kosmos · kosmos document v1.0 july 9, 2010 page 3 100 overview kosmos (kitt peak ohio state multi...
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KOSMOS Document v1.0
July 9, 2010
Page 1
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
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July 9, 2010
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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
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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
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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’
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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
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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
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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.
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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).
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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
Irregularity 2 0 5 Fringes
Thickness 15 0.2 mm
Wedge 0 30 arc sec
Meniscus Radius 1 78.8 8 Fringes
Radius 2 50.681 8 Fringes
Irregularity 1 0 3 Fringes
Irregularity 2 0 2 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
Radius 3 67.245 5 Fringes
Irregularity 1 0 2 Fringes
Irregularity 2 0 3 Fringes
Irregularity 3 0 1 Fringes
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Thickness 1 9 0.03 mm
Thickness 2 24 0.03 mm
Wedge 1 0 20 arc sec
Wedge 2 0 20 arc sec
Collimator Radius 1 Plano 5 Fringes
Singlet Radius 2 86.26 5 Fringes
Irregularity 1 0 1 Fringes
Irregularity 2 0 1 Fringes
Thickness 20 0.1 mm
Wedge 0 20 arc sec
Camera Radius 1 Section 530
Triplet #1 Radius 2 76.919 8 Fringes
Radius 3 67.019 8 Fringes
Radius 4 187.006 5 Fringes
Irregularity 1 Section 530
Irregularity 2 0 1.5 Fringes
Irregularity 3 0 1.5 Fringes
Irregularity 4 0 0.5 Fringes
Thickness 1 8 0.1 mm
Thickness 2 60 0.2 mm
Thickness 3 8 0.1 mm
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
Radius 3 869.959 8 Fringes
Radius 4 109.182 5 Fringes
Irregularity 1 0 0.5 Fringes
Irregularity 2 0 1.5 Fringes
Irregularity 3 0 1.5 Fringes
Irregularity 4 0 0.5 Fringes
Thickness 1 10 0.1 mm
Thickness 2 30 0.2 mm
Thickness 3 34 0.1 mm
Wedge 1 0 40 arc sec
Wedge 2 0 20 arc sec
Wedge 3 0 20 arc sec
Camera Radius 1 91 5 Fringes
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Doublet Radius 2 41.990 10 Fringes
Radius 3 Plano 5 Fringes
Irregularity 1 0 0.5 Fringes
Irregularity 2 0 1.5 Fringes
Irregularity 3 0 1 Fringes
Thickness 1 36 0.03 mm
Thickness 2 26 0.03 mm
Wedge 1 0 20 arc sec
Wedge 2 0 30 arc sec
Field Radius 1 80.721 6 Fringes
Flattener Radius 2 154.755 6 Fringes
Irregularity 1 0 2 Fringes
Irregularity 2 0 2 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.
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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.
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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
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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.
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Fig, 16. Radial profile of the pupil shown in Fig. 15.
Fig. 17. Pupil ghost in spectral direction.
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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.
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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.
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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.
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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.
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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.
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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