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

Deep Surveys:

Followup spectroscopy:

N S N S

~4 day period

SNAPSimple 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

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

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 : : : : : : : : : : : : : : : : : : : : : : : :

TMA62 spot diagrams

TMA55 Vignetting?

Ray Trace ResultsFive radii: +X, +XY, +Y, -XY, -X

Transmission vs off-axis angle,milliradians

0

10

20

30

40

50

60

70

80

90

100

0 5 10 15

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 Coefs TOL,RMS 3 microns

SECONDARY MIR disp,um shift,um rms,um disp(TOL),umX 10 -62 2 15

20 -125 4 1530 -187 6 15

Y 10 62 2 1520 124 4 1530 186 6 15

Z 10 0 16 220 0 32 230 0 47 2

disp,urad shift,um rms,um disp(TOL),uradPitch 16 134 3 16

32 268 5 19Tilt 16 134 3 16

32 268 5 1948 401 7 21

TMA56 sensitivity coefficients-secondary mirror-

45DEG MIR disp,urad shift,um rms,um disp(TOL),uradpitch 160 -350 5 96

320 -700 11 87Tilt 160 -173 23 21

320 -346 46 21

DETECTOR disp,um shift,um rms,um disp(TOL),umZ 100 0 13 23

200 0 26 23disp,urad shift,um rms,um disp(TOL),urad

Pitch 160 4 3 160320 9 5 192480 13 8 180

Tilt 160 0 3 160320 0 5 192480 0 8 180

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

mirror emissivity 0.015 each surfacepixel size 0.12 arcsec

Primary Secondary Tertiary Fold SMA StrutsTemperature 300 270 270 220 220Flux (fraction Zodiacal) 0.213 0.140 0.140 0.123 0.205

Total Blackbody Flux 0.821as a fraction of Zodicalshould be < 1

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

Materialshttp://www.minerals.sk.ca/atm_design and other sources

Young's density CTE(300) heat cond heat capy diffusivityMfr GPa g/cm3 ppm/K w/mK j/kg-K 1e-6 m2/s

ULE Corning 68 2.20 <0.03 1.31 776 0.8Zerodur Schott 91 2.53 <0.05 1.46 821 0.8silica many 73 2.20 0.52 1.38 703 0.84SiC many 466 3.05 2.37 300 660 146Borofloat Schott 63 2.22 3.2 1.1 830 0.6Pyrex Corning 64 2.23 3.2 1.3 726 0.7

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

X

Y

Z

V1G1

Initial design for primary mirror substrate: 334 kg

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

Deformations of mirror top face under pseudo-static launch loads: peak deflection

= 20 m

Design with locally thicker web platesStandard web thickness = 5 mm (orange)

Thickened plates = 10 mm (red)

Primary Mirror Substrate

• Free-free modes

• Sag during 1g figuring

– Sag is too large (>0.1m) on simple supports (3 pt vertical, strap horizontal)

– Will likely require vertical axis figuring on airbag supports

1g sag on 3pt supportvertical axis

P-P Z deflection = 2.3 m

1g sag in 180º strap supporthorizontal axis

P-P Z deflection = 0.5 m

Fundamental mode: 360 HzSecond mode: 566 Hz

1g front face ripple on perfect back-side support

P-P Z deflection = 0.018 m

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)

XY

Z

Lowest global mode of tertiary metering truss: 110Hz

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

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