telescope optical performance m.lampton ucberkeley space sciences lab 10 july 2002 draft 6, 7 june...

Telescope Optical Performance

M.Lampton

UCBerkeley Space Sciences Lab

10 July 2002

draft 6, 7 June 2002

Optical Performance: Overview

Status Review Image quality Diffracted Starlight Stray (scattered) Light Plans, Contracts, Manpower

Materials, manufacturing etc will be discussed in Pankow’s talk

Optical Performance: Review 1

• Light Gathering Power — must measure SNe 4 magnitudes fainter than 26 magnitude peak

— require SNR of 50:1 at peak brightness

— presence of zodiacal light foreground radiation

— time-on-target limited by revisit rate & number of fields

— spectroscopy demands comparable time-on-target

— requires geometric diameter ~ 2 meters

• Angular resolution— signal to noise ratio is driver

— diffraction limit imposes upper bound

— Airy disk at one micron wavelength is 0.12 arcseconds FWHM

— other blur contributions must be kept well below this limit.

• Field of View— determined by required supernova discovery rate

— volume of space is proportional to field of view

— one square degree area will deliver the requisite discovery rate

• Wavelength Coverage— 0.35 to 1.7 microns requires all-reflector optical train


• Aperture: 2 meters

• Field of View: 1 sq deg

• Wavelength Range: 0.35 to 1.7 microns

• Strehl: >91% at 1.0 microns

• WFE: <50 nm RMS

• Focal surface: flat

• EFL: 21.66 meters, f/11

• Stray light: << Zodiacal

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• Wide-field high-resolution telescopes are NOT new—Schmidt cameras (1930 to present)

—Wynne cameras (e.g. FAUST)

—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 (1980)

—Angel-Woolf-Epps three-mirror design (1982)

—McGraw three-mirror system (1982)

—Willstrop “Mersenne Schmidt” family (1984)

—Kodak “IKONOS” Earth Resources telescope = TMA

—LANL/Sandia/DOE Multispectral Thermal Imager = TMA


• 1999-2002: Suitability Assessments—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

—TMA55 (f/10): improved performance, shorter pri-sec, margins OK

—TMA59 (f/15): same but with longer focal length

—TMA62 (f/11): transverse rear axis with filter wheel

—TMA63 (f/11): transverse rear axis, no filter wheel

Payload Layout 1

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

Payload Layout 2


• Existing technologies are suitable for SNAP Optical Telescope Assembly• New materials, processes, test & evaluation methods are unnecessary• Mirror materials

— Corning ULE glass: extensive NRO flight history, but expensive

— Schott Zerodur glass/ceramic composite: lower cost, widely used in ground based astronomical telescopes; huge industrial base

— Astrium/Boostec SiC-100: newcomer; unproven in space optics; higher CTE; adoped for Herschel/FIRST in infrared

• Structural materials— M55J carbon fiber + cyanate ester resin; epoxy adhesive bonds

— full report in Pankow presentation

• Mirror finishing technology— conventional grind/polish/figure using abrasives

— ion-beam figuring available from two vendors

• Mirror surface metrology— same as other space telescopes, e.g. cassegrains

— standard interferometer setups will do the job for SNAP

— no unusual accuracy drivers have been encountered


• Prolate ellipsoid concave primary mirror• Hyperbolic convex secondary mirror• Flat folding mirror with central hole• Prolate ellipsoid concave tertiary mirror• Flat focal plane• Delivers < 0.06 arcsecond FWHM geometrical blur over annular field 1.37 sqdeg• Adapts to focal lengths 20 meters through 30 meters; baseline=21.66m• Provides side-mounted detector location for best detector cooling

Image Quality 1

TMA62/TMA63 configuration

Airy-disk zero at one micron wavelength

26 microns diam=0.244arcsec

Image Quality 2

• 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

Image Quality 3

• 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

Image Quality 4

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

Mirror surface specification

• 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..

Image Quality 5

• 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

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

Diffracted Starlight 2 (Four vanes)

-3 -2 -1 0 1 2 3

-5

-4

-3

-2

-1

0

1

Irradiance at 633nmlo

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it in

put

Angle from star, Arcsec

2000mm Aperture, 39.06mm vanes log10 focal plane irradiance

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

Circular 2meter aperture

5 x 5 arcsec

Circular 2meter aperture

0.7 meter central obscuration

Circular 2m aperture

Three radial legs, 50mm

Circular 2m aperture

central 0.7m obscuration

Three 50mm legs


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

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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 3.2 70000.00 4.2336E-07 16500.00 9.9792E-081 0.00031 12.4 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


• 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?

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.

Stray Light 2

Stray Light 3: Reverse Trace

Optical Performance: Throughput

• Protected silver

—provides highest NIR reflectance currently available

—durability is an issue: 3 years at sea level prior to launch

—needs further study

• 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

—needs further study

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

Telescope: R&D Phase Management

• Management objective: biddable Requirements Document that reflects all science requirements and trades

• Experienced team has been assembled

• Have begun dialogs with prospective vendors

• Have begun examining potential fab/test flows

• No need for high-risk “advanced” materials or processes

• Emphasize proven manufacturing & test techniques

• We plan on selection of contractor(s) with sufficient experience to bring successful delivery cost & schedule

• This contractor mix defines the overall acquisition plan

Telescope: CDR Preparations Plan

• Fully complete and document all trade studies

• Supplement these with industry commentary

• Use system-engineering “budgets” to identify optimum allocation of tolerances & resources

• Identify materials and fabrication alternatives taking into account schedule risk and overall cost

• Detail the acquisition plan and milestones

• Prepare acceptance test plan, including flight qual tests:— Structural stability, thermal, stiffness, normal modes, creep

— Alignment and focussing plan

— Thermal vacuum and optical stability

— Stray light

— Gravity unloading plan

— Full-aperture and limited-aperture test opportunities

— Facilities needed, facilities available

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

telescope optical performance m.lampton ucberkeley space sciences lab 10 july 2002 draft 6, 7 june...

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