KOSMOS · KOSMOS Document v1.0 July 9, 2010 Page 3 100 Overview KOSMOS (Kitt Peak Ohio State Multi...

KOSMOS Document v1.0

July 9, 2010

KOSMOS

Kitt Peak-Ohio State Multi-Object Spectrograph

Optical Design

Revision History

Version Author Date Description

1.1 Ross Zhelem July 9, 2010 Initial Draft

1.2 Paul Martini July 20, 2010 Minor Edits, Disperser

Designs


July 9, 2010

Table of Contents

100 Overview .................................................................................................................................. 3

200 Optical Design ......................................................................................................................... 4

210 Optical Layout ..................................................................................................................... 4

220 Optical Prescription ............................................................................................................. 5

230 Baseline Optical Properties .................................................................................................. 6

240 Field of View ....................................................................................................................... 6

250 Pupil aberrations .................................................................................................................. 7

300 Image Quality........................................................................................................................... 9

310 Design Optimization Details ................................................................................................ 9

320 Imaging Mode ...................................................................................................................... 9

330 Spectral Mode .................................................................................................................... 10

400 Instrument Throughput .......................................................................................................... 15

410 Throughput Budget ............................................................................................................ 15

420 Internal Transmittance ....................................................................................................... 15

430 AR Coatings ....................................................................................................................... 16

500 Tolerance Analysis................................................................................................................. 18

510 Fabrication Tolerances ....................................................................................................... 18

520 Alignment Tolerances ........................................................................................................ 20

530 Asphere Manufacturability ................................................................................................ 21

600 Parasitic Light Analysis ......................................................................................................... 22

610 Scattered Light ................................................................................................................... 22

620 Stray Light ......................................................................................................................... 22

630 Ghost Light ........................................................................................................................ 22

700 Thermal Performance............................................................................................................. 26

710 Focus Shift ......................................................................................................................... 26

710 Bonded Interfaces .............................................................................................................. 27

800 Alignment and Testing ........................................................................................................... 28

810 Acceptance Testing at Vendor’s Site ................................................................................. 28

820 Alignment and Testing at the OSU Lab ............................................................................. 28

830 Alignment and Testing Equipment .................................................................................... 29

900 Disperser Designs .................................................................................................................. 30

910 Direct Vision Triple Prism ................................................................................................. 30

920 VPH Grism......................................................................................................................... 31


July 9, 2010

100 Overview

KOSMOS (Kitt Peak Ohio State Multi-Object Spectrograph) is a wide field, multi-slit imaging

spectrograph for the 4-m Mayall telescope located on Kitt Peak near Tucson, AZ. The telescope scale

is 6.6 arcsec/mm, f-ratio is f/7.9. The telescope will deliver a 100 square arcminute field of view to

the KOSMOS instrument and the optical design will collimate and then reimage this input beam onto

a 4096x2048 CCD with 15 micron pixels at a plate scale of 0.29 arcseconds per pixel.


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200 Optical Design

210 Optical Layout

The KOSMOS optics are based on a similar instrument (OSMOS) built for the MDM 2.4m

Hiltner telescope. Both the Hiltner and Mayall telescopes have comparable f/ratios. The choice

of similar sized linear field of view allowed use of the as-designed OSMOS collimator. The

collimator produces a pupil of the same dimensions in the collimated space for both telescopes.

The collimator is an f/7.9 double Gauss design with a 14 degree FOV and contains a total of five

lenses, including a doublet. The optical design is shown in Figure 1 and the optical prescription

is provided in Table 1. The first collimator element is 73 mm from the telescope focal surface,

which provide ample space for the slit wheel. The collimated beam diameter is 54 mm and the

pupil is located 68 mm from the vertex of the last collimator element. The total collimated beam

space is 170 mm and will be used for dispersers and filters (not shown on drawing).

Fig.1 Layout of the KOSMOS optical configuration

Optical design with ray traces for field angles up to 5′ off axis. Light enters the instrument from

the left, where the first surface corresponds to the focal surface of the 4m Mayall telescope. This

is the location of the slit wheel in the optical path. The disperser and two filter wheels are in the

collimated beam space between the fifth and sixth lenses. The last lens on the right is the Dewar

window. The optical prescription is provided in Table 1.

The KOSMOS camera was designed to deliver the necessary plate scale and maintain constant

image quality over the entire area of detector. The camera is an f/2.7 Petzval design with an 26

degree FOV. The camera contains a total of 10 lenses, including two triplets, a doublet and

singlet field flattener. The first camera lens is aspheric with the maximum departure from a best


July 9, 2010

fit sphere of 170 microns over the clear aperture. The last lens is the window of the standard

KPNO Dewar. The optical design produces a final plate scale of 0.29′′/pixel on the 4096x2048

CCD with 15 micron pixels currently intended for use with KOSMOS. Multiple bonded elements are

used in the camera design to minimize the amount of air-glass surfaces.

220 Optical Prescription

Element Lens Surface Radius Thickness Material Diameter

Slit Mask

0 3330 73

103

Field Lens COL-1 1 Infinity 15 BSL7Y 128

2 -180.89 164

128

Meniscus COL-2 3 78.8 40.3 BSM51Y 94

4 50.681 50.5

72

Collimator doublet

COL-3 5 -50.681 9 BAL15Y 86

COL-4 6 152.3 24 CAF2 86

7 -67.245 19.285

86

Collimator singlet COL-5 8 Infinity 20 CAF2 96

9 -86.26 170

96

Camera Triplet #1

(aspheric)

CAM-1* 10 111.278 8 BSM51Y 114

CAM-2 11 76.919 60 CAF2 114

CAM-3 12 67.019 8 BAL35Y 114

13 187.006 1

130

Camera Triplet #2 CAM-4 14 662.224 10 BAL35Y 146

CAM-5 15 143.802 30 CAF2 146

CAM-6 16 869.959 34 BSM51Y 146

17 109.182 1

146

Camera Doublet CAM-7 18 91 36 BSM51Y 120

CAM-8 19 41.990 26 CAF2 75.2

20 Infinity 11

Field Flattener CAM-9 21 80.721 8 PBM18Y 85

22 154.755 9.568

70

Dewar window CAM-10 23 Infinity 6.35 SILICA 100

24 Infinity 10

100

CCD

25 Infinity 0

61 x 31

Table 1. KOSMOS Optical Prescription.

*Even Asphere, coefficient on r4: -7.16678E-8, coefficient on r

6: -2.08578E-12


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All dimensions in Table 1 are in mm. The slit mask is located at the focal surface of the 4m

Mayall telescope. KOSMOS is fed by light from the telescope (a Ritchey-Chretien design),

parameters of which were supplied by NOAO. The complete prescription is included in the

ZEMAX file.

230 Baseline Optical Properties

Instrument Design All refractive, one aspheric surface used in camera

Focal Station f/7.9 RC at Mayall 4m telescope, radius of curvature 3330mm

Focal Surface Scale 6.6 arcsec/mm

Slit Mask

103 mm diameter (section 240), TBD shape/material/long slit

etc

Collimator Design

f/7.9, 14 degree FOV Double Gauss optimized for telescope

input

Collimator Focal Length 430 mm

Pupil Location 68 mm from collimator, 102 mm from camera

Collimated Beam Diameter 54 mm

Collimated Beam Length 170 mm

Camera Design

f/2.7, 26 degree FOV Petzval with pupil relief and focal plane

relief

Camera Focal Length 146.7 mm

Imaging Plate Scale 0.29 arcsec/pixel (15 micron pixels)

(De)magnification 2.93x reimaging of telescope focal surface

CCD 4096x2048 (15 micron pixels)

Operating Modes Multi-object and single object spectroscopy, imaging

Wavelength Range 350 - 1000 nm

Field of View

12 arc minute diameter circle, cropped by CCD to 100 square

arc minutes (section 240)

Design Image Quality (FWHM)

Imaging mode: U-band: 0.4 arcsec (zenith)

BVRI: 0.25 arcsec (section 320)

Polychromatic: 0.32 arcsec

Spectral mode:

single object 0.2 arcsec (section 330)

multi-object 0.5 arcsec (section 330)

Throughput TBD

Dispersers VPH grism, Surface Relief Grism, Direct Vision Triple Prism

Filters 4 inch square, two 6 position filter wheels tilted by 8 degrees

Refocus between filters None required (section 320)

240 Field of View

Field of view is limited in the field direction (along a slit) by the edge of the 2K x 4K CCD

(Fig.2). To ensure 100 square arc minute field coverage, the maximum field of view will be 5.82’


July 9, 2010

in the spectral direction while constrained to 4.95’ in field direction. This field of view

configuration maps onto the footprint in the telescope focal plane, which is defined by the 103

mm diameter in spectral direction and 90 mm in field direction.

Fig. 2. 100 square arc minute imaging field of view on 4K x 2K CCD is shown in red

250 Pupil aberrations

According to the online optical prescription of the Mayall telescope, the entrance pupil is located

on the primary mirror. Collimator reimages the pupil at the paraxial distance 68 mm from the last

lens. First order size of the pupil is 54 mm circular.

The collimator was originally designed for a smaller aperture telescope. When used with the

Mayall telescope at the same linear field of view in the telescope focal plane, pupil aberrations

increase for objects over 3 arc minute off axis. This results in field and wavelength dependent

pupil shift (Fig. 3). The collimated beam footprints for objects within 3 arc minutes radius

overlap on the disperser to ±1mm due to chromatic aberration. Monochromatic aberrations shift

the pupil radially on the disperser by an average of 5mm. The spectrum of an off-axis object is

produced by a slightly different area of the disperser.

2Kx2K

5.82’

spectral direction

fiel

d d

irec

tion


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Fig. 3. Pupil shift vs field of view


July 9, 2010

300 Image Quality

310 Design Optimization Details

The KOSMOS camera was initially designed for perfect collimated input using five object fields

uniformly distributed over the CCD area. The camera was then fed by the Mayall telescope and

OSMOS collimator. Object fields correspond to different image heights in the telescope focal

plane or slit mask. Imaging mode is characterized in the next section 320 by image quality

dependence on object field on sky.

The camera and collimator were designed for perfect input, are well matched in performance,

and deliver excellent image quality over the imaging field of view (Fig. 2). In spectral mode,

light from every object is dispersed between the edges of the CCD in the spectral direction. Due

to the significance of the spectral mode, KOSMOS was reoptimized for the best spectral

performance. This was accomplished by dispersing light into 5 equidistant spectral fields

according to Fig. 4.

Fig. 4. Object fields #1 through to #7 and spectral fields (color coded) for spectral optimization.

320 Imaging Mode

Imaging performance is shown at the best focus for V-filter or Stroemgren y filter.

4

6

7

5

3

2

1


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Fig.5. KOSMOS imaging performance in broadband filters

Fig. 6. KOSMOS imaging performance in narrowband filters, namely, Stroemgren uvy, Hα, Hβ.

330 Spectral Mode

Spectral performance is shown for object and spectral fields defined in section 310 (Fig.4). The

goal of spectral optimization is to deliver uniform performance in multi-object mode while

maintaining excellent image quality for a spectrum of an on-axis object. The single object case


July 9, 2010

corresponds to the object field #1. Plots for object fields #1, #4, #5 contain 3 spectral field curves

due to obvious symmetry. The table of baseline optical properties (section 230) contains values

averaged over fields and wavelengths.

Fig. 7. KOSMOS spectral performance


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Fig. 7. KOSMOS spectral performance (continued)


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Fig. 8. KOSMOS performance with seeing


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KOSMOS on sky performance is the convolution of the geometric image quality with

atmospheric seeing. Seeing was taken into account via a quadrature sum. The results are shown

in Fig. 8 for an on-axis object at 0.9 arcsec seeing, an off-axis object at 0.9 arcsec seeing, and an

on-axis object at 0.6 arcsec seeing. The latter case provides evidence for effective use of 2 pixel

spectral slit. Simulated performance with atmospheric seeing is uniform over the entire CCD

area and wavelength region.


July 9, 2010

400 Instrument Throughput

410 Throughput Budget

The main factors affecting throughput are:

Internal absorption in the glass lenses

Reflection losses for bonded surfaces

Reflection losses for the AR coatings

This budget does not include any losses at the filters since these depend on the particular kind of

filter used. Spectral efficiencies are also disperser specific; they should be taken into account

separately.

Loss Source Notes

1.7% Internal absorption at 500 nm

1.7% Reflectance of

bonded surfaces at 500 nm

23.8% Broadband AR

(BBAR) coating

1.5% each,

18 surfaces including

dewar window

72.8% Total Throughput

Table 2. Throughput budget

The throughput table indicates that the throughput is sufficiently high to satisfy the 40% requirement

for the entire system including disperser and detector.

420 Internal Transmittance

Internal transmittance is determined by absorption of light in the bulk of material as it passes

through. For bonded elements, it includes Fresnel losses on bonded surfaces. Calcium Fluoride is

chosen as a crown material; its internal absorption is small. To maximize throughput in the blue

wavelength range, a set of Ohara i-line glasses is used. They ensure high transmittance over the

design wavelengths (Fig. 9).


July 9, 2010

Fig. 9 Internal transmittance of KOSMOS optical elements

430 AR Coatings

The transmittance of different AR coatings on all KOSMOS lenses is shown in Fig. 10. BBAR

performance is modeled as constant over the entire wavelengths range. Single layer coatings and

uncoated transmittance are given for reference. Overall throughput of KOSMOS optical system

is shown in Fig. 11. It does not include filters, dispersers or CCD quantum efficiency. Design

performance of modern BBAR coatings is very efficient (Fig. 12). It confirms that the

throughput goal in the Table 2 can be achieved.

Fig. 10. Transmittance of AR coatings on KOSMOS optics


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Fig. 11. KOSMOS throughput with different types of AR coatings

Fig. 12. Theoretical reflectivity of a BBAR coating


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500 Tolerance Analysis

KOSMOS optics deliver excellent image quality over the entire field of view and wavelength

range (section 300). The purpose of tolerancing is to maintain the image quality by setting limits

to fabrication and alignment errors. A sensitivity analysis was run using the RMS image

diameter averaged over the field as an image criterion. Camera refocus is allowed as a

compensator for axial tolerances, namely, radii, air and glass thicknesses.

All spherical surfaces will be fitted to the manufacturer’s test plates in order to reduce cost/time of

optical fabrication. Test plate fits insure very close matches to the design radii without significant

effort.

KOSMOS optics are based on high blue transmission i-line glasses from Ohara, Japan. Standard

glass data refer to visible wavelengths. Dispersion may be different from catalog values close to the

blue or red end of the design wavelength range. It is important to order blanks along with melt index

measurements. Upon the delivery of blanks, the design will be reoptimized for actual indices of

glasses. This strategy eliminates the need to tolerance on index of refraction.

510 Fabrication Tolerances

Element Parameter Nominal Tolerance Unit

Field Lens Radius 1 Plano 15 Fringes

Radius 2 180.89 15 Fringes

Irregularity 1 0 5 Fringes


Thickness 15 0.2 mm

Wedge 0 30 arc sec

Meniscus Radius 1 78.8 8 Fringes




Thickness 40.3 0.1 mm

Wedge 0 20 arc sec

Collimator Radius 1 50.681 5 Fringes

Doublet Radius 2 152.3 10 Fringes






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Thickness 1 9 0.03 mm


Wedge 1 0 20 arc sec


Collimator Radius 1 Plano 5 Fringes

Singlet Radius 2 86.26 5 Fringes



Thickness 20 0.1 mm

Wedge 0 20 arc sec

Camera Radius 1 Section 530

Triplet #1 Radius 2 76.919 8 Fringes



Irregularity 1 Section 530

Irregularity 2 0 1.5 Fringes






Wedge 1 0 arc sec

Wedge 2 0 arc sec

Wedge 3 0 arc sec

Camera Radius 1 662.224 5 Fringes

Triplet #2 Radius 2 143.802 8 Fringes













Camera Radius 1 91 5 Fringes


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Doublet Radius 2 41.990 10 Fringes

Radius 3 Plano 5 Fringes








Field Radius 1 80.721 6 Fringes

Flattener Radius 2 154.755 6 Fringes



Thickness 8 0.2 mm

Wedge 0 30 arc sec

520 Alignment Tolerances

Element

Axial

displacement

mm

Tilt

arcminutes

Decenter

mm

Col. Field Lens 0.5 5’ 0.5

Col. Meniscus 0.1 1’ 0.02

Col. Doublet 0.1 2’ 0.02

Col. Singlet 0.05 1’ 0.02

Cam. Triplet #1 0.1 1’ 0.02

Cam. Triplet #2 0.1 1’ 0.02

Cam. Doublet 0.1 2’ 0.06

Cam. Field Lens 0.1 1’ 0.04

Collimator

assembly Focus 20’ 2

Camera

assembly Focus 30’ 2

Fabrication and alignment tolerances are similar for KOSMOS and OSMOS systems. Both

designs make use of an identical collimator. The KOSMOS camera includes two triplets of equal

alignment sensitivity. Their alignment tolerances are twice as high as those for the only triplet of

the OSMOS camera. KOSMOS asphere is sensitive to decenter to within 0.03 mm that is

commensurable with the allowable limit for triplet #1. Tolerances down to 5 µm are achievable

with modern manufacturing techniques.


July 9, 2010

530 Asphere Manufacturability

The first surface of KOSMOS camera is an even asphere. It corrects the 3rd

and 5th

order

spherical aberration in the system. The aspheric profile is shown in Fig. 13 with respect to the

best fit sphere (radius of 116.7 mm). The best fit sphere is tangent at the vertex of an aspheric

surface. It is optimized for minimum volume removal over the clear aperture of the surface.

Maximum departure from best fit sphere is 0.171 mm at the radial height 40mm.

Fig. 13. Profile of the even asphere in the KOSMOS camera

Asphericity of the first camera lens is within manufacturing capabilities of modern metrology

and fabrication equipment that is designed to generate aspheric departures up to 200 micron on

parts up to 200 mm in size.

The asphere will be generated with the help of a small area tool. Upon finishing the large scale

profile, the surface will be covered by residual texture of the size of the tool. This is similar to

polishing large astronomical mirrors with undersized laps. The technique of tolerancing the

aspheric surface by matching to atmospheric seeing is developed via a structure function

approach. The KOSMOS camera asphere is toleranced to cause the degradation of image quality

by no more than 0.07 arcsec FWHM.

OSU has experience in procuring highly aspheric optics from the MODS project. It is important

to read and analyze interferograms from manufacturer to evaluate surface quality and verify the

structure function specification. The software package FRINGESOFT is available for $2000. It is

required to provide efficient vendor support in regard to the fabrication of KOSMOS camera

aspheric lens.


July 9, 2010

600 Parasitic Light Analysis

610 Scattered Light

Parasitic light in an optical system can be generated by multiple effects. It can be scattered by

distributed or local features such as glass defects, dust particles, surfaces of mechanical

structures, etc. The materials chosen for the KOSMOS optics are highly homogeneous across the

blank with regard to index of refraction, air inclusions and striae. Lenses will be fabricated to

high scratch and dig tolerances and mounted in enclosed barrels of cylindrical shape. Scattering

in the instrument is negligibly small as compared to the telescope.

620 Stray Light

KOSMOS will be mounted at the Cassegrain focus of the Mayall telescope. At this location, the

focal surface of the instrument may be directly illuminated by the sky. Baffles are mounted on

the primary and secondary mirrors of the telescope to block stray light. Their size and relative

separation are shown in Fig. 14. Based on the information about the baffles, the focal plane of

the telescope is shielded from stray light up to 15 arcminute diameter. The KOSMOS field of

view is under 12 arcminutes, therefore, the instrument is protected from stray light.

Fig.14. Baffling of the Mayall telescope.

630 Ghost Light

Ghost light is caused by undesirable reflection from specular or nearly specular surfaces. In a

refractive system these are coated air-glass surfaces. The KOSMOS optical train was analyzed


July 9, 2010

for the effect of double bounce reflections redirecting light toward the detector. Every pair of

surfaces generating a ghost reflection is identified by surface numbers in Section 220. A constant

reflectivity of 1.5% for each surface was assumed at all wavelengths. Reflection from the CCD

as the first surface is also considered. The effect of reflections can be calculated with the

following formula (MacFarlane, M.J. and E.W. Dunham, “Optical design of the Discovery

Channel Telescope”, Ground-Based Telescopes, J.M. Oschmann, Jr., ed., SPIE, 5489, 796,

2004):

,)1( 22

2

21G

SRRIG

where R1 and R2 are the reflectivities of the two surfaces, S is the image diameter, is the

relative obscuration of the secondary and G is the ghost image diameter.

Pupil ghosts are produced by double reflections with focus point which is far from CCD. The

intensity is evaluated based on the contribution of 100 objects of equal brightness uniformly

distributed in the field of view in imaging mode. The strongest ghost pupil was created by the

surfaces 22 and 1 with peak relative intensity 0.14x10-6

(Fig. 15-16).

In the KOSMOS optical train there are over 144 pairs of air-glass surfaces that generate

defocused images on the CCD. Overlap of all these double reflections results in the formation of

a ghost pupil (Fig. 17) with roll off edges and central peak intensity 1.3x10-6

with respect to the

intensity of the parent images. It is below the requirement of 10-4

for allowed ghost intensity.

Fig. 15. Pupil from surfaces 22 and 1 on central 2Kx2K area of CCD.


July 9, 2010

Fig, 16. Radial profile of the pupil shown in Fig. 15.

Fig. 17. Pupil ghost in spectral direction.


July 9, 2010

Field ghosts are focused close to the detector surface. They can produce spurious images of

objects or spectral lines. KOSMOS filters are located in the collimated space next to the camera.

The double bounce path “CCD-filter” requires special attention because it yields a nearly

perfectly focused ghost. Reflection off the CCD surface is recollimated by the camera, then

reflected by the flat surfaces of the filter and reimaged by the camera again. Filters are designed

with a tilt of 8 degrees to the optical axis in order to remove ghost images from the detector. The

tilt is implemented in the field direction (Fig. 2).

Double reflection from the ¼ inch fused silica dewar window generates the defocused image 3.2

mm across. There are two field ghosts of smaller size listed in Table 3.

Surfaces of double reflection On axis diameter, mm Max. relative brightness

22-20 0.7 3x10-5

23-18 1.3 1x10-5

Table 3. Characteristics of the strongest field ghosts

Maximum relative brightness is specified for an off-axis location where ghosts are imaged with

large aberrations.

The cross section of all identified pupil and field ghost images is larger than 6 arcsec and relative

brightness is less than the requirement of 10-4

. Field ghosts may be registered on a frame with

oversaturated objects or spectral lines. They will be recognized due to their oversized

dimensions.

The characteristics of ghosts depend on the actual radii of an optical system and their separation.

Test plate fits of lens radii will change the optical prescription to a certain extent. Ghost light

analysis will be repeated to ensure comparable performance.


July 9, 2010

700 Thermal Performance

710 Focus Shift

KOSMOS operational temperature range is from -10 to +30 C. In a lens system of multiple

elements, optical properties vary with temperature for two main reasons, namely, i) each

individual element’s index of refraction depends on temperature of the medium in which the lens

is immersed ii) both lenses and mount undergo thermal expansion/contraction.

To understand the effects of temperature variation, a soak condition thermal analysis of the

KOSMOS optics was performed using the “rubber tube” model in the optical design. Results

indicate that the camera requires a small motion with respect to the CCD dewar to compensate

for both index of refraction and isotropic change of dimensions. Once the lens is refocused, the

original performance is restored because the effects of temperature and pressure perturbations

result in defocus as the primary optical aberration.

The rate of thermal focus drift is 12.5 micron per 1 degree C or 0.5mm over the operational

temperature range. According to Fig. 18 the geometric depth of focus across the field of view is

±0.1 mm. Based on the depth of focus, KOSMOS image quality will be stable over 8 degrees C

of temperature change, therefore it can be focused once at the beginning of nighttime

observations. However, seasonal variations of focus may be required.

Fig. 18. Focus curves for five field positions

The thermal performance will depend on the details of the mechanical mounts. Once they are

finalized by the vendor, a more accurate model will be created for image quality variations with

temperature or pressure.


July 9, 2010

710 Bonded Interfaces

KOSMOS optics include two doublets and two triplets. Lenses will be cemented with the help of

Sylgard 184. This is a silicon encapsulant for electronics components with good transmission and

elastic properties. Optical bonds join elements up to 145 mm in diameter made of glasses with

different coefficients of thermal expansion (CTE).

During assembly of the SOAR Imager triplet there was an issue with bonding the fused silica

lens to calcium fluoride. At the same time, the bond between calcium fluoride and BAK2 was

trivial due to both reduced differential expansion and eventually the low adhesion of Sylgard 184

to fused silica. KOSMOS lenses are smaller and made of glasses with similar CTE to that of

BAK2 glass. Table 4 contains comparison of the main characteristics affecting the stability of an

optical bond. OSMOS elements were joined by a layer 0.1 mm thick. KOSMOS materials and

parameters are similar.

The behavior of a cemented interface depends on details of the process; the strength of a bond

may be reduced by surface contamination. It is important to thermally cycle all KOSMOS

doublets and triplets within survival temperature range -20 to +50 C.

Instrument Bonded

materials

Diameter

mm

Bond

thickness

mm

Δ CTE, 10-6

Survival

temperature

range, C

SOAR Imager Silica/CaF2 203 0.16 18.4

-5 to +25 CaF2/BAK2 203 0.16 10.9

OSMOS BAL15Y/CaF2 86 0.1 11.3

-15 to +50 BSM51Y/CaF2 120 0.1 12.6

KOSMOS

BAL15Y/CaF2 86 0.1 11.3

-20 to +50 BSM51Y/CaF2 145 0.1 12.6

BAL35Y/CaF2 145 0.1 13.2

Table 4. Parameters of cemented interfaces.


July 9, 2010

800 Alignment and Testing

810 Acceptance Testing at Vendor’s Site

The main acceptance test will be a test of the entire system assembly that simulates real

operating conditions. This test will include a light source, narrow band filters, the optical

assembly, and a test CCD. The light source should have a broad band spectrum to produce

measurable input over the entire wavelength range. A pinhole source ( 5 microns) is suitable for

testing the imaging performance of the entire system assembly. It would be translated along a

curved path representative of the telescope focal surface to test the assembly off axis and would

require a precision XYZ stage with an XY travel of 100 mm and a Z travel (optical axis) of

1mm. A stop aperture with 29 mm would be placed 68 mm from the collimator singlet at the

location of the collimated pupil.

The test CCD should be able to record the fine structure of the light source image produced by

the optics. A 10 micron (or less) pixel size is required to reliably measure the image quality. The

test CCD will therefore need to be mounted on a precision XY stage to sample the image quality

across imaging field of view.

820 Alignment and Testing at the OSU Lab

KOSMOS lenses will be integrated by a vendor into collimator and camera assemblies according

to high tolerances (Section 520). Mutual alignment of collimator and camera is less demanding.

It will be verified that the slit mask, collimator, disperser wheel cell, camera, and CCD mount

plate share the same opto-mechanical axis.

The collimator will be focused by means of autoreflection from a mirror mounted in the

disperser wheel. The camera will be focused on-axis by reimaging a sieve mask onto the CCD.

In order to accelerate focusing and detector alignment it would be useful for the NOAO data

taking system to implement a focus series frame taking sequential exposures and placing them on

a single frame with charge shift 20-30 pixels per exposure. Camera is refocused between

exposures that results in a focus sequence of multiple objects in the same fits file.

Dewar will be aligned in tip/tilt using a sieve mask. It consists of 5 micron pinholes attached to

spherical surface following the telescope curved focal plane. Pinholes are uniformly distributed

over 103 mm area. The focus sequence will be run with the z’ filter for the entire sieve mask and

the tip/tilt of CCD dewar will be determined. The correction is made with the help of a spacer

machined to acquire the necessary compensating wedge.

Throughput of the system will be measured in monochromatic filters for various field positions.

Chromatic focal shift will be measured to confirm dependence of focus upon wavelength. An

illuminator with variable filters will be used to take focus sequences at different wavelengths.


July 9, 2010

830 Alignment and Testing Equipment

The cost of alignment and testing equipment, the fabrication of test fixtures, and software have

been included in the budget and project plan.


July 9, 2010

900 Disperser Designs

KOSMOS is designed as an imaging spectrograph with dispersers operating in transmission.

Given that the optical axes of the collimator and camera are coincident and the camera allows

only focus adjustment, the common requirement is to restore the path of the chief ray upon

exiting a disperser. The Triple Prism and VPH Grism satisfy this condition.

910 Direct Vision Triple Prism

The Triple Prism is shown in Fig. 19 along with filters in the collimated space between the last

element of the collimator and the first element of camera. The middle prism is made of Ohara

glass PBL2Y. It is surrounded by identical prisms made of S-FPL51Y from the same

manufacturer. Cemented surfaces provide dispersion while external facets steer the beam into the

camera.

Fig. 19 Triple Prism

The resolving power of the prism increases if higher index materials are used with larger

difference in dispersive power. In both cases the prism angle and size increases as well. The

current prism design was used for OSMOS. It can be implemented in KOSMOS due to the fact

that collimated beam space is retained. Spectral resolution of the prism is evaluated in Fig. 20

for an on axis object based on a 3 pixels slit.


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Fig. 20 Triple Prism resolution

920 VPH Grism

VPH Grism is shown in Fig. 21 along with filters in the collimated space between the last

element of the collimator and the first element of camera. A VPH grating is cemented between

two right angle prisms. The front facet provides the incident angle for the grating according to

the Littrow condition. The back surface steers the dispersed beam into the camera.

Fig. 21. VPH Grism

The resolving power is determined by the grating line frequency. Higher dispersion requires

steeper facets or higher index material to satisfy the Littrow condition. The grism shown in Fig.


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21, corresponds to the limiting case of an equilateral assembly (60º apex angle) due to

packaging. The prism material is BK7 and the grating constant is 1185 lines/mm. The spectral

resolution of the grism is R=1600 for an on-axis object based on 1 arcsec slit. The spectral

coverage in single object/long slit mode is 320 – 600 nm. Absolute peak efficiency is 95% at 450

nm (Fig. 22) and ~60% close to the CCD edge. There is a slight variation of grating efficiency

for off-axis objects that should be taken into account when considering the multi-object mode.

With spectral coverage of 260 nm per grating, the entire wavelength range 350 -1000 nm will

require the total of three VPH grisms of similar design.

Fig. 22. VPH Grism efficiency

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