Sana01.01.1970
Hajmi
#1989


SNAP OTA Baseline TMA62


SNAP Mission Plan

  • Preselect ~20 study fields, both NEP and SEP

  • Discoveries & photometric light curves from repeated deep images

    • huge multiplex advantage with “batch” observations, 1E9 pixels
  • Spectroscopy near maximum light from followup pointings



SNAP



Payload Layout *transverse rear axis *shortest length



Annular Field Three Mirror Anastigmat

  • Aperture: 2 meters

  • Field of view: > 1 square degree

    • 1.37 square degrees in TMA62
  • Diffraction limited longward of one micron

    • 2 microns RMS, 15microns FWZ geometric
  • Flat field

  • Folded to obtain short overall length

    • 3.3 meters in TMA62


Wide-Field Telescope: History

  • Wide-field high-resolution telescopes are NOT new

    • Schmidt cameras (1930 to present)
    • 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 (1972...)
    • Angel-Woolf-Epps three-mirror design (1982)
    • McGraw three-mirror system (1982)
    • Willstrop “Mersenne Schmidt” family (1984)
    • Dark Matter Telescope (1996+)
    • New Planetary Telescope (1998)
    • IKONOS Earth resources satellite (1999)
    • FAME astrometric TMA
    • Multispectral Thematic Imager (1999)


Three-mirror anastigmat (TMA)

  • Identified as best choice for SNAP

  • Can deliver the required FOV

  • Can deliver the required resolution

  • Inherently achromatic, no correctors needed

  • Inherently flat field

  • Inherently elastic: 9 d.o.f. to meet 4 Seidel conditions plus focus & focal length

  • Can meet packaging requirements



Telescope: Downselection

  • 1999-2001: Suitability Assessments

    • sought 1 sq deg with diffraction limited imaging (< 0.1 arcsec)
    • low obscuration is highly desirable
    • 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; lateral axis between tertiary & detector
    • TMA55 (f/10): improved performance, margins positive, common axes for pri, sec, tertiary.
    • TMA56 (f/10) like TMA55 but stretched
    • TMA59 (f/15): same but with longer focal length
    • TMA62 (f/10.5) lateral axis between tertiary & detector


Baseline Telescope

  • Baseline Optical System: Annular Field TMA62

    • prolate ellipsoid concave primary mirror
    • hyperbolic convex secondary mirror
    • flat annular folding mirror
    • prolate ellipsoid concave tertiary mirror
    • flat focal plane
    • provides side-mounted detector location for best detector cooling
    • EFL = 21.66m matches 10.5 micron SiCCD pixel to 0.1 arcsec angular scale
      • plate scale is 105 microns per arcsecond
    • delivers annular field 1.37 sqdeg
    • average geometrical blur 2.5umRMS = 6umFWHM; 16um worst case FWZ
      • compare: SiCCD pixel = 10.5 um; HgCdTE pixel 18.5um
    • angular geometrical blur 0.023arcsecRMS =0.06arcsecFWHM
      • compare: Airy disk, 1um wavelength: FWHM=0.12arcsec=13um




Annular Field Dimensions

  • Outer radius: 0.745 degrees

    • corresponds to 283.56 mm at detector
  • Inner Radius: 0.344 degrees

    • corresponds to 129.1 mm at detector
  • Sky coverage 1.37 square degree

    • corresponds to 1957 cm2 detector area
  • Field Blockages-- none

  • Can go to larger radii but image quality degrades rapidly

  • Can go to smaller radii but vignetting becomes severe



TMA62 Optics Prescription

  • Primary Mirror (concave prolate ellipsoid) located at origin:

    • diameter= 2000 mm; hole= 450mm
    • curvature= -0.2037586, radius=4.907768m; shape=+0.0188309, asphericity= -0.981169
  • Secondary Mirror (convex hyperboloid) located at Z=-2.000 meters:

    • diameter= 450mm
    • curvature= -0.9103479, radius=1.0984811m; shape= -0.8471096, asphericity= -1.8471096
  • Folding flat mirror located on axis, Z=+0.91 meters:

    • oval, 700mm x 500mm; central hole 190 x 120mm
  • Tertiary Mirror (concave prolate ellipsoid) located at Z=+0.91, X= -0.87meters:

    • diameter=680mm
    • curvature= -0.7116752, radius=1.405135m; shape=+0.40203, asphericity= -0.59797
  • Filter/Window located along beam toward detector

    • nominal thickness 5mm, fused silica
  • Annular Detector Array located at Z=+0.91, X=+0.90 meters:

    • inner diameter 129mm, outer diameter 283.6mm


TMA62 Prescription -- BEAM FOUR format



TMA62 spot diagrams



TMA55 Vignetting?



Ray Trace Results Five radii: +X, +XY, +Y, -XY, -X Transmission vs off-axis angle,milliradians



TMA62 Vignetting and Image quality issues

  • Nominal annulus 6 to 13mrad

    • no vignetting, but little or no tolerance
    • 2 um rms average image blur over this field
  • At 5mrad: approx 50% of rays are lost at edge of hole in 45deg flat mirror

  • At 14mrad: vignetting losses depend critically on element sizing; geometrical blur about 40um FWZ.



TMA56 sensitivity coefficients -secondary mirror-



TMA56 sensitivity coefficients -fold mirror & detector-



Glare & Stray Light Sources

  • Ecliptic Poles places Sun 70 to 110deg off axis

    • sunshade design “straightforward”
  • Earth, moon can be up to 15 deg off axis

    • needs careful baffle study, now in work
  • Stars, Zodiacal dust, diffuse Galactic light

    • concerns are optics scatter, dirt, structure
  • Stray light specification: must be small compared to natural NIR foreground

  • Thermal emission from optics must also be small



Baffle treatment: outer tube, secondary cone, inner tube



Stray Light Baffle Concept



Diffuse NIR foreground



Mirror emissivity



Optical Mirror Technologies

  • Open-back weight relieved Zerodur or silica

    • offers 75% to 80% LW
    • potentially quicker procurement cycle
  • Ultralight core+face+back: 90-95%LW

    • typically use Corning ULE
    • requires ion milling
    • requires in-chamber metrology
  • SiC technologies

    • evolving; under study


Materials http://www.minerals.sk.ca/atm_design and other sources



Primary Mirror Substrate

  • Key requirements and issues

    • Dimensional stability over time
    • Dimensional stability in thermal gradient
    • High specific stiffness (1g sag, acoustic response)
    • Stresses during launch
    • Design of supports
  • Prefer < 100kg/m2

  • Variety of materials & technologies



Primary Mirror Substrate

  • Stresses from pseudo-static launch loads

    • 6.5g axial, 0.5g transverse
    • 3-point supports
  • Baseline

    • Face sheets (12 mm)
    • Locally thickened web walls (10 mm)
    • Thicker outer ring (8 mm)
  • Mass (330 kg)

  • Fundamental mode 360 Hz

  • Conclusions

    • 80% lightweighted design is workable
    • 3 pt support may be usable for launch
    • Vertical axis airbag support required for figuring


Primary Mirror Substrate

  • Free-free modes

  • Sag during 1g figuring

    • Sag is too large (>0.1m) on simple supports (3 pt vertical, strap horizontal)
    • Will likely require vertical axis figuring on airbag supports


Secondary Metering Structure

  • Key requirements:

    • Minimize obscuration (<3.5%) & interference spikes
    • Dimensional stability
    • 35 Hz minimum fundamental frequency
  • Baseline design: hexapod truss with fixed end

    • Simple design with low obscuration (3.5%)
    • 6-spiked diffraction pattern
    • Ø 23 mm by 1 mm wall tubular composite (250 GPa material) struts with invar end-fittings.


Secondary Metering Structure



Tertiary Metering Structure

  • Key requirements:

    • Dimensional stability
    • 35 Hz minimum fundamental frequency
  • Easier design problem than secondary metering structure

    • Overall dimensions much smaller than secondary metering truss
    • No obscuration concerns
    • Use strut design from secondary metering structure (cost effective)


Telescope: Focussing

  • 13 mechanical adjustments is minimum set

    • focussing
    • collimation
    • centering
    • alignment
    • on orbit, may only need secondary to be articulated
  • Least squares optimization for focussing and collimation

  • Alternatives: Zernike defocus analysis



GIGACAM 1 billion pixel detector

  • 132 large format silicon CCDs

  • 25 2Kx2K HgCdTe NIR detectors

  • Larger than SDSS array

  • Smaller than BABAR silicon vertex detector

  • Outside diameter 480mm

  • Each chip has dedicated bandpass filter

  • Located within 150K cryostat

  • Accommodates guiding and spectroscopy feeds



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