# Presentation: 535997

## 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

## 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

## Image Quality     1TMA62/TMA63  configuration  

- Airy-disk zero at one micron wavelength
- 26 microns diam=0.244arcsec

## Image Quality     2

## 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**

## Image Quality:  Distortion

## 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**

- Marechal’s equation relates WFE and Strehl

## 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**

## Image Quality   5

## 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..**

## 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 **

## Tolerance to Primary curvature

## Tolerance to misplaced secondary mirrorExample assumes 3 micron growth in image blur

## Tolerance to misplaced tertiary mirrorExample assumes 3 micron growth in image blur

## Diffracted Starlight 1

## Diffracted Starlight   2  (Four vanes)

## Diffracted Starlight   3  (Eight vanes)

## Circular 2meter aperture

- 5 x 5 arcsec

## Circular 2meter aperture

- 0.7 meter central obscuration

## Circular 2m aperture

- Three radial  legs, 50mm x 1 meter

## Circular 2m aperture

- central 0.7m obscuration
- Three legs,  50mm x 1meter

## Diffracted Starlight    8

## Diffracted Starlight    9

## Diffracted Starlight    10

## 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?**

## 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

## Slide 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**

## 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**

## 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**

## Reflex Test Configuration

## 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**