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					Telescope Optical Performance
       Breakout Session


           M.Lampton
   UCBerkeley Space Sciences Lab
           10 July 2002




                                   1
Optical Performance: Overview



    Review
    Image quality

    Diffracted Starlight

    Stray (scattered) Light

    Acquisition Plan




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


                                              2
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
                                          3
Image Quality          1
TMA62/TMA63 configuration
                        Airy-disk zero at one micron wavelength
                        26 microns diam=0.244arcsec




                                                              4
                 Image Quality             2

PSF study TMA63
Contains ideal optimum surface aberrations, no mfg errors, no misalignments

                        One Dim One Dim         Two Dim    Two Dim       One Dim
             microns microns microns             microns milliArcsec   milliArcsec
 sinTheta      Rfinal radialRMS tangRMS        total RSS  total RSS         FWHM
    0.006    129122         3.32     1.6             3.69      35.09         58.29
    0.007    150838         3.33     1.6             3.69      35.17         58.44
    0.008    172649         3.18    1.59             3.56      33.85         56.23
    0.009    194565         2.83    1.51             3.21      30.54         50.74
     0.01    216600         2.28    1.37             2.66      25.32         42.07
    0.011    238769         1.57    1.35             2.07      19.71         32.75
    0.012    261086         1.18    1.89             2.23      21.21         35.24
    0.013    283565         2.09    3.23             3.85      36.63         60.85

                                RSS, 2D=           3.12       29.69           49.33



                                                                               5
            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

                                                                               6
     Image Quality: Distortion



TMA62 Distortion -- M.Lampton 07 Feb 2002

sinTheta    R microns LinModel   diff,
                                 microns
    0.006     129122    129960         -838
    0.007     150838    151620         -782
    0.008     172650    173280         -630
    0.009     194567    194940         -373
     0.01     216600    216600            0
    0.011     238769    238260          509
    0.012     261085    259920         1165
    0.013     283563    281580         1983


                                              7
                      Image Quality 3

                                                             1.2
•   Science SNR drives Strehl ratio                           1


     — Imperfections in delivered wavefront cause            0.8

                                                             0.6
       central PSF intensity to be less than ideal           0.4

       diffraction-limited PSF                               0.2

                                                              0
     — This ratio is the “Strehl Ratio”                     -0.2 0    5        10     15


•   Systems Engineer manages WFE budget
     —   geometrical aberrations                     Marechal’s equation relates WFE and Strehl
     —   manufacturing figure errors & cost
     —   alignment errors in 1-g environment           Strehl  exp[ (2  WFE /  ) 2 ]
     —   gravity release in mirrors & structure               
     —   launch induced shifts & distortions           WFE     ln(1 / Strehl )
                                                             2
     —   on-orbit thermal distortion
     —   ageing & creep of metering structure                                  Percent Energy in...
     —   how many on-orbit adjustments?              rms WFE/lam Strehl        Airy disk Rings
                                                                0          1         0.84      0.16
•   Primary mirror dominates WFE budget
                                                           0.018        0.99         0.83      0.17
    because it is the most expensive to figure.            0.036        0.95         0.80       0.2
•   Non-optical factors:                                    0.07        0.82         0.68      0.32
     — Attitude control system stability                      0.1       0.67         0.55      0.45
     — Transparency & optical depth in silicon              0.14        0.46         0.40       0.6
                                                              0.2       0.21         0.20   8 0.8
                   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

                 1.2
                  1
                 0.8
        Strehl




                 0.6
                 0.4
                 0.2
                  0
                       0       0.5       1       1.5        2
                                wavelength, microns
                                                                     9
                    Image Quality 5


                   Strehl ratio vs RMS WFE
          black=0.633um red=0.825um yellow=1.0um

1.2
 1
0.8
0.6
0.4
0.2
 0
      0      0.02       0.04         0.06         0.08   0.1   0.12
                               RMS WFE, microns
                                                               10
              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..
                                                                          11
                 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


                                                                      12
      Tolerance to Primary curvature



TMA62/63 IMAGE BLUR SENSITIVITY TO PRIMARY CURVATURE
          row in BOLD is the nominal design

Primary     Primary     UNADJUSTED SECY        ADJUSTED SECY
curvature   radius      zPosition rmsBlur      zPosition rmsBlur
 meter^-1    meters       meters   microns      meters    microns
 0.203740    4.908216    -2.000000      414     -2.000217        10
 0.203745    4.908096    -2.000000      303     -2.000158         8
 0.203750    4.907975    -2.000000      193     -2.000100         5
 0.203755    4.907855    -2.000000        83    -2.000042         3
 0.203759    4.907759    -2.000000         2    -2.000000         2
 0.203760    4.907735    -2.000000        23    -1.999988         2
 0.203765    4.907614    -2.000000      143     -1.999925         3
 0.203770    4.907494    -2.000000      260     -1.999867         5
 0.203775    4.907373    -2.000000      366     -1.999808         7
 0.203780    4.907253    -2.000000      478     -1.999750        10



                                                                  13
  Tolerance to misplaced secondary mirror
      Example assumes 3 micron growth in image blur

TMA56 Sensitivity Coefs                 TOL,RMS           3 microns

SECONDARY MIR        disp,um shift,um rms,um      disp(TOL),um
       X                     10        -62      2        15
                             20       -125      4        15
                             30       -187      6        15
          Y                  10         62      2        15
                             20        124      4        15
                             30        186      6        15
          Z                  10          0     16         2
                             20          0     32         2
                             30          0     47         2
                     disp,urad shift,um rms,um    disp(TOL),urad
          Pitch              16        134      3        16
                             32        268      5        19
          Tilt               16        134      3        16
                             32        268      5        19
                             48        401      7        21



                                                                      14
  Tolerance to misplaced tertiary mirror
   Example assumes 3 micron growth in image blur



TERTIARY MIR      disp,um shift,um rms,um      disp(TOL),um
         X               100       -252      2       150
                         200       -505      3       200
                         300       -757      5       180
        Y                100        252      2       150
                         200        504      4       150
        Z                100          0     21        14
                         200          0     40        15
                  disp,urad shift,um rms,um    disp(TOL),urad
        Pitch            160        701      6        80
                         320      1403      11        87
        Tilt             160        698      6        80
                         320      1396      11        87




                                                                15
    Diffracted Starlight 1
3X 4cm                               6X 2cm
                     Ø2m                          Ø2m



             Ø45cm                        Ø45cm




  Three 4cm Thick              Six 2cm thick
   Radial Vanes              Tangential Vanes

3X 4cm                               6X 2cm
                     Ø2m                          Ø2m



             Ø45cm                        Ø45cm




                                                        16
  Four 4cm Thick              Eight 2cm thick
   Radial Vanes              Tangential Vanes
                      Diffracted Starlight 2 (Four vanes)

                                                                         Irradiance at 633nm

                                 1
log10(I), scaled to unit input


                                 0
                                 -1
                                 -2

                                 -3
                                 -4
                                 -5

                                      -3           -2           -1                0              1             2                3
                                                                       Angle from star, Arcsec

                                      2000mm Aperture, 39.06mm vanes                             log10 focal plane irradiance




                                                                                                                                    17
                Diffracted Starlight 3 (Eight vanes)

                                                                         Irradiance at 633nm
log10(I), scaled to unit input




                                 0


                                 -2


                                 -4


                                 -6
                                      -3           -2           -1                0              1             2                3
                                                                       Angle from star, Arcsec

                                      2000mm Aperture, 19.53mm vanes                             log10 focal plane irradiance




                                                                                                                                    18
Circular 2meter aperture
5 x 5 arcsec




                  19
Circular 2meter aperture
0.7 meter central obscuration




                           20
Circular 2m aperture
Three radial legs, 50mm x 1 meter




                         21
Circular 2m aperture
central 0.7m obscuration
Three legs, 50mm x 1meter




                       22
Diffracted Starlight 8




                         23
           Diffracted Starlight 9




Assuming a 2.0 meter telescope,  1m, and a 2m  5cm obstruction :
    The Airy disk irradiance ratio vs angle envelope is
       I( )  I 0 10-4 -3 with in arcseconds
                                               .
    The spike irradiance ratio vs angle envelope is
       I( )  I 0 10-3 -2 with in arcseconds
                                               .
    The centralintensity I 0  3E10 100.4V ph/sec.pixel.m
    The Zodi intensity is 1 photon/sec.pixel.m
    Airy disk area above Zodi   2  7E4 10-0.267V
    12 - spike area above Zodi  12  w   6.6E4  w 10-0.2V


                                                                  24
                    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-spike
V 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-08
     1    0.00031           12          37852.80    8.87118E-07               10410.80         2.43987E-07
     2     0.0014           56          20469.07    2.16645E-06                6568.77         6.95238E-07
     3     0.0048          192          11068.74    4.01662E-06                4144.61            1.504E-06
     4      0.018          720           5985.47    8.14502E-06                2615.07         3.55859E-06
     5       0.05         2000           3236.67    1.22346E-05                1650.00         0.000006237
     6      0.141         5640           1750.24    1.86569E-05                1041.08         1.10975E-05
     7        0.4        16000            946.45    2.86207E-05                 656.88          1.9864E-05
     8        1.1        44000            511.80    4.25611E-05                 414.46         3.44666E-05
     9        2.9       116000            276.76    6.06761E-05                 261.51         5.73329E-05
    10        8.7       348000            149.66    9.84326E-05                 165.00         0.000108524
    11       21.9       876000             80.93    0.000133987                 104.11         0.000172365
    12       58.9      2356000             43.76    0.000194866                  65.69         0.000292497
    13        141      5640000             23.66    0.000252255                  41.45         0.000441799
    14        339     13560000             12.80    0.000327959                  26.15         0.000670202
    15        813     32520000              6.92    0.000425315                  16.50         0.001014136
    16       1738     69520000              3.74    0.000491665                  10.41         0.001367904
    17       3467    138680000              2.02    0.000530364                   6.57         0.001721708
    18       6918    276720000              1.09    0.000572269                   4.14         0.002167636
    19      10471    418840000              0.59     0.00046839                   2.62         0.002070112
    20      17783    711320000              0.32    0.000430155                   1.65         0.002218251

                                   Airy Fraction=   0.004104045      12-spike fraction=             25
                                                                                               0.012380234
               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?




                                                           26
                 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.          27
Stray Light 2




                28
Stray Light 3: Reverse Trace




                               29
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

                                                              31
          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
                                                                         32
                 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




                                                 33
Reflex Test Configuration




                            34
         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              35

				
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