SNAP OTA Baseline TMA62

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SNAP OTA Baseline TMA62

• M.Lampton
• Jan 2002
• UC Berkeley Space Sciences Lab

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SNAP Mission Plan

• Preselect 20 study fields, both NEP and SEP
• Discoveries photometric light curves from
  repeated deep images
• huge multiplex advantage with batch
  observations, 1E9 pixels
• Spectroscopy near maximum light from followup
  pointings

Deep Surveys
N
S
N
S
Followup spectroscopy
4 day period
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SNAP
Simple Observatory consists of 1) 3 mirror
telescope w/ separable kinematic mount 2) Baffled
Sun Shade w/ body mounted solar panel and
instrument radiator on opposing side 3)
Instrument Suite 4) Spacecraft bus supporting
telemetry (multiple antennae), propulsion,
instrument electronics, etc No moving parts (ex.
filter wheels, shutters), rigid simple structure.

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Payload Layouttransverse rear axisshortest
length
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Annular Field Three Mirror Anastigmat

• Aperture 2 meters
• Field of view gt 1 square degree
• 1.37 square degrees in TMA62
• Diffraction limited longward of one micron
• 2 microns RMS, 15microns FWZ geometric
• Flat field
• Folded to obtain short overall length
• 3.3 meters in TMA62

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Wide-Field Telescope History

• Wide-field high-resolution telescopes are NOT new
• Schmidt cameras (1930 to present)
• Field-widened cassegrains, Gascoigne (1977-)
  SDSS
• Paul three-mirror telescopes (1935) and
  Baker-Paul
• Cook three-mirror anastigmats (1979)
• Williams TMA variants (1979)
• Korsch family of TMAs (1972...)
• Angel-Woolf-Epps three-mirror design (1982)
• McGraw three-mirror system (1982)
• Willstrop Mersenne Schmidt family (1984)
• Dark Matter Telescope (1996)
• New Planetary Telescope (1998)
• IKONOS Earth resources satellite (1999)
• FAME astrometric TMA
• Multispectral Thematic Imager (1999)

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Three-mirror anastigmat (TMA)

• Identified as best choice for SNAP
• Can deliver the required FOV
• Can deliver the required resolution
• Inherently achromatic, no correctors needed
• Inherently flat field
• Inherently elastic 9 d.o.f. to meet 4 Seidel
  conditions plus focus focal length
• Can meet packaging requirements

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Telescope Downselection

• 1999-2001 Suitability Assessments
• sought 1 sq deg with diffraction limited imaging
  (lt 0.1 arcsec)
• low obscuration is highly desirable
• off-axis designs attractive but unpackagable
  rejected
• four, five, and six-mirror variants explored
  rejected
• eccentric pupil designs explored rejected
• annular field TMA concept rediscovered
  developed
• TMA43 (f/10) satisfactory performance but
  lacked margins for adjustment lateral axis
  between tertiary detector
• TMA55 (f/10) improved performance, margins
  positive, common axes for pri, sec, tertiary.
• TMA56 (f/10) like TMA55 but stretched
• TMA59 (f/15) same but with longer focal length
• TMA62 (f/10.5) lateral axis between tertiary
  detector

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Baseline Telescope

• Baseline Optical System Annular Field TMA62
• prolate ellipsoid concave primary mirror
• hyperbolic convex secondary mirror
• flat annular folding mirror
• prolate ellipsoid concave tertiary mirror
• flat focal plane
• provides side-mounted detector location for best
  detector cooling
• EFL 21.66m matches 10.5 micron SiCCD pixel to
  0.1 arcsec angular scale
• plate scale is 105 microns per arcsecond
• delivers annular field 1.37 sqdeg
• average geometrical blur 2.5umRMS 6umFWHM
  16um worst case FWZ
• compare SiCCD pixel 10.5 um HgCdTE pixel
  18.5um
• angular geometrical blur 0.023arcsecRMS
  0.06arcsecFWHM
• compare Airy disk, 1um wavelength
  FWHM0.12arcsec13um

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Annular Field Dimensions

• Outer radius 0.745 degrees
• corresponds to 283.56 mm at detector
• Inner Radius 0.344 degrees
• corresponds to 129.1 mm at detector
• Sky coverage 1.37 square degree
• corresponds to 1957 cm2 detector area
• Field Blockages-- none
• Can go to larger radii but image quality degrades
  rapidly
• Can go to smaller radii but vignetting becomes
  severe

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TMA62 Optics Prescription

• Primary Mirror (concave prolate ellipsoid)
  located at origin
• diameter 2000 mm hole 450mm
• curvature -0.2037586, radius4.907768m
  shape0.0188309, asphericity -0.981169
• Secondary Mirror (convex hyperboloid) located at
  Z-2.000 meters
• diameter 450mm
• curvature -0.9103479, radius1.0984811m
  shape -0.8471096, asphericity -1.8471096
• Folding flat mirror located on axis, Z0.91
  meters
• oval, 700mm x 500mm central hole 190 x 120mm
• Tertiary Mirror (concave prolate ellipsoid)
  located at Z0.91, X -0.87meters
• diameter680mm
• curvature -0.7116752, radius1.405135m
  shape0.40203, asphericity -0.59797
• Filter/Window located along beam toward detector
• nominal thickness 5mm, fused silica
• Annular Detector Array located at Z0.91,
  X0.90 meters
• inner diameter 129mm, outer diameter 283.6mm

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TMA62 Prescription -- BEAM FOUR format
8 surfaces TMA62.OPT f/10.83, optim 6 to
14mrad, use 6 to 13mrad index X Z
pitch Curvature shape Diam diam
Mirr? --------.---------.----------.---------
-.--------------------------- 0
0.0 -0.2037586 0.0188309 2.01
mir pri 0 -2.0
-0.9103479 -0.8471096 mir sec
0 0.1
iris 0 0.91
45 mir fold
-0.87 0.91-90 -0.7116752
0.4020288 mir tert 0.25
0.91 90 0 0.3
lensFilter 1.456 0.255 0.91 90 0
0.3 lensFilter
0.9 0.91 90 0
0.65 CCDarray




EFL21.66meters




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TMA62 spot diagrams
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TMA55 Vignetting?
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Ray Trace ResultsFive radii X, XY, Y, -XY,
-XTransmission vs off-axis angle,milliradians
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TMA62 Vignetting and Image quality issues

• Nominal annulus 6 to 13mrad
• no vignetting, but little or no tolerance
• 2 um rms average image blur over this field
• At 5mrad approx 50 of rays are lost at edge of
  hole in 45deg flat mirror
• At 14mrad vignetting losses depend critically on
  element sizing geometrical blur about 40um FWZ.

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TMA56 sensitivity coefficients-secondary mirror-
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TMA56 sensitivity coefficients-fold mirror
detector-
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Glare Stray Light Sources

• Ecliptic Poles places Sun 70 to 110deg off axis
• sunshade design straightforward
• Earth, moon can be up to 15 deg off axis
• needs careful baffle study, now in work
• Stars, Zodiacal dust, diffuse Galactic light
• concerns are optics scatter, dirt, structure
• Stray light specification must be small compared
  to natural NIR foreground
• Thermal emission from optics must also be small

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Baffle treatment outer tube, secondary cone,
inner tube
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Stray Light Baffle Concept
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Diffuse NIR foreground
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Mirror emissivity
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Optical Mirror Technologies

• Open-back weight relieved Zerodur or silica
• offers 75 to 80 LW
• potentially quicker procurement cycle
• Ultralight corefaceback 90-95LW
• typically use Corning ULE
• requires ion milling
• requires in-chamber metrology
• SiC technologies
• evolving under study

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Materialshttp//www.minerals.sk.ca/atm_design
and other sources
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Primary Mirror Substrate

• Key requirements and issues
• Dimensional stability over time
• Dimensional stability in thermal gradient
• High specific stiffness (1g sag, acoustic
  response)
• Stresses during launch
• Design of supports
• Prefer lt 100kg/m2
• Variety of materials technologies

Initial design for primary mirror substrate 334
kg
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Primary Mirror Substrate

• Stresses from pseudo-static launch loads
• 6.5g axial, 0.5g transverse
• 3-point supports
• Baseline
• Face sheets (12 mm)
• Locally thickened web walls (10 mm)
• Thicker outer ring (8 mm)
• Mass (330 kg)
• Fundamental mode 360 Hz
• Conclusions
• 80 lightweighted design is workable
• 3 pt support may be usable for launch
• Vertical axis airbag support required for figuring

Design with locally thicker web plates Standard
web thickness 5 mm (orange) Thickened plates
10 mm (red)
Deformations of mirror top face under
pseudo-static launch loads peak deflection 20
?m
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Primary Mirror Substrate

• Free-free modes
• Sag during 1g figuring
• Sag is too large (gt0.1?m) on simple supports (3
  pt vertical, strap horizontal)
• Will likely require vertical axis figuring on
  airbag supports

1g sag on 3pt support vertical axis P-P Z
deflection 2.3 ?m
1g sag in 180º strap support horizontal axis P-P
Z deflection 0.5 ?m
1g front face ripple on perfect back-side
support P-P Z deflection 0.018 ?m
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Secondary Metering Structure

• Key requirements
• Minimize obscuration (lt3.5) interference
  spikes
• Dimensional stability
• 35 Hz minimum fundamental frequency
• Baseline design hexapod truss with fixed end
• Simple design with low obscuration (3.5)
• 6-spiked diffraction pattern
• Ø 23 mm by 1 mm wall tubular composite (250 GPa
  material) struts with invar end-fittings.

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Secondary Metering Structure
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Tertiary Metering Structure

• Key requirements
• Dimensional stability
• 35 Hz minimum fundamental frequency
• Easier design problem than secondary metering
  structure
• Overall dimensions much smaller than secondary
  metering truss
• No obscuration concerns
• Use strut design from secondary metering
  structure (cost effective)

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Telescope Focussing

• 13 mechanical adjustments is minimum set
• focussing
• collimation
• centering
• alignment
• on orbit, may only need secondary to be
  articulated
• Least squares optimization for focussing and
  collimation
• Alternatives Zernike defocus analysis

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GIGACAM1 billion pixel detector

• 132 large format silicon CCDs
• 25 2Kx2K HgCdTe NIR detectors
• Larger than SDSS array
• Smaller than BABAR silicon vertex detector
• Outside diameter 480mm
• Each chip has dedicated bandpass filter
• Located within 150K cryostat
• Accommodates guiding and spectroscopy feeds