore, if the instrument were to degrade the slit width from 2 to 2.3 pixels, the maximal image FWHM intrinsic to the spectrograph optics is about (2.32-22)1/2=1.14 pixels (or 15.4µm). The following pair of figures show how the present design achieves an image quality (in both imaging and spectroscopic modes) much better than that since 85% of the light is enclosed in diameters smaller than 0.6 pixels in both, encircled (imaging) or enslited (spectroscopy) polychromatic energy. Remember that a 2-D Gaussian (imaging) FWHM contains 50% of the energy, while a 1-D Gaussian (spectroscopic extraction) FWHM contains about 76% of the energy.

ESOPO



PÁGINA: 39 DE 66

• Figure 18. Same as in the previous figure but for the ESOPO red arm. The red system produces even better overall image quality than the blue am, so good in both cases that the image quality degradation of the ESOPO optics is negligible for all practical purposes. Therefore, the present design can deliver actual resolutions only limited by the grating properties and the slit width, so gratings delivering up to for times the resolution of the nominal 1200 l/mm can be used in ESOPO to reach R probably higher than 15,000 in the blue and 25,000 in the red.

The following figure graphically shows that the spectroscopic image quality does vary with wavelength and along the slit, but not sensibly so, and given the small size of the images (less than 0.5 pix FWHM) the variations will be totally negligible even with the narrowest slit of 0.9”. Therefore, the resolution variations along the slit will be limited by the slit mechanical quality as well as by the quality of the grating ruling (but not by the ESOPO optics) which are expected to be better than the required goal for 5%.

Figure 14. Monochromatic images produced by the ESOPO spectrograph with a grating covering the 350-700nm octave of the blue arm (right) and for a grating covering the 450-900nm octave of the red system (left). The RMS size and variations of the images FHWM are negligible small relative to the projected size of a 0.8¨wide slit.

ESOPO



PÁGINA: 40 DE 66

Conclusions:

• The present design concept delivers supreme image quality along the whole field and wavelength range.

• This warranties delivering of the required maximum resolution (R=5000) from 350nm to 900nm along a 10’ slit with an slit aperture of 0.8¨ and conventional 1200 ll/mm gratings.

• A 2.3 pix/FWHM resolution sampling can be actually achieved with a 0.9”-1” slit, with the consequent light-gathering gain from a 0.8” opening.

• The instrument can deliver much higher resolution than required, even through narrower slit, working on second order, or better yet through gratings of a higher ruled-density. Conventional gratings easily reach at least 2400 ll/mm ruled densities (volume phase holographic gratings can deliver much higher performances). Therefore, R=15,000 can be easily achieved with ESOPO, closing the gap between ESOPO and the Echelle SPM spectrograph.

• The present design yields 2-D and spectroscopic FHWM images well within a pixel (13.5 µm).

• These image sizes broaden the FWHM of the project 0.8” wide slit insignificantly (< 4%), far less than the allowed degradation to reach a 2.3 pixel/FHWM sampling (57%).

THROUGHPUT

The following table summarizes the expected throughput of ESOPO under different circumstances. The following subsections present a breakdown (easier to digest) discussion of these expectations.

ESOPO



PÁGINA: 41 DE 66

3.11 Optics: Transmission and AR Efficiencies

ESOPO Optics Efficiency

0.20.30.40.50.60.70.80.91.0

0.30 0.40 0.50 0.60 0.70 0.80 0.90

Wavelength

Effic

ienc

y Transmission BATransmission RABlue Arm (T+AR)Red Arm (T+AR)Blue Arm (T*AR*Mirror)Red Arm (T*AR*M)

3.12 Instrument (Single arm operation): Optics + Grating + Detector + Folder Mirror

To estimate the spectroscopic performance of ESOPO we use a couple of RGL gratings actually available and in use with the B&Ch at SPM.

ESOPO & 300 ll/mm Gratings

0.00.10.20.30.40.50.60.70.8

0.30 0.40 0.50 0.60 0.70 0.80 0.90

Wavelength

Effic

ienc

y

Blue SpecOptics+CCDRed SpecOptics+CCDBlue Grating & Spec

Red Grating & Spec

ESOPO



PÁGINA: 42 DE 66

3.13 Instrument (Dual arm operation): Optics + Grating + Detector + Dichroic

ESOPO on Telescope (dual beam operation)

-0.10.00.10.20.30.40.50.6

0.30 0.40 0.50 0.60 0.70 0.80 0.90

Wavelength

Effic

ienc

y

Blue Grating & ArmRed Grating & Arm

3.14 Actual Efficiency: Instrument on Telescope & comparison with Brera B&Ch

ESOPO



PÁGINA: 43 DE 66

Figure 15 Actual efficiency of the B&Ch Brera’s Spectrogrpah on the 2.1 mts (includes telescope and SiTe3 CCD)

ESOPO & B&Ch (net efficiencies with 300 ll/mm gratings)

0.00.10.20.30.40.50.6

0.30 0.40 0.50 0.60 0.70 0.80 0.90

Wavelength [nm]

Effic

ienc

y

ESOPO BlueSpectrographESOPO RedSpectrographB&Ch Blue Grating

B&Ch Red Grating

ESOPO-B&Ch Efficiency Gain

1.0

10.0

100.0

0.30 0.40 0.50 0.60 0.70 0.80 0.90

Wavelength [nm]

ESO

PO/B

&C

h

Blue GratingRed Grating

The B&Ch efficiencies (next figure) are “as observed”, that is the combined effect of telescope, optics, grating and detector efficiencies. For a fair comparison with ESOPO, the following comparison is the above instrument on telescope efficiencies selecting gratings.

ESOPO



PÁGINA: 44 DE 66

4. DESCRIPCIÓN DEL PROCESO DE DISEÑO ÓPTICO 4.1 General

El estado actual del diseño óptico de ESOPO ha requerido de un proceso previo que puede describirse así:

1. El concepto de ESOPO se inicia con un proceso que, sucintamente, consiste en que: Se define un conjunto de proyectos y objetivos científicos que deben describir, lo más claramente posible, para que tipo de problemas científicos el espectrógrafo, a ser diseñado, constituya una herramienta fundamental para la investigación astronómica basada en la observación. Hay que decidir, por ejemplo, si se quiere que sea de uso general ó un instrumento, con un alto grado de especialización, dedicado a un número relativamente limitado de tipo de observaciones. Asimismo, en la medida en que se vayan decidiendo los objetivos científicos a alcanzar, se van a ir tomando decisiones respecto al cubrimiento espectral, tamaño del campo, dispersión espectral deseada, etc.

2. Esta etapa inicial del proyecto se lleva a cabo, primordialmente, mediante un proceso iterativo en el que los astrónomos del proyecto, a través de una serie de consultas y discusiones con diversos usuarios del Observatorio Astronómico Nacional y con la formación de un Comité Asesor Científico, van definiendo las características del espectrógrafo, que dan como resultado la generación de una primera lista de Requerimientos de Alto Nivel (RAN) para ESOPO.

3. Los RAN incluyen: rango espectral; en que configuración del telescopio se quiere acoplar (F/7.5 en nuestro caso); la eficiencia <throughput> del instrumento, sin telescopio y con detector, a diferentes longitudes de onda; resoluciones mínima y máxima; modos de operación; resolución espectral muestreada, con una rejilla de cuantas líneas por mm y tamaño de la misma y para que tamaño de rendija nominal; variaciones máximas en la resolución aceptables a lo largo de la rendija; tamaño del campo (que tan larga es la rendija); ancho de la rendija; escala sobre el detector (segundos-arco/pixel, definiendo tamaño del pixel), prioridades entre muestreo espectroscópico y espacial; dimensiones del detector; definición de requerimientos ambientales de diseño, operación y supervivencia; necesidad o no de definir si se requiere un corrector de dispersión atmosférica; etc.

4. A los anteriores requerimientos se agregan otros (también de alto nivel) que se relacionan con características de linealidad y ruido de lectura del detector, y otros más relacionados con la operación del instrumento, cambios de configuración (número de rejillas intercambiables sin necesidad de abrir el instrumento durante la noche de observación) y definición de tiempo muerto máximo entre cambios de configuración; calibraciones del espectrógrafo al principio y final de la noche, y comportamiento ante flexiones, cambios de temperatura, etc.

5. De los puntos anteriores se desprende lo siguiente: Se hizo un esfuerzo intenso y consumidor de tiempo, en aras de definir con la mayor precisión posible, el uso astronómico del instrumento y que características se esperan de él. Se parte de la convicción de que es un esfuerzo que se verá compensado por la calidad del instrumento, siempre que se tenga un minucioso cuidado en las otras etapas del proyecto: diseño, manufactura, ensamble, pruebas y un aprovechamiento de la etapa de “commisioning” en el telescopio que realmente permitan caracterizar bien el espectrógrafo ya construido.

ESOPO



PÁGINA: 45 DE 66

6. Estos primeros requerimientos fueron ampliamente discutidos con el Comité Científico Asesor, y siguen siendo revisados de manera iterativa conforme el diseño avanza. Los RAN actuales se muestran en el Anexo 1.

Vale la pena aclarar que el lujo de detalle, al describir el proceso de elaboración de los RAN, tiene la intención de establecer que, si se quiere tener al final un instrumento de excelencia -pues el costo es alto y mucho mayor el de posibles instrumentos para un futuro gran telescopio mexicano-, es preciso tener una metodología mucho más cuidadosa y detallada que la que tradicionalmente hemos usado, lo cual implica mucho mas trabajo.

Una vez definidos claramente los RAN, hay que traducirlos a las características técnicas a considerarse en los diseños óptico, mecánico, etc. Se empieza con el diseño óptico, pues éste va a definir, en gran medida, las dimensiones del instrumento.

Es éste también un proceso iterativo en el que se proponen y evalúan diseños conceptuales que -en principio- deben cumplir con las especificaciones y requerimientos, para que posteriormente los comités respectivos vuelvan a revisar y, de ser necesario modificar los RAN (generalmente sólo si se encontrara que algún requerimiento no fuera viable). Esta forma de trabajo es una de las que mas frecuentemente se siguen en proyectos sujetos a la normatividad internacional.

4.2 Diseño Óptico

El diseño óptico de ESOPO tuvo como puntos de partida:

Con base en la experiencia que tuvimos en el proyecto OSIRIS, tenemos claro que:

Se requiere, en ESOPO, de un sistema óptico Reductor Focal, consistente de:

• un colimador que forme haces de luz, lo más paralelos posibles, para rayos provenientes de cada punto del campo lineal (rendija) y en cada longitud de onda y que, además, forme una imagen del espejo primario de un diámetro deseado (tamaño de pupila), y

• una cámara

El comportamiento espectroscópico del diseño iba a estar modulado por el grado de colimación alcanzado en el diseño del colimador, tanto para cada campo (lineal en la rendija) como para varias longitudes de onda en el rango espectral requerido. Mientras mejor sea la colimación de los ha

También iba a ser importante que el tamaño de la pupila se mantuviera lo más cercano posible al deseado (100 mm de diámetro),

La Figura 1 muestra el concepto básico para la distribución del intervalo espectral requerido en ESOPO, dividido en los dos brazos del instrumento mediante un dicroico (parte baja), y el cubrimiento espectral en el que cada brazo debe estar optimizado por sí mismo (parte alta de la figura).

ESOPO



PÁGINA: 46 DE 66

• El diseño óptico se hizo utilizando, principalmente, el paquete Zemax, y bases de datos conteniendo las características detalladas de los vidrios ópticos comerciales, principalmente de las fábricas Schott y O´Hara.

•

• A continuación se muestran los parámetros más importantes que definen el diseño óptico del espectrógrafo:

• Rendija

– 10´ de largo: 46.04 mm (76.74 µm/”). Para diseño: 8’ de largo: 36.83 mm – Apertura desde 0.2” – Imagen del telescopio -ideal- en el campo: 3.8 µm (0.05”)

• Lente de campo (pre-colimador)

– Doblete a 47.6 mm de la rendija compartido por ambos brazos. D=70mm – Ayuda a homogenizar los campos divergentes que llegan al colimador

• Colimador

– Triplete refractor (por brazo) a 891.45 mm <azul> y 895.35 mm <rojo> de la rendija

– Diámetro: 120 mm en ambos brazos – F/7.5 produciendo un haz colimado de 10 mm (f=791.53 mm <azul>, 791.35 mm

<rojo>) – Colimación RMS: 0´.029 <azul> y 0.´038 <rojo> (telescopio = 0´.02)

• Dicroico y espejo doblador (entre lente de campo y colimador)

– Posición nominal 556 mm y 706 mm de la rendija (mecánica) – Ángulo de inclinación 22.5° (con margen para mecánica) – Apertura en posición nominal:

• Rejillas de Reflexión

– A 1167.00 mm <azul> y 1164.00 mm <rojo> de la rendija (225.39 mm <azul> y 217.39mm del colimador <rojo>)

– Montura cónica (22°.5) en Littrow – Apertura clara: 103x117 mm (Lxλ) – Rayada y/o holográfica con blaze

• Cámaras (Figura 2)

– Escala 0.45”/pix: f = 309.4 mm <azul> y 343.8 mm <rojo> (F/5 y F/5.6 ) – 1a superficie a 150 mm de la rejilla en ambos brazos

ESOPO



PÁGINA: 47 DE 66

– 2 dobletes y 3 singletes (esféricas). Diámetro máximo: 172 mm <azul> y 157 mm <rojo>

– Diámetro mínimo: 76 mm (ventana del criostato) – Amplio espacio para obturador y compensación térmica pasiva – Ventana de criostato idéntica en ambos brazos

• Detectores e2v (Marconi) back-illuminated, NIMO

– A 9.2 mm de la ventana del criostato – Azul: 2048x4608 13.5µm pixeles, adelgazado, broad-band astro – Rojo: 2048x4096 15 µm pix, deep-depletion, low-fringing, Mid-band astro

Los resultados más relevantes ya considerados en el diseño óptico final son:

• Cumplimiento, con margen de seguridad, de todos los requerimientos de alto nivel. • Eficiencia entre 3 y 20 veces mayor a la del espectrógrafo actual, además de cubrir todo

el rango espectral en una sola exposición al doble de resolución. • Operación estable sin ajustes finos, sin necesidad de enfocar cámara o colimador • Distorsión: < 0.8% (azul) y < 0.7% (rojo) • Imagen instrumental (FWHM) en 8’ (todas dispersiones):

o Azul: 3.2µm (1.3 µm RMS) mn-mx: 0.8-7.7 µm (ver Figura 3) o Rojo: 3.5µm (1.4 µm RMS) mn-mx: 1.3-7.8 µm

• Amplio margen de construcción/operación: degradación hasta 15 µm • Muestreo final: 2.1 pix/FWHM (rendija 0.8”) Imagen instrumental final: 1.12pix (15.1

µm y 16.8µm) • Contingencia de intercambio de detectores entre ambos brazos • Acoplamiento de multipletes con gels curables (espesor ~80 µm)

Y a considerar a futuro: • Recubrimiento antireflejante (14 interfaces aire/vidrio) ~ 0.8%. • Compensación térmica pasiva.

ESOPO



PÁGINA: 48 DE 66

Cámara Roja Cámara Azul


0.00.10.20.30.40.50.6

0.30 0.40 0.50 0.60 0.70 0.80 0.90

Wavelength [nm]

Effic

ienc

y


B&Ch Red Grating

ESOPO



PÁGINA: 49 DE 66


0.00.10.20.30.40.50.6

0.30 0.40 0.50 0.60 0.70 0.80 0.90

Wavelength [nm]

Effic

ienc

y


B&Ch Red Grating

4.2.1 Particular: Colimadores

4.2.1.1 COLIMADOR (BRAZO AZUL)

MATERIAL DIÁMETRO (mm) ESPESOR (mm) PROVEEDOR

SILICA* 70 7.8 SCHOTT

CaF2* 70 15.5 SCHOTT

SILICA 118 12.7 SCHOTT

S-FPL53 118 26.4 OHARA

N-BAK2 118 9.9 SCHOTT

* común a los dos brazos: azul y rojo.

4.2.1.2 COLIMADOR (BRAZO ROJO)


SILICA* 70 7.8 SCHOTT

CaF2* 70 15.5 SCHOTT

S-BAL11 118 12.9 OHARA


S-BAL11 118 11.8 OHARA

* común a los dos brazos: rojo y azul.

ESOPO



PÁGINA: 50 DE 66

4.2.2 Particular: Cámaras

4.2.2.1 CÁMARA (BRAZO AZUL)


N-BAK1 155 15 SCHOTT


S-FPL53 162.5 39.6 OHARA

BSM51Y 162.5 24 OHARA

PBL26Y 170.5 30 OHARA

PBL26Y 130.5 30 OHARA

PBL6Y 80 7 OHARA


4.2.2.2 CÁMARA (BRAZO ROJO)


N-SSK5 144 11 SCHOTT

S-FPL53 144 27 OHARA

S-FPL53 151.5 42 OHARA

N-LAK8 151.5 17 SCHOTT

SF5 157 17.6 SCHOTT

N-SSK5 142.5 26.1 SCHOTT

SF5 77.5 7 SCHOTT


ESOPO



PÁGINA: 51 DE 66

Figura 3. Imágenes Espectroscópicas de ESOPO (brazo azul)

ESOPO



PÁGINA: 52 DE 66

5. CONCEPT DIRECTRICES TOWARDS A FINAL DESING The present design concept is just that a preliminary set of parameters to build a system in principle capable of fulfilling the requirements for the ESOPO spectrograph. Nevertheless the system can be adjusted (or tuned up) to improve in the following directions:

1) Increase wavelength coverage by at least a factor of 1.25

a. This is highly desirable, since

i. System just falls short to cover range combinations of high astronomical interest (see table)

ii. Increases the minimal resolutions that cover the full wavelength range (3500-9000 angs), the main (3500-7000 & 4500-9000) or any other octaves of interest.

iii. May reduce the number of required gratings.

ESOPO



PÁGINA: 53 DE 66

iv. Reduces observation overheads (of course) .

b. Larger detectors are an unlikely option (expense) but the presented design of the cameras has more than enough leverage to accommodate more coverage on larger CCD formats.

c. Reduction of the Camera-Collimator angle (“spectrograph” angle). A reduction from 45 to about 36 degrees is still mechanically and optically possible and would increase wavelength coverage by 10% (interesting!)

d. Hybrid grating mount: a given camera-collimator angle can be achieved with a combination of a conical (off-plane) and a Non-Ebert configurations. As an example, at the expense of small variable anamorphic amplification and off-littrow performance losses (never as large as in the B&Ch mount though), 35 system, with X off-plane and Y non-ebert deviations increases the wavelength coverage of the present design by 25% (substantially better, increasing the already astronomically interesting coverage).

e. Faster camera (i.e. more arcseconds –and angstroms- per pixel). The present design delivers such small images that this is quite possible without noticeably degrading the actual size of the slit-width on the detector (i.e. the resolution that the astronomer measures is still determined by the slit even at the highest R or narrower widths). This makes a “more efficient” spectrograph -more light per pixel- since the resolution element is projected into less pixels, but decreases the ability to measured higher order moments (line centers, widths, asymmetry, kurtosis, etc) of spectral lines with a natural width close to the used resolution. A change from 0.4”/px to 0.5”/px, increases wavelength coverage by a factor of 1.25 (very nice!) and only produces under-sampled spectra when closing the slit below 1”.

2) Increase the maximal resolution covering the whole spectral range

a. There is no reason to consider gratings of lower resolution than this (since coarser pixelization can be done with software or while readout) and, at this resolution threshold, there is no need to fine-tune the central wavelength. Therefore, a system with less fine and high-precision adjustable mechanism can be conceived.

b. This may require a CCD format with more pixels along the dispersion (e.g., 2048x4096, 2048x6144, etc). This is highly desirable and convenient also to increase the wavelength coverage.

ESOPO



PÁGINA: 54 DE 66

c. With a 2048x4096 CCD format can double the wavelength coverage, with a faster camera it is possible to cover the whole 3500-9000 range with a resolution close to 3000.

d. Under c), each arm will only have three or four (fixed) configurations: one for R≈3000 and two or so more for R=5000, with fixed central wavelengths (avoiding the very expensive and hardly repeatable fine-tilt mechanism of the gratings).

e. Having a finite (hopefully one) configuration set of gratings, makes the instrument lighter, mechanically simpler and sturdier, and more stable with standard calibrations applicable to all users.

3) Simplification to a single arm system

a. Blue arm already supersedes the B&Ch performance significantly (red does not).

b. There would be some throughput reductions. If blue CCD is kept as proposed (UV-AR coated & not fringe optimized) the red end (beyond 7000) will be less efficient by about or so, and may suffer more severe fringes. A “broader” CCD AR coating can split throughput losses between blue and red ends but is less desirable given the high value of UV photons and high sky emission in the red.

c. There is at least one option to still cover the whole 3500-9000 A range with the blue arm: implementation of a cross-dispersed low-order Echelle system (also know as “echelette”) which are based on gratings ruled for optimal performance between 3rd and 6th orders (unlike typical ehelle spectrographs that work much higher orders). This option can be implemented at the expense of reducing the slit length (form 10’ to around 5-6´) and to alter the collimator design somewhat (unless Vphs are used). This system will also yield lower total efficiencies across the whole wavelength range than the 2-arm system, and then again even more so beyond 6000A.

4) Imaging Capabilities

a. Given its refractive collimator, the present optical design provides excellent fully-polychromatic 2D images, allowing for the possibility of direct broad and –more importantly- narrow band imaging.

b. This only requires demanding the slit to even be retrieved or to open to the full field.

ESOPO



PÁGINA: 55 DE 66

c. Even if the system is not required to accommodate more filters (besides the minimal order blockers for spectroscopy) this feature is of high interest.

d. This feature also allows for multi-object-spectroscopy (MOS) capabilities in ESOPO. Although MOS is not a powerful enough in small-aperture telescopes (faint-object crowded fields), it can increase multiplexing observations of not very crowded and more “classical” fields (galaxy pairs or small groups, notches of nebular emission, some stellar fields, etc.). Given the small physical size of the ESOPO field of view (10’x10’ represent at the telescope focal plane) the following considerations may be of relevance:

i. Avoid need to cut masks (cutters are expensive and not necessary in this case)

ii. Masks can be made with photographic film (or other substrate –silica p.e- to minimize uv losses)

iii. A small number of fixed-length and fixed-width slitlets, covering the 10’ field length, but movable along the dispersion direction (like the MOS unit of the HET’s LRS) is a simple and inexpensive concept that gives powerful and enough flexibility to this small telescope. Being far beyond the scope –and budget- of ESOPO and the SPM 2.1m telescope, solutions like the FORS –IFU systems are not necessary.

iv. Lenslet-based Integrating Field Units (IFU) are the most powerful 3-D systems to date and could be designed to used the 2.1m/ESOPO system as a fully-powered 3-D spectrograph, but then again these are too complex, too expensive, and scientifically uncalled for small telescopes unless designed to cover unprecedented fields of views.

ESOPO



PÁGINA: 56 DE 66

OPTICAL ELEMENTS: Dimensions and Weights

The total weight of the glass lenses ia about 10kgs for each arm (as detailed in the next table). Each grating (185x220x35 mm BK substrate) weighs 3.2 kg. It is expected that the opto-mechanic weight needed to hold the lenses (cells and barrels) is about 3 times the glass weight (30-40 kgs). The optomechnics weight to hold the grating is expected to be 4-5 times the grating weight (about 15kgs).

Lens

Element

Material Ctr/EdgeDiam volumedensityWeightMaterialCtr/EdgeDiam volume densityWeight

FLa Silica

FLb

C1

C2

C3

D1a

D1b

D2a

D2b

S1

S2

W

Grating

Dichoic

Mirror

BLANK GLASSES, PROPERTIES AND DIMENSIONS: ESTIMATED COST BUDGET

BLUE SYSTEM RED SYSTEM

Lens

ID

Diameter

(mm)

Material

$/cm3

Central

Thick

(mm)

Curv.

Radii

(mm)

Blank

Volume

~Cost

Material

$/cm3Thick Radii

Blank

Volume

Cost

F F 69 4 Sili 72 17

ESOPO



PÁGINA: 57 DE 66

1.29 480.8 69.2

$90

Fb 69.4 CaF2

3.22 16.7

480.8

-75.28

72x21

86

$275

I

E

L

D

Test plates $1000.00

$1356.60

Lens Diameter Material Radii Blank Material Radii Blank

C1 128.4 Silica

1.29 7.6

3991

425.6

131x19

256.1

$330

N-K5

1.20 15.8

1181

317.2

131x24

323.5

$390

C2 128.4 CaF2

3.22 28.2

425.6

-192.6

131x33

444.8

$1432

CaF2

3.22 24.6

317.2

-283.3

131x29

390.9

$1260

C3 128.4 N-K5

1.20 16

-192.6

-351.5

131x36

485.2

$585

N-BAK1

1.67 12.5

-282.3

-404.0

131x23

310

$520

C

O

L

L

I

M

A

T

O

R Test plates

$2000.00 $2000.00

$4344.90 $4164.60

Lens Diameter Material T Radii Blank Material Thick Radii Blank

D1a 142.2 BAK4

1.34 13

+391.9

+111.9

145x43

710.1

$950

SSK2

2.76 20

+370.5

+112.4

145x54

891.7

$2460

D1b 142.2 N-FK56

10.0 42

+111.9

-905.2

145x46

759.6

$7600

N-FK56

10.0 40

+112.4

-1008.7

145x44

726.6

7270

D2a 142.2 CaF2

3.22 45

+192.6

-141.3

145x49

809.1

$2600

N-FK56

10.0 47

+148.1

-187.3

145x51

842.2

$8420

D2b 142.2 I-BSM51Y

4.55 15

-141.3

plane

145x27

445.9

$2030

N-LAK8

8.67 20

-187.3

plane

145x44

726.6

6300

S1 133 I-PBL26Y

4.55 25

-420.7

-166.4

135x34*

486.7

$2215

SF5

1.98 20

-385.6

-187.3

135x31

443.7

$880

C

A

M

A

E

R

A

S

S2 45 LLF1 5 -76.9 47x9 SF1 4.5 -71.6 47x13

ESOPO



PÁGINA: 58 DE 66

2.12 -317.2 15.6

$33

2.24 -172.6 22.6

$51

Dewar

Window 40.5

Silica

1.29 3 planes

43x5

7.3

$10

N-BK7

1.00 3.0 planes

43x5

7.3

$7

22 15438.7 25385.0

Test Plates 1000 4000.0 4000.0

Fc Field Lens 1365 1365

Collimator 4350 4200

Camera 19450 29400

Whole Arm 25165 34965

Both arms 58765

SOME RELEVANT ISSUES REMAINING

• Shutter number and location: single shutter (close to the slit) or a shutter per arm

(in camera?)

• Focusing element(s)

• Plane or cylindrical slit?

• Passive thermal compensation (image motion and plate scale changes)

• Location and number of order-blocking filters: close to slit, after collimator, close to or within camera, ect?

• Slit-masks: needed or required (nod&shuffle p.e.)?

• AR coatings detailed specifications

• Verify whether dichroics with incidences less than 450 are actually simpler to make, more efficient, etc.

• Dichroic specifications, manufactures, etc.

6. OPTICAL ELEMENTS: DIMENSIONS & WEIGHTS

ESOPO



PÁGINA: 59 DE 66

The total weight of the glass lenses ia about 10kgs for each arm (as detailed in the next table). Each grating (185x220x35 mm BK substrate) weighs 3.2 kg. It is expected that the opto-mechanic weight needed to hold the lenses (cells and barrels) is about 3 times the glass weight (30-40 kgs). The optomechnics weight to hold the grating is expected to be 4-5 times the grating weight (about 15kgs).

Lens

Element

Material Ctr/Edge Diam volume density Weight Material Ctr/Edge Diam volume density Weight

FLa Silica

FLb

C1

C2

C3

D1a

D1b

D2a

D2b

S1

S2

W

Grating

Dichoic

Mirror

7. BLANK GLASSES, PROPERTIES AND DIMENSIONS: ESTIMATED COST BUDGET

BLUE SYSTEM RED SYSTEM

Lens

ID

Diameter

(mm)

Material

$/cm3

Central

Thick

(mm)

Curv.

Radii

(mm)

Blank

Volume

~Cost

Material

$/cm3Thick Radii

Blank

Volume

Cost

ESOPO



PÁGINA: 60 DE 66

Fa 69.4 Silica

1.29

480.8

72x17

69.2

$90

Fb 69.4 CaF2

3.22 16.7

480.8

-75.28

72x21

86

$275

F

I

E

L

D

Test plates $1000.00

$1356.60

Lens Diameter Material Radii Blank Material Radii Blank

C1 128.4 Silica

1.29 7.6

3991

425.6

131x19

256.1

$330

N-K5

1.20 15.8

1181

317.2

131x24

323.5

$390

C2 128.4 CaF2

3.22 28.2

425.6

-192.6

131x33

444.8

$1432

CaF2

3.22 24.6

317.2

-283.3

131x29

390.9

$1260

C3 128.4 N-K5

1.20 16

-192.6

-351.5

131x36

485.2

$585

N-BAK1

1.67 12.5

-282.3

-404.0

131x23

310

$520

C

O

L

L

I

M

A

T

O

R Test plates

$2000.00 $2000.00

$4344.90 $4164.60

Lens Diameter Material T Radii Blank Material Thick Radii Blank

D1a 142.2 BAK4

1.34 13

+391.9

+111.9

145x43

710.1

$950

SSK2

2.76 20

+370.5

+112.4

145x54

891.7

$2460

D1b 142.2 N-FK56

10.0 42

+111.9

-905.2

145x46

759.6

$7600

N-FK56

10.0 40

+112.4

-1008.7

145x44

726.6

7270

D2a 142.2 CaF2

3.22 45

+192.6

-141.3

145x49

809.1

$2600

N-FK56

10.0 47

+148.1

-187.3

145x51

842.2

$8420

D2b 142.2 I-BSM51Y

4.55 15

-141.3

plane

145x27

445.9

$2030

N-LAK8

8.67 20

-187.3

plane

145x44

726.6

6300

C

A

M

A

E

R

A

S

S1 133 I-PBL26Y

4.55 25

-420.7

-166.4

135x34*

486.7

$2215

SF5

1.98 20

-385.6

-187.3

135x31

443.7

$880

ESOPO



PÁGINA: 61 DE 66

S2 45 LLF1

2.12 5

-76.9

-317.2

47x9

15.6

$33

SF1

2.24 4.5

-71.6

-172.6

47x13

22.6

$51

Dewar

Window 40.5

Silica

1.29 3 planes

43x5

7.3

$10

N-BK7

1.00 3.0 planes

43x5

7.3

$7

22 15438.7 25385.0

Test Plates 1000 4000.0 4000.0

Fc Field Lens 1365 1365

Collimator 4350 4200

Camera 19450 29400

Whole Arm 25165 34965

Both arms 58765

8. REQUIREMENT COMPLIANCE MATRIX

1. OAN Institutional Project to substitute the B&Ch (Brera) on a short time scale, keeping at least its functions and capabilities and using present-day technology and under already proven concepts.

PARTIAL COMPLIANCE (N/A to optics only): Optical design out of schedule; no innovations in optical design.

2. Optical low-dispersion & long-slit spectrograph, optimized for the 350-900 nm spectral range

(no integrated field or imaging capabilities required).

COMPLIANCE EXCEDED: range fulfilled but design permits excellent imaging and MOS capabilities.

3. The spectrograph must couple to the f/7.5 focus (with guider) of the SPM 2.1 m Telescope.

COMPLIANCE

4. High efficiency is of highest priority. Efficiencies (without telescope but including detector): 3500 Å > 15%, 4500 Å > 35%, 5500 Å > 36%, 7500 Å > 40%, 9000 Å > 15% (goal without grating or CCD: >80% from 3500-9000 Å).

ESOPO



PÁGINA: 62 DE 66

PARTIAL plus CLOSE-TO-GOAL COMPLIANCE:

Efficiencies (everything but telescope): 3500=0.14, 4500=0.53, 5500=0.53, 7500=0.42%, 9000=0.19 (grating dependent)

Optics (no mirrors, grating nor CCD): 3500=0.62, 4000=0.81, 5000=0.82, 6000=0.80, 7000=0.80, 8000=0.80, 9000=0.80

5. The whole spectral range must be observable at once, at least at the minimum resolution

(R~300 or higher).

COMPLIANCE EXCEDED: the whole spectral range is observable with R≈1600

6. If a 2-arm design is adopted, operation must allow use of either arm alone or both arms simultaneously.

COMPLIANCE

7. A maximum real (actual) FHWM resolution R=5000 must be achieved with 1200 ll/mm

grating, with a maximum cross section of 154 x 206mm, and a nominal slit width of 0.8’’.

COMPLIANCE EXCEDED: At 4000 real R≈5500, at 6600 real R≈9900 and higher at 8000, with such a gratting

8. The spectral resolution sampling: from 2.3 to 4.0 píxels per FWHM (0.8” slit).

COMPLIANCE EXCEDED: 2 pixel sampling for construction degradation or, even better, a

wider slit (0.9”-1”)

9. Maximum variation of actual resolution along the slit: less than 10% (goal: <5%).

COMPLIANCE: Optical Design Complies (including grating and optical distortions) final

compliance depends on Slit design (mechanics package)

10. Pupil size limited by the size of commercial (off-shelf) gratings.

COMPLIANCE EXCEDED

11. Gratings must be interchangeable.

N/A to optics (Mechanics) but optical design accommodates for that.

12. At least two gratings should be usable without need of opening the instrument (goal: 3

gratings per arm).

N/A to optics (Mechanics) but optical design accommodates for goal of 3/arm.

ESOPO



PÁGINA: 63 DE 66

13. The field around target must be visible and it should be possible to center target on the slit

during exposures.

N/A to optics (Mechanics) but optical design accommodates for that.

14. Slit length: a minimum 8’ field and a goal of 10’.

GOAL COMPLIANCE: Slit-length=10’

15. Detector plate-scale: ≤ 0.5 ’’/píxel (a goal, since spectroscopic sampling has priority over

spatial sampling).

COMPLIANCE EXCEDED: actual scale 0.4”/pixel

16. Design shall consider cameras for a CCD format of 2048 píxels.

COMPLIANCE

17. Read out noise: < 8e- (goal of <5).

COMPLIANCE NA to optics, but detector complies (≈ 3.0 e-)

18. Linear detector response up to a 250 signal-to-noise (full-well capacity > 62,500 e-/pixel).

COMPLIANCE NA to optics, but detector complies (>99,000 e-/pixel).

19. Minimum slit width less than or close to the spectrograph diffraction limit.

COMPLIANCE NA to optics (Mechanics).

20. Maximum slit width: > 9” (goal >10”).

COMPLIANCE NA to optics (Mechanics) but optical design accommodates for full-field

opening (10’).

21. Dead time between configuration changes: comparison arcs ≤ 1 min; other (grating, slit,

etc.): ≤ 3-5 min.

COMPLAINCE NA to optics (Mechanics).

22. Repetitivelity and stability such that: a. Resolution should not degrade by more than 2.5% over 30 min exposures, b. Resolution constant within 5% after changing and coming back to a given

configuration (goal: 3%), c. Relative calibrations at beginning/end of night (relative to spectral dispersion and

resolution, plate scale and responses along slit and spectral directions) should apply to all within-night data with a confidence level better than 10% (in relative functional shape, but not in an absolute sense or zero point) under the nocturnal temperature changes (+/-6˚C), flexures or other derivatives (goal: 5%).

ESOPO



PÁGINA: 64 DE 66

PARTIAL COMPLIANCE; TOTAL COMPLAINCE TBD: Not only applicable to Optics (Mechanics drives compliance) but Optical Designs considers passive thermal compensation as part of its package.

23. Design, operation and survival ambient requirements:

d. Optimization temperature and atmospheric pressure: T=3˚C; P=562 mmHg. e. Minimum range of operation (fulfilling requirements): from -10˚C to 16˚C and

545-570 mmHg. f. Minimum survival range: from -16˚C to 34˚C, and from 500 to 1100 mmHg.

PARTIAL COMPLIANCE; TOTAL COMPLIANCE TBD: Optical design optimized to required values, but designs still needs to consider operational and survival ranges (shared compliance with mechanics and control)

ESOPO



PÁGINA: 65 DE 66

9. OBJETIVO El objetivo de este documento es:

1. Concentrar todas las especificaciones y requerimientos de la óptica para ESOPO, describir las diferentes configuraciones geométricas de la óptica y los diferentes materiales de la que esté compuesta, pero en el contexto de explicar cual ha sido el proceso para arribar al diseño actual, explicando además en que etapa nos encontramos y cuales etapas faltan por cubrir.

10. INTRODUCCIÓN

10.1 ANTECEDENTES

El Instituto de Astronomía ha hecho espectroscopía astronómica durante varias décadas, así como desarrollo instrumental, en especial instrumentos para observaciones en el óptico y en el infrarrojo con detectores dedicados. En el aspecto astronómico gran parte del trabajo se ha realizado, en el Observatorio Astronómico Nacional en San Pedro Mártir, con dos equipos: El espectrógrafo Boller & Chivens -en préstamo temporal del Observatorio de Brera- y el espectrógrafo Echelle de alta resolución espectral. En la medida de lo posible, se han modernizado los espectrógrafos existentes. Sin embargo, el espectrografo Boller & Chivens no tiene la eficiencia necesaria para muchos proyectos astronómicos; así mismo el espectrógrafo Echelle es un instrumento especializado para trabajo astronómico que requiere de alta resolución espectral.

El IA-UNAM, a través de su Consejo Interno y después de un concurso con arbitraje interno y externo, determinó proceder al diseño y construcción de un Espectrógrafo Óptico de Mediana Resolución para el telescopio de 2.1 m del Observatorio Astronómico Nacional en San Pedro Mártir, esencialmente con la idea de sustituir al espectrógrafo Boller & Chivens. El proyecto fue otorgado al grupo responsable del instrumento al que actualmente se denomina ESOPO, ganador de ese concurso. El concepto básico de este espectrógrafo se basa en que consiste de dos brazos, que conjuntamente cubren un intervalo espectral amplio, siguiendo la recomendación de uno de los árbitros externos de esa licitación. El otorgamiento no incluía presupuesto asignado al proyecto. El propósito básico del proyecto es: contar con un espectrógrafo propio que partiendo, como mínimo, de una equivalencia en cuanto a su utilidad al actualmente en uso (propiedad del Observatorio de Brera de la Universidad de Milán), sea más eficiente y moderno en su concepción y diseño y permita llevar a cabo una gran diversidad de observaciones astronómicas, incluidas las que requieran de amplia cobertura espectral en el intervalo óptico, con resolución espectral 500 < R < 5000.

La información que se obtiene de un espectrógrafo de esas características es necesaria para el estudio de objetos tanto estelares como extendidos, que pueden pertenecer a nuestra Galaxia o a galaxias externas. En suma, se trata de un espectrógrafo de propósito general que debe mejorar la resolución y el cubrimiento espectrales, la cobertura de campo y la eficiencia óptica y de operación del espectrógrafo italiano con el que actualmente se cuenta.

ESOPO ha sido considerado por el Instituto como un proyecto de alta prioridad, no solo por su utilidad astronómica, sino también por ser no sólo un proyecto piloto de gestión, en el que se

ESOPO



PÁGINA: 66 DE 66

están aplicando procedimientos de operación y normatividad empleados en proyectos de competencia internacional, sino porque en él se está aplicando una metodología de trabajo que, a nuestro juicio, si se aplican adecuadamente, garantizan que el instrumento sea de gran calidad y satisfaga las expectativas en cuanto a su desempeño. Esta forma de trabajo implica estructuras de organización y funcionamiento que incluyen aspectos como las figuras de investigador principal, de gerencia de proyectos, de ingeniería de sistemas, de control de calidad, la creación de documentación mínima en cada etapa, así como las de asesoría y evaluación por parte de comités como el científico y el técnico especializado. Este proyecto ha sido financiado desde un principio por el CONACYT y por la UNAM: a través de DGAPA, CTIC, y el Instituto de Astronomía.

Título Conceptual Preliminary Optical Design

Documents

Transcript of Título Conceptual Preliminary Optical Design