Telescope Optical Performance Breakout Session

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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


• Example: overall telescope 50 nm RMS WFE




• 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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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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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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• 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

Documents

Telescope Optical Performance Breakout Session