Transcript Document 7442553

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:
N
S
Followup spectroscopy:
~4 day period
N
S
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
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
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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)
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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
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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
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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 :
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EFL=21.66meters :
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TMA62 spot diagrams
TMA55 Vignetting?
Ray Trace Results
Five radii: +X, +XY, +Y, -XY, -X
Transmission vs off-axis angle,milliradians
100
90
80
70
60
50
40
30
20
10
0
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 coefficients
-secondary mirrorTMA56 Sensitivity Coefs
SECONDARY MIR
X
Y
Z
Pitch
Tilt
TOL,RMS
3 microns
disp,um shift,um rms,um
disp(TOL),um
10
-62
2
15
20
-125
4
15
30
-187
6
15
10
62
2
15
20
124
4
15
30
186
6
15
10
0
16
2
20
0
32
2
30
0
47
2
disp,urad shift,um rms,um
disp(TOL),urad
16
134
3
16
32
268
5
19
16
134
3
16
32
268
5
19
48
401
7
21
TMA56 sensitivity coefficients
-fold mirror & detector45DEG MIR
pitch
Tilt
DETECTOR
Z
Pitch
Tilt
disp,urad shift,um rms,um
disp(TOL),urad
160
-350
5
96
320
-700
11
87
160
-173
23
21
320
-346
46
21
disp,um shift,um rms,um
disp(TOL),um
100
0
13
23
200
0
26
23
disp,urad shift,um rms,um
disp(TOL),urad
160
4
3
160
320
9
5
192
480
13
8
180
160
0
3
160
320
0
5
192
480
0
8
180
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
pixel size
0.015 each surface
0.12 arcsec
Primary Secondary Tertiary
Temperature
300
270
270
Flux (fraction Zodiacal)
0.213
0.140
0.140
Total Blackbody Flux
as a fraction of Zodical
should be < 1
0.821
Fold
SMA Struts
220
220
0.123
0.205
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
Mfr
ULE
Zerodur
silica
SiC
Borofloat
Pyrex
Corning
Schott
many
many
Schott
Corning
Young's
GPa
68
91
73
466
63
64
density
g/cm3
2.20
2.53
2.20
3.05
2.22
2.23
CTE(300) heat cond heat capy diffusivity
ppm/K
w/mK
j/kg-K
1e-6 m2/s
<0.03
<0.05
0.52
2.37
3.2
3.2
1.31
1.46
1.38
300
1.1
1.3
776
821
703
660
830
726
0.8
0.8
0.84
146
0.6
0.7
Primary Mirror Substrate
V1
G1
• 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
Z
X
Y
Initial design for primary mirror
substrate: 334 kg
Primary Mirror
Substrate
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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
Substrate
Fundamental mode: 360 Hz
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Second mode: 566 Hz
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 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 backside support
P-P Z deflection = 0.018 m
Secondary Metering Structure
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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
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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)
Z
Y
X
Lowest global mode of tertiary
metering truss: 110Hz
Telescope: Focussing
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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