Download document

Telescope Optical Performance
Breakout Session
M.Lampton
UCBerkeley Space Sciences Lab
10 July 2002
1
Optical Performance: Overview
Review
 Image quality
 Diffracted Starlight
 Stray (scattered) Light
 Acquisition Plan


Materials, manufacturing etc will be
discussed in Pankow’s talk
2
Review
Telescope is a three-mirror anastigmat
2.0 meter aperture
1.37 square degree field
Lightweight primary mirror
Low-expansion materials
Optics kept near 290K
Transverse rear axis
Side Gigacam location
passive detector cooling
combines Si & HgCdTe detectors
Spectrometers share Gigacam focal plane
Minimum moving parts in payload
shutter for detector readouts
3
Image Quality
1
TMA62/TMA63 configuration
Airy-disk zero at one micron wavelength
26 microns diam=0.244arcsec
4
Image Quality
2
PSF study TMA63
Contains ideal optimum surface aberrations, no mfg errors, no misalignments
sinTheta
0.006
0.007
0.008
0.009
0.01
0.011
0.012
0.013
One Dim One Dim
microns microns microns
Rfinal radialRMS tangRMS
129122
3.32
1.6
150838
3.33
1.6
172649
3.18
1.59
194565
2.83
1.51
216600
2.28
1.37
238769
1.57
1.35
261086
1.18
1.89
283565
2.09
3.23
RSS, 2D=
Two Dim
Two Dim
microns milliArcsec
total RSS
total RSS
3.69
35.09
3.69
35.17
3.56
33.85
3.21
30.54
2.66
25.32
2.07
19.71
2.23
21.21
3.85
36.63
3.12
29.69
One Dim
milliArcsec
FWHM
58.29
58.44
56.23
50.74
42.07
32.75
35.24
60.85
49.33
5
Image Quality 2 continued
•
Although the range of radii in use within the focal plane is the nominal
design range 129 to 283mm, the extremes are poorly populated with
pixels
6
Image Quality: Distortion
TMA62 Distortion -- M.Lampton 07 Feb 2002
sinTheta
0.006
0.007
0.008
0.009
0.01
0.011
0.012
0.013
R microns LinModel
129122
150838
172650
194567
216600
238769
261085
283563
129960
151620
173280
194940
216600
238260
259920
281580
diff,
microns
-838
-782
-630
-373
0
509
1165
1983
7
Image Quality 3
•
Science SNR drives Strehl ratio
— Imperfections in delivered wavefront cause
central PSF intensity to be less than ideal
diffraction-limited PSF
— This ratio is the “Strehl Ratio”
•
•
1
0.8
0.6
0.4
0.2
0
-0.2 0
5
10
15
Systems Engineer manages WFE budget
—
—
—
—
—
—
—
—
•
1.2
geometrical aberrations
manufacturing figure errors & cost
alignment errors in 1-g environment
gravity release in mirrors & structure
launch induced shifts & distortions
on-orbit thermal distortion
ageing & creep of metering structure
how many on-orbit adjustments?
Primary mirror dominates WFE budget
because it is the most expensive to figure.
Non-optical factors:
— Attitude control system stability
— Transparency & optical depth in silicon
Marechal’s equation relates WFE and Strehl
Strehl  exp[ (2 WFE /  ) 2 ]
WFE 

ln( 1 / Strehl )
2
rms WFE/lam Strehl
0
1
0.018
0.99
0.036
0.95
0.07
0.82
0.1
0.67
0.14
0.46
0.2
0.21
Percent Energy in...
Airy disk Rings
0.84
0.16
0.83
0.17
0.80
0.2
0.68
0.32
0.55
0.45
0.40
0.6
0.20
8 0.8
Image Quality 4
•
•
For diffraction-limited optics, rmsWFE or Strehl @0.633um is
usually the governing procurement specification
SNAP exposure-time-critical science is at wavelengths > 0.63um
Science team needs to be aware of cost/schedule/quality trades
Strehl for rmsWFE= 20, 40, 60 nm
1.2
1
Strehl
•
0.8
0.6
0.4
0.2
0
0
0.5
1
1.5
2
wavelength, microns
9
Image Quality 5
Strehl ratio vs RMS WFE
black=0.633um red=0.825um yellow=1.0um
1.2
1
0.8
0.6
0.4
0.2
0
0
0.02
0.04
0.06
0.08
0.1
0.12
RMS WFE, microns
10
Image Quality
•
•
•
•
•
6
Example: overall telescope 43 nm RMS WFE
— gives Strehl= 0.93 at 1000 nm
— gives Strehl=0.90 at 830 nm
— gives Strehl=0.83 at 633 nm
Example: overall telescope 50 nm RMS WFE
— gives Strehl=0.91 at 1000 nm
— gives Strehl=0.87 at 830 nm
— gives Strehl=0.77 at 633 nm
WFE to be budgeted among pri, sec, flat, and tertiary mirrors
— detailed breakdown to be determined
How sensitive are cost & schedule to our WFE specification?
Encircled Energy specification needs to be defined
— central obstruction 40% radius, 16% area
— with this obstruction alone, EE=50% at 0.088arcsec diam @633nm or
EE=80% at 0.23arcsec diam @633nm
— Budget lower EE for aberrations, spider, figuring, thermal, gravity..
11
Image Quality 7
•
•
•
•
•
Strehl vs Aperture Trade
— Strehl (image quality) costs time & money
— Aperture (image quantity) costs time & money
— Central obscuration trades off with stray light
— NIR (not visible) is where SNR demands the most observing time
— Is 77% Strehl and 2.0 meters aperture the right mix?
Encircled Energy Specification
— High spatial frequency figure errors lose photons
— Low spatial frequency figure errors broaden the encircled energy
— Steeper EE curves demand absence of LSF amplitudes
— Is 70% EE at 0.1 arcsecond the right target?
Quantitative answers require modelling
Our sim team can deal with image quality trades
We expect to resolve these issues during R&D phase
12
Tolerance to Primary curvature
TMA62/63 IMAGE BLUR SENSITIVITY TO PRIMARY CURVATURE
row in BOLD is the nominal design
Primary
curvature
meter^-1
0.203740
0.203745
0.203750
0.203755
0.203759
0.203760
0.203765
0.203770
0.203775
0.203780
Primary
radius
meters
4.908216
4.908096
4.907975
4.907855
4.907759
4.907735
4.907614
4.907494
4.907373
4.907253
UNADJUSTED SECY
zPosition rmsBlur
meters
microns
-2.000000
414
-2.000000
303
-2.000000
193
-2.000000
83
-2.000000
2
-2.000000
23
-2.000000
143
-2.000000
260
-2.000000
366
-2.000000
478
ADJUSTED SECY
zPosition rmsBlur
meters
microns
-2.000217
10
-2.000158
8
-2.000100
5
-2.000042
3
-2.000000
2
-1.999988
2
-1.999925
3
-1.999867
5
-1.999808
7
-1.999750
10
13
Tolerance to misplaced secondary mirror
Example assumes 3 micron growth in image blur
TMA56 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
14
Tolerance to misplaced tertiary mirror
Example assumes 3 micron growth in image blur
TERTIARY MIR
X
Y
Z
Pitch
Tilt
disp(TOL),um
disp,um shift,um rms,um
150
2
-252
100
200
3
-505
200
180
5
-757
300
150
2
252
100
150
4
504
200
14
21
0
100
15
40
0
200
disp(TOL),urad
disp,urad shift,um rms,um
80
6
701
160
87
11
1403
320
80
6
698
160
87
11
1396
320
15
Diffracted Starlight 1
3X 4cm
Ø2m
Ø45cm
Six 2cm thick
Tangential Vanes
Ø2m
Ø45cm
Four 4cm Thick
Radial Vanes
Ø2m
Ø45cm
Three 4cm Thick
Radial Vanes
3X 4cm
6X 2cm
6X 2cm
Ø2m
Ø45cm
Eight 2cm thick
Tangential Vanes
16
Diffracted Starlight 2 (Four vanes)
log10(I), scaled to unit input
Irradiance at 633nm
1
0
-1
-2
-3
-4
-5
-3
-2
-1
2000mm Aperture, 39.06mm vanes
0
Angle from star, Arcsec
1
2
3
log10 focal plane irradiance
17
Diffracted Starlight 3 (Eight vanes)
log10(I), scaled to unit input
Irradiance at 633nm
0
-2
-4
-6
-3
-2
-1
2000mm Aperture, 19.53mm vanes
0
Angle from star, Arcsec
1
2
3
log10 focal plane irradiance
18
Circular 2meter aperture
5 x 5 arcsec
19
Circular 2meter aperture
0.7 meter central obscuration
20
Circular 2m aperture
Three radial legs, 50mm x 1 meter
21
Circular 2m aperture
central 0.7m obscuration
Three legs, 50mm x 1meter
22
Diffracted Starlight 8
23
Diffracted Starlight 9
Assuming a 2.0 meter tele scope,   1m, and a 2m  5cm obstructio n :
The Airy disk irradiance ratio vs angle envelope is
I( )  I 0 10-4 -3 with  in arcseconds .
The spike irradiance ratio vs angle envelope is
I( )  I 0 10-3 -2 with  in arcseconds .
The central intensity I 0  3E10 10 0.4V ph/sec.pix el. m
The Zodi intensity is 1 photon/sec .pixel. m
Airy disk area above Zodi   2  7E4 10-0.267V
12 - spike area above Zodi  12  w   6.6E4  w 10-0.2V
24
Diffracted Starlight 10
STAR NUMBERS AND STARLIGHT; Diffracted light above Zodi
V mag stars/sqdeg
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
0.00008
0.00031
0.0014
0.0048
0.018
0.05
0.141
0.4
1.1
2.9
8.7
21.9
58.9
141
339
813
1738
3467
6918
10471
17783
Nstars/sky
per mag
3
12
56
192
720
2000
5640
16000
44000
116000
348000
876000
2356000
5640000
13560000
32520000
69520000
138680000
276720000
418840000
711320000
Airy area/star Total Airy area
sqarcsec fraction of sky
12 spikes * 0.25 arcsec * spikeLength
12 spike area/star
Total 12-spike
sq arcsec
fraction of sky
70000.00
37852.80
20469.07
11068.74
5985.47
3236.67
1750.24
946.45
511.80
276.76
149.66
80.93
43.76
23.66
12.80
6.92
3.74
2.02
1.09
0.59
0.32
4.2336E-07
8.87118E-07
2.16645E-06
4.01662E-06
8.14502E-06
1.22346E-05
1.86569E-05
2.86207E-05
4.25611E-05
6.06761E-05
9.84326E-05
0.000133987
0.000194866
0.000252255
0.000327959
0.000425315
0.000491665
0.000530364
0.000572269
0.00046839
0.000430155
16500.00
10410.80
6568.77
4144.61
2615.07
1650.00
1041.08
656.88
414.46
261.51
165.00
104.11
65.69
41.45
26.15
16.50
10.41
6.57
4.14
2.62
1.65
9.9792E-08
2.43987E-07
6.95238E-07
1.504E-06
3.55859E-06
0.000006237
1.10975E-05
1.9864E-05
3.44666E-05
5.73329E-05
0.000108524
0.000172365
0.000292497
0.000441799
0.000670202
0.001014136
0.001367904
0.001721708
0.002167636
0.002070112
0.002218251
Airy Fraction=
0.004104045
12-spike fraction=
25
0.012380234
Diffracted Starlight 11
•
•
•
•
•
•
•
Extensive work with sim team
Modelling PSF for SNR, exposure times...
Modelling wings of diffraction pattern
Algorithms for photometry in presence of diffraction
Determination of effective SNR
Inputs from our known sky, down to V=19 (SDSS)
How well can these effect be modelled?
26
Stray Light
•
•
•
•
•
•
•
•
•
1
Guiding principle: keep total stray light FAR BELOW natural Zodi
R.O.M. assessment gives...
— Natural Zodi (G.Aldering) = 1 photon/pixel/sec/micron
— Starlight+Zodi scattered off primary mirror = 0.002
— Starlight+Zodi scattered off support spider < 0.001
— Sunlight scattered off forward outer baffle edge = 2E-5
— Earthlight scattered off forward outer baffle inner surface = 0.02
— Total stray = 0.02 photon/pixel/sec/micron
ISAL conclusion: “manageable”
Long outer baffle is clearly preferred
— limit is launch fairing and S/C size
ASAP software in place
ASAP training begun
Preliminary telescope ASAP models being built
ASAP illumination environment models not yet started
Our intension is to track hardware & ops changes as they occur,
27
allowing a “system engineering management” of stray light.
Stray Light 2
28
Stray Light 3: Reverse Trace
29
30
Optical Performance: Throughput
• Protected silver
—provides highest NIR reflectance currently available
—durability is an issue: 3 years at sea level prior to launch
—this is our baseline
—new developments at LLNL: Thomas & Wolfe process
• Protected aluminum
—highly durable coating
—slight reflectance notch at 0.8 microns wavelength
—after four reflections, amounts to 30-40% loss at 0.8 um
—prefer to retain high reflectance at 0.8 microns
—not our first choice
31
Telescope Acquisition Plan
•
•
•
•
Potential Vendors Identified
— Ball Aerospace Systems Division (Boulder)
— Boeing-SVS (Albuquerque/Boulder)
— Brashear LP (Pittsburgh)
— Composite Optics Inc (San Diego)
— Corning Glass Works (Corning NY)
— Eastman Kodak (Rochester)
— Goodrich (Danbury)
— Lockheed-Martin Missiles & Space Co (Sunnyvale)
— SAGEM/REOSC (Paris)
These vendors have been briefed on SNAP mission
Each has responded to our Request for Information
Identify a route (materials, fabrication, test, integration, test)
— Milestones with appropriate incentives
— Visibility into contractor(s) activities
32
Test Plans
• Individual Mirror Testing
• Assembly into metering structure
• Assembled optical testing
— interferometric
— reflex testing against reference flat
• Integration with focal plane assembly
• End-to-end testing
— in air at room temperature
— in vacuum or dry N2 with cold focal plane
— reflex testing against reference flat
33
Reflex Test Configuration
34
Telescope: Summary
•
•
Pre-R&D
— converted science drivers into telescope requirements
— reviewed existing optical telescope concepts
— developed annular-field TMA configuration
— preliminary materials assessment
— begun to explore vendor capabilities
— started a budget for image quality
R&D Phase
— engineering trade studies and “budgets”
— manufacturing process risk assessments
— test plans and associated cost/risk trades
• facilities; equipment
— prepare the acquisition plan
— performance specifications & tolerance analysis
— create draft ICDs
— develop preliminary cost & schedule ranges
35