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SNAP OTA Baseline TMA62. M.Lampton Jan 2002 UC Berkeley Space Sciences Lab. 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

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snap ota baseline tma62

SNAP OTA Baseline TMA62

M.Lampton

Jan 2002

UC Berkeley Space Sciences Lab

snap mission plan
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

slide3
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.
annular field three mirror anastigmat
Annular Field Three Mirror Anastigmat
  • Aperture: 2 meters
  • Field of view: > 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
wide field telescope history
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)
three mirror anastigmat tma
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
telescope downselection
Telescope: Downselection
  • 1999-2001: Suitability Assessments
    • sought 1 sq deg with diffraction limited imaging (< 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
baseline telescope
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: FWHM=0.12arcsec=13um
annular field dimensions
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
tma62 optics prescription
TMA62 Optics Prescription
  • Primary Mirror (concave prolate ellipsoid) located at origin:
    • diameter= 2000 mm; hole= 450mm
    • curvature= -0.2037586, radius=4.907768m; shape=+0.0188309, asphericity= -0.981169
  • Secondary Mirror (convex hyperboloid) located at Z=-2.000 meters:
    • diameter= 450mm
    • curvature= -0.9103479, radius=1.0984811m; shape= -0.8471096, asphericity= -1.8471096
  • Folding flat mirror located on axis, Z=+0.91 meters:
    • oval, 700mm x 500mm; central hole 190 x 120mm
  • Tertiary Mirror (concave prolate ellipsoid) located at Z=+0.91, X= -0.87meters:
    • diameter=680mm
    • curvature= -0.7116752, radius=1.405135m; shape=+0.40203, asphericity= -0.59797
  • Filter/Window located along beam toward detector
    • nominal thickness 5mm, fused silica
  • Annular Detector Array located at Z=+0.91, X=+0.90 meters:
    • inner diameter 129mm, outer diameter 283.6mm
tma62 prescription beam four format
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 :

: : : : : : : : :

: : : : : : : : :

: : : : : : : : :

: EFL=21.66meters : : : : : :

: : : : : : : : :

: : : : : : : : :

ray trace results five radii x xy y xy x transmission vs off axis angle milliradians
Ray Trace ResultsFive radii: +X, +XY, +Y, -XY, -XTransmission vs off-axis angle,milliradians
tma62 vignetting and image quality issues
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.
glare stray light sources
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
optical mirror technologies
Optical Mirror Technologies
  • Open-back weight relieved Zerodur or silica
    • offers 75% to 80% LW
    • potentially quicker procurement cycle
  • Ultralight core+face+back: 90-95%LW
    • typically use Corning ULE
    • requires ion milling
    • requires in-chamber metrology
  • SiC technologies
    • evolving; under study
primary mirror substrate
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 < 100kg/m2
  • Variety of materials & technologies

Initial design for primary mirror substrate: 334 kg

primary mirror substrate1
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

primary mirror substrate2

Second mode: 566 Hz

Fundamental mode: 360 Hz

Primary Mirror Substrate
  • Free-free modes
  • Sag during 1g figuring
    • Sag is too large (>0.1m) on simple supports (3 pt vertical, strap horizontal)
    • Will likely require vertical axis figuring on airbag supports

1g front face ripple on perfect back-side support

P-P Z deflection = 0.018 m

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

secondary metering structure
Secondary Metering Structure
  • Key requirements:
    • Minimize obscuration (<3.5%) & 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.
tertiary metering structure

Lowest global mode of tertiary metering truss: 110Hz

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)
telescope focussing
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
gigacam 1 billion pixel detector
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
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