telescope optical performance breakout session
DESCRIPTION
Telescope Optical Performance Breakout Session. M.Lampton UCBerkeley Space Sciences Lab 10 July 2002. Optical Performance: Overview. Review Image quality Diffracted Starlight Stray (scattered) Light Acquisition Plan Materials, manufacturing etc will be discussed in Pankow’s talk. - PowerPoint PPT PresentationTRANSCRIPT
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Telescope Optical PerformanceBreakout Session
M.Lampton
UCBerkeley Space Sciences Lab
10 July 2002
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Optical Performance: Overview
ReviewImage qualityDiffracted StarlightStray (scattered) LightAcquisition Plan
Materials, manufacturing etc will be discussed in Pankow’s talk
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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
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Image Quality 1
TMA62/TMA63 configuration
Airy-disk zero at one micron wavelength
26 microns diam=0.244arcsec
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Image Quality 2
PSF study TMA63Contains ideal optimum surface aberrations, no mfg errors, no misalignments
One Dim One Dim Two Dim Two Dim One Dimmicrons microns microns microns milliArcsec milliArcsec
sinTheta Rfinal radialRMS tangRMS total RSS total RSS FWHM0.006 129122 3.32 1.6 3.69 35.09 58.290.007 150838 3.33 1.6 3.69 35.17 58.440.008 172649 3.18 1.59 3.56 33.85 56.230.009 194565 2.83 1.51 3.21 30.54 50.740.01 216600 2.28 1.37 2.66 25.32 42.07
0.011 238769 1.57 1.35 2.07 19.71 32.750.012 261086 1.18 1.89 2.23 21.21 35.240.013 283565 2.09 3.23 3.85 36.63 60.85
RSS, 2D= 3.12 29.69 49.33
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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
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Image Quality: Distortion
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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”
• Systems Engineer manages WFE budget— 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
Percent Energy in...rms WFE/lam Strehl Airy disk Rings
0 1 0.84 0.160.018 0.99 0.83 0.170.036 0.95 0.80 0.20.07 0.82 0.68 0.320.1 0.67 0.55 0.45
0.14 0.46 0.40 0.60.2 0.21 0.20 0.8
)/1ln(2
])/2(exp[ 2
StrehlWFE
WFEStrehl
Marechal’s equation relates WFE and Strehl
-0.2
0
0.2
0.4
0.6
0.8
1
1.2
0 5 10 15
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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
0
0.2
0.4
0.6
0.8
1
1.2
0 0.5 1 1.5 2
wavelength, microns
Str
ehl
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Image Quality 5
Strehl ratio vs RMS WFEblack=0.633um red=0.825um yellow=1.0um
0
0.2
0.4
0.6
0.8
1
1.2
0 0.02 0.04 0.06 0.08 0.1 0.12
RMS WFE, microns
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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..
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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
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Tolerance to Primary curvature
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Tolerance to misplaced secondary mirrorExample assumes 3 micron growth in image blur
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
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Tolerance to misplaced tertiary mirrorExample assumes 3 micron growth in image blur
TERTIARY MIR disp,um shift,um rms,um disp(TOL),umX 100 -252 2 150
200 -505 3 200300 -757 5 180
Y 100 252 2 150200 504 4 150
Z 100 0 21 14200 0 40 15
disp,urad shift,um rms,um disp(TOL),uradPitch 160 701 6 80
320 1403 11 87Tilt 160 698 6 80
320 1396 11 87
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Diffracted Starlight 1
Three 4cm ThickRadial Vanes
Six 2cm thickTangential Vanes
Ø2m
Ø45cm
3X 4cm
Ø45cm
Ø2m6X 2cm
Ø2m
Radial VanesFour 4cm Thick
Ø45cm
3X 4cmØ2m
Ø45cm
Tangential VanesEight 2cm thick
6X 2cm
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Diffracted Starlight 2 (Four vanes)
-3 -2 -1 0 1 2 3
-5
-4
-3
-2
-1
0
1
Irradiance at 633nmlo
g10(
I),
scal
ed t
o un
it in
put
Angle from star, Arcsec
2000mm Aperture, 39.06mm vanes log10 focal plane irradiance
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Diffracted Starlight 3 (Eight vanes)
-3 -2 -1 0 1 2 3-6
-4
-2
0
Irradiance at 633nm
log1
0(I)
, sc
aled
to
unit
inpu
t
Angle from star, Arcsec
2000mm Aperture, 19.53mm vanes log10 focal plane irradiance
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Circular 2meter aperture
5 x 5 arcsec
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Circular 2meter aperture
0.7 meter central obscuration
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Circular 2m aperture
Three radial legs, 50mm x 1 meter
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Circular 2m aperture
central 0.7m obscuration
Three legs, 50mm x 1meter
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Diffracted Starlight 8
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Diffracted Starlight 9
0.2V-
0.267V-2
4.00
2-3-0
3-4-0
10w6.6E4w12 Zodiabove area spike-12
107E4 Zodiabove areadisk Airy
m.pixel.photon/sec 1 isintensity ZodiThe
mel.ph/sec.pix10103Iintensity central The
.arcsecondsin with 10I)I(
is envelope angle vsratio irradiance spike The
.arcsecondsin with 10I )I(
is envelope angle vsratio irradiancedisk Airy The
:nobstructio 5cm2m a and m,1 scope,meter tele 2.0 a Assuming
VE
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Diffracted Starlight 10
STAR NUMBERS AND STARLIGHT; Diffracted light above Zodi 12 spikes * 0.25 arcsec * spikeLength
Nstars/sky Airy area/star Total Airy area 12 spike area/star Total 12-spikeV mag stars/sqdeg per mag sqarcsec fraction of sky sq arcsec fraction of sky
0 0.00008 3 70000.00 4.2336E-07 16500.00 9.9792E-081 0.00031 12 37852.80 8.87118E-07 10410.80 2.43987E-072 0.0014 56 20469.07 2.16645E-06 6568.77 6.95238E-073 0.0048 192 11068.74 4.01662E-06 4144.61 1.504E-064 0.018 720 5985.47 8.14502E-06 2615.07 3.55859E-065 0.05 2000 3236.67 1.22346E-05 1650.00 0.0000062376 0.141 5640 1750.24 1.86569E-05 1041.08 1.10975E-057 0.4 16000 946.45 2.86207E-05 656.88 1.9864E-058 1.1 44000 511.80 4.25611E-05 414.46 3.44666E-059 2.9 116000 276.76 6.06761E-05 261.51 5.73329E-05
10 8.7 348000 149.66 9.84326E-05 165.00 0.00010852411 21.9 876000 80.93 0.000133987 104.11 0.00017236512 58.9 2356000 43.76 0.000194866 65.69 0.00029249713 141 5640000 23.66 0.000252255 41.45 0.00044179914 339 13560000 12.80 0.000327959 26.15 0.00067020215 813 32520000 6.92 0.000425315 16.50 0.00101413616 1738 69520000 3.74 0.000491665 10.41 0.00136790417 3467 138680000 2.02 0.000530364 6.57 0.00172170818 6918 276720000 1.09 0.000572269 4.14 0.00216763619 10471 418840000 0.59 0.00046839 2.62 0.00207011220 17783 711320000 0.32 0.000430155 1.65 0.002218251
Airy Fraction= 0.004104045 12-spike fraction= 0.012380234
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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?
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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,
allowing a “system engineering management” of stray light.
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Stray Light 2
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Stray Light 3: Reverse Trace
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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
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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
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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
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Reflex Test Configuration
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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