Telescope optical performance breakout session
Download
1 / 35

Telescope Optical Performance Breakout Session - PowerPoint PPT Presentation


  • 112 Views
  • Uploaded on

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.

loader
I am the owner, or an agent authorized to act on behalf of the owner, of the copyrighted work described.
capcha
Download Presentation

PowerPoint Slideshow about 'Telescope Optical Performance Breakout Session' - job


An Image/Link below is provided (as is) to download presentation

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.


- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -
Presentation Transcript
Telescope optical performance breakout session

Telescope Optical PerformanceBreakout Session

M.Lampton

UCBerkeley Space Sciences Lab

10 July 2002


Optical performance overview
Optical Performance: Overview

  • Review

  • Image quality

  • Diffracted Starlight

  • Stray (scattered) Light

  • Acquisition Plan

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


Review
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
Image Quality 1TMA62/TMA63 configuration

Airy-disk zero at one micron wavelength

26 microns diam=0.244arcsec



Image quality 2 continued
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 3
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
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 6
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
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 misplaced secondary mirror example assumes 3 micron growth in image blur
Tolerance to misplaced secondary mirrorExample assumes 3 micron growth in image blur


Tolerance to misplaced tertiary mirror example assumes 3 micron growth in image blur
Tolerance to misplaced tertiary mirrorExample assumes 3 micron growth in image blur






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 11
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
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.




Optical performance throughput
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
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
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



Telescope summary
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