Snap ota baseline tma62 M. Lampton Jan 2002

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

• Spectroscopy near maximum light from followup pointings

SNAP

Payload Layout *transverse rear axis *shortest length

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 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)

• 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

• 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 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

• 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

• 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 spot diagrams

TMA55 Vignetting?

Ray Trace Results Five radii: +X, +XY, +Y, -XY, -X Transmission vs off-axis angle,milliradians

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 -secondary mirror-

TMA56 sensitivity coefficients -fold mirror & detector-

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

Baffle treatment: outer tube, secondary cone, inner tube

Stray Light Baffle Concept

Diffuse NIR foreground

Mirror emissivity

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

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

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

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

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.

Secondary Metering Structure

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

• 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

• 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