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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


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.


Payload layout transverse rear axis shortest length
Payload Layout*transverse rear axis*shortest length


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.



Tma56 sensitivity coefficients fold mirror detector
TMA56 sensitivity coefficients-fold mirror & detector-


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


Materials http www minerals sk ca atm design and other sources
Materialshttp://www.minerals.sk.ca/atm_design and other sources


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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