# Presentation: 400192

## Point-spread function stability of the SNAP telescope

**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**

## 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 light’s redshift gives the universe’s 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**

**Notes:**
Add timeline to this slide 3 months comissioning, 18 months supernova program and 15deg^2 deep WL survey, then, 1000deg/year WL survey

## Abell 2218 (HST)

## WL PSF stability metrics

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

## 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 size****2****: **
**100milli-arcsec=sqrt(10****4*****0.0003)****→****2 milli-arcsec**
**Recall: ellipticity is a second moment**
**As-defined whisker is a linear quantity**
**Change in whisker length should be ~<2 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**

## SNAP Observatory Configuration

**Significant engineering work has gone into the design of the SNAP observatory**

## 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**

**Notes:**
20 mas mean whisker, 80 mas max whisker, 2x2 visible pixels, ~1 infrared pixel

## 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?**

## 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**

**Notes:**
A widefield TMA has greater absolute whisker and effective anisotropy than a narrowfield imager
Diffraction is not considered in this analysis, so effective anisotropy numbers are larger than will actually be realized (whisker effects are expected to be small)
Nearfield stars are essentially point sources, and may be used to quantify telescope-induced shear (ellipticity or whisker)

## WL effects of SN budgeted SM despace

**WL effects of SN budgeted SM despace**

**Whisker distribution is axisymmetric**
**WL effects are small!**

## Thermal transients

**Earth-Sun L2 Lagrange point provides supreme thermal stability**
**Orbit designed to avoid Earth’s shadow**
**Numerous thermal loading cases analyzed by P. Jelinsky, most significant of which are:**
**Seasonal 1/R****2**** variations in solar flux (1298W/m****2**** to 1384W/m****2****)**
**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 2.5x10****5**

## Predicted thermal transients

**Whisker change allowable <2 milli-arcsec for all thermal transients**
**Predicted thermal varations in observatory geometry fall well within stability constraints of WL survey**

| Event | Note | Single Leg Δl (nm) | Max. ΔS
   (milli arcsec) | Max Δχeff change (%) |
| --- | --- | --- | --- | --- |
| 90o roll (every 3 months) | View to sun changes | 15.0 | 0.181 | 0.010 |
| Communication slew | View to sun changes | 10.0 | 0.120 | 0.007 |
| Seasonal 1/R2 variations in solar flux | 1298W/m2 minimum
1384W/m2 maximum | 8.0 | 0.096 | 0.005 |

## HST composite structure dryout

- _[http://](http://www.stsci.edu/hst/observatory/focus/focushistory.html)__[www.stsci.edu/hst/observatory/focus/focushistory.html](http://www.stsci.edu/hst/observatory/focus/focushistory.html)_

- SM

- PM

**Notes:**
Hubble launched in 1990
3-year timescale

## HST composite structure dryout

- _[http://](http://www.stsci.edu/hst/observatory/focus/focushistory.html)__[www.stsci.edu/hst/observatory/focus/focushistory.html](http://www.stsci.edu/hst/observatory/focus/focushistory.html)_

## HST –vs- SNAP composites comparison

**HST’s all-composite metering structure was revolutionary**
**Comparison between HST & SNAP**
**PM-SM despace on SNAP=2m, 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 <0.2% by weight**
**Coefficient of moisture expansion (CME) <200 PPM/%H****2****O**
**Composite 1/e timescales shortened to 1 month if one uses ****≤2mm thick layups**
**Mission timeline**
**Launch to L+3 months: commissioning and composites dryout**
**L+3 to L+21 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**
**L+21 months on: 1000 square wide-field WL survey **
**Composite dryout largely complete: shrinkage effects are small (<2 milli-arcsec whisker change per day)**
**Foreground stars still available to map telescope effects**

**Notes:**
150nm per WL survey
Timescale of <50 days

## 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 75****μ****m shrinkage over life of mission).**
**L+3 to L+21 months (photometric survey and deep WL): shrinkage is significant, but predictable, monotonic and axi-symmetric**
**L+21 months on (WL survey): Whisker changes <2 milli-arcsec per 24 hours.**
**WL PSF stability requirements are more stringent than those demanded by the SN program, but a****ll analyses performed show that the as-designed SNAP mission will have unprecedented stability, and meets performance requirements for both SN photometry and WL surveys.**