Pointspread function stability of the SNAP telescope

1
Point-spread function stability of the SNAP
telescope
M.J. Sholl R. Besuner P. Jelinsky M.L.
Lampton http//snap.lbl.gov astro-ph/0405232
2
How to measure dark energy?

• Standard candles
• a standard candle has a calibrated luminosity
  (Type Ia supernova, no H or He, strong Si,
  uniform 1062 photons)
• Inverse square law flux luminosity/distance2
• a measured flux gives the distance hence the
  lookback time
• the lights redshift gives the universes scale
  at lookback time

• Weak gravitational lensing
• matter has mass, hence gravity
• over cosmic history, matter aggregates through
  gravitation
• mass aggregation w.r.t. redshift is determined by
  expansion history
• mapping lensing vs. redshift can constrain
  expansion models
• http//www.cita.utoronto.ca/hoekstra/lensing.html

3
Abell 2218 (HST)
4
WL PSF stability metrics

• Weighted moments
• 
  Weighting
  factor
• Whisker (arcsec) Ellipticity
  Anisotropy
• 
  
• Whisker vector subtraction Effective
  anisotropy

5
Weak lensing metrics and proposed requirements

• WL measurements seek very small changes in a
  large number of images
• Telescope stability is of crucial importance
• Final requirements are yet to be determined for
  the WL campaign
• The following are initial estimates of the
  stability necessary for WL
• Thanks to Prof. Gary Bernstein (U. Penn)
• One desires to measure ellipticity to 0.1
• Divide by 3 for margin 0.03
• Multiply by nominal galaxy size2
• 100milli-arcsecsqrt(1040.0003)?2 milli-arcsec
• Recall ellipticity is a second moment
• As-defined whisker is a linear quantity
• Change in whisker length should be lt2
  milli-arcsec (goal)
• Notes
• This is a goal, not a final requirement!
• This value may be exceeded if changes are
  preticable or foreground stars available

6
SNAP Observatory Configuration
Significant engineering work has gone into the
design of the SNAP observatory
7
Ellipticity variation across the SNAP focal plane
(pristine telescope)

• (Spots scaled 10,000x)
• Ellipticity is a second moment (discussed later)
• PSF is oriented radially near the center of the
  FOV
• Azimuthal orientation near outer extreme of FOV
• At telescope sweet spot, 0.65 off-axis,
  orientation changes from radial to azimuthal

8
Wavefront error (WFE) budget for supernova program

• Alignment drift tolerances extracted from
  observatory WFE budget
• Margin exists, and these tolerances could be
  relaxed for supernova program
• How does drift of the structure (optics) within
  these tolerance bands affect WL?

9
SN telescope drift budget

• As expected, stability requirements for a WL
  survery are more stringent than those of the SN
  program
• Next step examine predicted stability

10
WL effects of SN budgeted SM despace

• Whisker distribution is axisymmetric
• WL effects are small!

11
Thermal transients

• Earth-Sun L2 Lagrange point provides supreme
  thermal stability
• Orbit designed to avoid Earths shadow
• Numerous thermal loading cases analyzed by P.
  Jelinsky, most significant of which are
• Seasonal 1/R2 variations in solar flux (1298W/m2
  to 1384W/m2)
• Daily slews to point body-mounted Ka band antenna
  toward Earth
• (No mechanisms and associated reliability issues
  with fixed antenna)
• Cartesian tiling scheme requires 90 rotation
  every three months
• Thermo-mechanical-optical analysis tool developed
  (R. Besuner, P. Jelinsky)

Distortion exaggerated 2.5x105
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Predicted thermal transients

• Whisker change allowable lt2 milli-arcsec for all
  thermal transients
• Predicted thermal varations in observatory
  geometry fall well within stability constraints
  of WL survey

13
HST composite structure dryout
SM
PM
http//www.stsci.edu/hst/observatory/focus/focushi
story.html
14
HST composite structure dryout
http//www.stsci.edu/hst/observatory/focus/focushi
story.html
15
HST vs- SNAP composites comparison

• HSTs all-composite metering structure was
  revolutionary
• Comparison between HST SNAP
• PM-SM despace on SNAP2m, while HST despace was
  4m
• PM-SM system on SNAP TMA is f/1 (more
  sensitivity)
• Composite structure technology has advanced
  significantly since HST design (Blair, et al,
  2002)
• Advances in the state of the art of graphite
  fiber cyanate ester laminates have reduced the
  moisture retention level to lt0.2 by weight
• Coefficient of moisture expansion (CME) lt200
  PPM/H2O
• Composite 1/e timescales shortened to 1 month if
  one uses 2mm thick layups
• Mission timeline
• Launch to L3 months commissioning and
  composites dryout
• L3 to L21 months photometric survey and deep
  WL survey
• Dryout rate is predictable, and whisker variation
  axisymmetric
• Foreground stars may be used to map
  telescope-induced ellipticity
• L21 months on 1000 square wide-field WL survey
• Composite dryout largely complete shrinkage
  effects are small (lt2 milli-arcsec whisker change
  per day)
• Foreground stars still available to map telescope
  effects

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Conclusions

• A combined thermal-mechanical-optical tool was
  developed to analyze the suitability of the
  as-designed SNAP observatory.
• Worst-case thermal variations include seasonal
  flux changes, daily pointing of the antenna
  toward Earth, and 90 roll maneuvers were
  analyzed, and found to fall with WL survey
  requirements
• Composite structural shrinkage cannot be ignored
  (possible 75µm shrinkage over life of mission).
• L3 to L21 months (photometric survey and deep
  WL) shrinkage is significant, but predictable,
  monotonic and axi-symmetric
• L21 months on (WL survey) Whisker changes lt2
  milli-arcsec per 24 hours.
• WL PSF stability requirements are more stringent
  than those demanded by the SN program, but all
  analyses performed show that the as-designed SNAP
  mission will have unprecedented stability, and
  meets performance requirements for both SN
  photometry and WL surveys.