SNAP Telescope

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SNAP Telescope
http//snap.lbl.gov http//xxx.lanl.gov/pdf/astro-
ph/0405232
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SNAP Telescope
M. Sholl9, M. Lampton9, G.Aldering1, W.
Althouse2, R. Amanullah3, J. Annis4, P. Astier5,
C. Baltay6, E. Barrelet5, S. Basa7, C. Bebek1, L.
Bergström3, G. Bernstein8, M. Bester9, R.
Besuner, B. Bigelow10, R. Blandford2, R.
Bohlin11, A. Bonissent12, C. Bower13, M. Brown10,
M. Campbell10, W. Carithers1, E. Commins9, W.
Craig2, C. Day1, F. DeJongh4, S. Deustua14, T.
Diehl4, S. Dodelson4, A. Ealet7, R. Ellis15, W.
Emmet6, D. Fouchez12, J. Frieman4, A. Fruchter11,
D. Gerdes10, L. Gladney8, G. Goldhaber9, A.
Goobar3, D. Groom1, H. Heetderks9, M. Hoff1, S.
Holland1, M. Huffer2, L. Hui4, D. Huterer16, B.
Jain8, P. Jelinsky9, A. Karcher1, S. Kahn2, S.
Kent4, A. Kim1, W. Kolbe1, B. Krieger1, M. Krim,
G. Kushner1, N. Kuznetsova1, R. Lafever1, J.
Lamoureux1, O. Le Fèvre7, M. Levi1, P. Limon4, H.
Lin4, E. Linder1, S. Loken1, W. Lorenzon10, R.
Malina7, J. Marriner4, P. Marshall2, R. Massey17,
A. Mazure7, T. McKay10, S. McKee10, R. Miquel1,
N. Morgan6, E. Mörtsell3, N. Mostek13, S.
Mufson13, J. Musser13, P. Nugent1, H. Oluseyi1,
R. Pain5, N. Palaio1, D. Pankow9, J. Peoples4, S.
Perlmutter1, E. Prieto7, D. Rabinowitz6, A.
Refregier18, J. Rhodes15, N. Roe1, D. Rusin8, V.
Scarpine4, M. Schubnell10, G. Smadja19, R. M.
Smith15, G. Smoot9, J. Snyder6, A. Spadafora1, A.
Stebbins4, C. Stoughton4, A. Szymkowiak6, G.
Tarlé10, K. Taylor15, A. Tilquin12, A. Tomasch10,
D. Tucker4, D. Vincent5, H. von der Lippe1, J-P.
Walder1, G. Wang1, W. Wester4 1Lawrence
Berkeley National Laboratory 11Space Telescope
Science Institute 2Stanford Linear Accelerator
Center 12CPPM, CNRS-IN2P3, Marseille, France
3University of Stockholm 13Indiana University
4Fermi National Accelerator Laboratory 14American
Astronomical Society 5LPNHE, CNRS-IN2P3, Paris,
France 15California Institute of Technology
6Yale University 16Case Western Reserve
University 7LAM, CNRS-INSU, Marseille,
France 17Cambridge University 8University of
Pennsylvania 18CEA, Saclay, France 9University
of California at Berkeley 19IPNL, CNRS-IN2P3,
Villeurbanne, France 10University of Michigan
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SNAP Mission Goals

• Supernova Acceleration Probe (SNAP)
• Proposed mission to study dark energy and
  acceleration of the universe
• Goals
• 16(x2) month supernova program (Type Ia supernova
  (SNe) Hubble diagram) 7.5 square degrees, North
  and South ecliptic poles
• 1-year, 300-1000 square degree Wide-field weak
  gravitational lensing survey
• Optional 3-year 7000-10,000 square degree
  panoramic survey

• Scientific requirements
• 0.35-1.7µm band
• 20mas pointing control
• Measure SNe 4 magnitudes fainter than 26
  magnitude peak, out to z1.7
• S/N 301 at peak brightness
• Stray light lt10 foreground Zodiacal light level
  at dark ecliptic poles
• Stable PSF (lt1) for weak lensing shear
  measurements
• FOV vs- critical sampling ? Dithering strategy
  and focal length currently being optimized via
  simulation

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SNAP Reference Mission

• Spacecraft
• L2 orbit
• 20mas pointing control ? focal plane guiders (1/5
  ccd pixel)
• Ka band antenna (2.35Mbps, 3.4dB link margin)
• Telescope
• Three Mirror Anastigmat (Korsch, Lampton)
• 2m aperture, f/11 (130mas imaging at 1µm)
• Telescope design accommodates EFL in 15-30m range
  (currently 21.66)
• 1.4 square degree optical FOV (0.7 square degree
  instrumented)
• 0.9 Strehl at 1µm
• Room temperature optics (test and fly warm)
• Low CTE structure (carbon fiber)
• Instruments
• 36 LBNL CCDs (4kx4k, 10.5µm pixels, 0.35-1µm
  band)
• 36 HgCdTe (2kx2k, 18µm pixels, 0.9-1.7µm band)
• 3x6 integral field unit (IFU) spectrograph
  (Ealet, et. al.)
• Significant engineering work has gone into the
  design of the SNAP observatory

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Observatory configuration
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Integration Test

• The SNAP observatory was designed to be simple to
  test, calibrate, and verify
• Interface ring between payload and spacecraft
  (spacecraft bolts on)
• Direct insertion and removal of science package
  from telescope without telescope deintegration
• Semi-kinematic mount for focal plane assembly and
  spectrograph
• Comprehensive test plan developed

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Focal Plane

• 140K operating temperature
• Four fine guidance trackers
• 36 CCDs (6.5 arcmin)
• 36 HgCdTe detectors (6.5 arcmin)
• Six filter panels for photo-Z measurement

• Spectrograph light passes through fixed entrance
  port on focal plane
• Built in test equipment (BITE) illuminates
  mirrors on secondary spider for stability check.

C. Bebek
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Optical DesignMike Lampton

• Korsch AF-TMA configuration
• Prolate ellipsoid concave primary mirror (PM)
• Hyperbolic convex secondary mirror (SM)
• Flat folding mirror (FM) with central hole
• Prolate ellipsoid concave tertiary mirror
• 1µm 10mas 1/10 pixel (ccd)
• Telephoto advantage1/7
• Side-mounted detector (place near radiator for
  passive cooling)
• Huge diffraction-limited field (Ofov/Odiffraction
  blur3x109)
• Near-Hubble angular resolution at visible

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Wavefront Error Budget

• Flowdown from science requirements
• WFE tracked in OPD in the pupil plane
• Incoherence between aberrations assumed
• Error categories
• Figure manufacturing errors
• Mount mirror distortion caused by clamping glass
• Positioning residual alignment errors remaining
  after secondary correction
• Stability
• Thermal
• Longterm (dryout, creep)
• 1-G-release
• Effects of 1-G testing not eliminated by
  offloaders
• Metrology errors budgeted individually
• Marechal approximation used to cast pupil-plane
  errors on focal plane (or vice versa)
• All elements of this optical system fall well
  within existing state of the art

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Mechanical Tolerances

• Initial alignment to surveying levels
• Secondary mirror used for final alignment
• Interferometer (terrestrial)
• Starfield (on orbit)
• Allowable mechanical tolerances are determined as
  follows
• Generate WFE budget line items corresponding to
  misalignment
• Misalign/SM correct until corrected WFE
  approaches budget allocation
• Initial mechanical misalignment becomes tolerance
• Tolerances are well within capability of
  surveying equipment
• Leica TM5100A theodolite (3 units) 10µm linear,
  2.5µrad angular
• Leica Laser tracker 25µm linear (10ppm), µrad
  angular

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Reference design mirrorsR. Besuner
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1-G testing

• Full end to end test planned (avoid surprises)
• Vertical testing preferred
• Due to 1-g sag, testing to full Strehl
  requirements is not practical
• Reduced figure requirements during 1-g test
• 96 Offloaders on PM
• Prefer to not offload SM, FM, TM

PM sag without offloaders 770nm RMS
PM sag with 96 offloaders 8.8nm RMS
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SiC fold mirror

• Optical design of TMA requires tight packaging
  between FM and passive cold stop
• Because of central hole, FM experiences excessive
  1-g sag when constructed from ULE or Zerodur
• Silicon carbide (SiC), in an open-back
  configuration meets 1-g test figure requirements
• Closed back ULE designs meet gravity sag
  requirements for SM and TM, without offloaders

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Thermal effects on mirrors

• Bulk temperature changes considered
• Radius of curvature of powered mirrors changes
  with temperature
• Separate budgets developed for bulk temperature
  changes which may be corrected by motion of the
  SM, and instantaneous control requirements for
  the mirrors

• CTE/k ratios for ULE, Zerodur and SiC suggest
  comparable gradient performance

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Passive Cold Stop

• Reverse raytrace shown
• Passive Cold Stop (PCS) extended to waist
• Pixels see PCS mainly (see Table)
• PCS at 140K
• Zodi 0.3 counts/s

P. Jelinsky
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Flat-field lamps

• Desired
• Dome flat (diffuse light source) for interpixel
  variation quantification
• Source should be in pupil plane (no features
  imaged on focal plane)
• Source should be removed from view during
  observations
• Constraints
• Passive Cold Stop (PCS) extends to internal
  pupil, and is at 140K (cold mechanisms are to be
  avoided)
• Fold mirror blocks access from front of internal
  pupil
• Solution
• Illuminate ring of diffuse material within PCS
  (in pupil plane, external to pupil)
• S. Mufson (Indiana) characterizing sources
• Angular input histogram, integrated through
  multilayer filters show minimal throughput
  variation between starlight and ring of light

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Cassegrain Baffle Shutter

• Cassegrain stray light baffle effectively shadows
  inactive areas of focal plane.
• Shutter moved from internal pupil to Cassegrain
  focus
• Allows PCS to extend without interruption to
  internal iris
• Baffle is warm, but not visible to detector pixels

• Redundant limited angle torque motor drives
• Magnetic resolvers and feedback loop in design
• Shutters open/close in lt100ms

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Stray light

• L2 orbit removes bright Earth (main stray light
  source)
• Developing stray light budget
• ASAP model of observatory developed
• Visible and Critical surfaces PST analysis
• Mirror roughness
• Mirror particulate contamination
• Ghosting (filters)
• Baffle BRDF

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Conclusions

• The SNAP mission is designed to study the dark
  energy and alternate explanations for the
  accelerating universe
• Significant engineering effort has gone into
  design of the observatory in areas of
• Ease of alignment, calibration and test
• Strength, dynamics, thermal and optomechanical
  design
• Stray light analysis
• Comprehensive test plan for the SNAP telescope
  has been developed
• The SNAP observatory design is
• Mature
• low risk
• straightforward to calibrate
• The SNAP observatory can be built using todays
  technology