WFC3 UVIS Shutter Blade Kailash C. Sahu STScI STScI Calibration Workshop August 13, 2014

1. WFC3 UVIS Shutter Blade Kailash C. Sahu STScI STScI Calibration Workshop August 13, 2014 With thanks to Sylvia Baggett and Megan Sosey

2. OUTLINE • WFC3 Shutter Blade : Introduction • PSF Variation in Short Exposures due to Shutter Blade • Science Impact • Previous Strategy • New Strategy: BLADE=A for Better Image Quality • Summary WFC3 UVIS Shutter Blade

3. WFC3 UVIS Shutter Blade WFC3 UVIS uses a shutter blade with two sides, A and B. Blade sides A and B are used alternately in consecutive exposures. WFC3 UVIS shutter mechanism. The blade rotates half a turn for each exposure.

4. PSF Variation in Short Exposuresdue to Shutter Blade Texp ~ 0.5 sec PSFs in short-exposures taken with side B show larger FWHM and lower sharpness compared to those taken with side A, due to shutter vibrations. The PSF width is larger by ~10% for the shortest (0.5 sec) exposures.

5. PSF Variation in Short Exposures The difference decreases as the exposure time increases. This difference is: -- ~ 1% for exposure time of 10 seconds, and -- negligible for exposure times of 30 seconds or larger.

6. Science Impact • Which shutter will be used is unknown a priori. • The PSF variation affects science programs with Texp < 30sec, where the shape and stability of the PSF is important. Some Examples: • Mass determination of nearby lenses (WDs and M stars) through Astrometric Microlensing, where the exposure times are short for the lens and long for the source (e.g. Sahu et al.). • Resolving Disks and Jets around stars (e.g. Bochanski, HST-GO-12208) • Several programs for finding SNe. • Search for missing low-mass companions of massive stars (e.g. Nancy Evans, HST-GO-12215) • Orbital evolution of Uranus moons (short exp for psf, longer exp for the moons).

7. Mass of Proxima Centauri through AstrometricMicrolensing 0.5s for Proxima, ~100 sec for source. PSFs critical for the science.

8. Previous Strategy • The usual trick to avoid PSF variation is to take 2 consecutive exposures in the same filter, so that one will be with the better shutter. • But this is very inefficient. The overhead to select the shutter is only ~2 sec. But the effective overhead in repeating the short-exposures is large since in most proposals with short exposures, the number of exposures is large, resulting in large overheads due to extra buffer dumps. • The measurement accuracy, which are often crucial for the science, can be improved if the short exposures can be taken by the better shutter.

9. Exposure Time Distributions

10. Exposure Time Distributions Science exposures with Texp < 10 sec: 11% Texp < 30 sec: 21%

11. Possible solution • The fraction of short exposures isnot large enough to cause shutter life-time issues. • In many cases, the PSF shape is critical for the science. • The overhead is small (<< double exposures). • An option to choose the shutter for shortexposures would result in improved accuracy for the science results. • So it was decided to add an option to choose BLADE=A.

12. Implementation of BLADE=A option • Option to use BLADE=A was implemented, and the first science observations with commanded BLADE=A was taken in Oct, 2013. • A calibration proposal (13088) was also used to test this capability. • The PSFs of the affected short exposures are as expected from blade A. No other anomalous behavior was found. • Option to use BLADE=A is now available through APT. The nearby WD+MS star pair, Stein 2051A+B, as observed with commanded blade A. The PSF shape is nominal.

13. SUMMARY • There are several GO programs with short exposures where it is critical to the science goals to have a sharp and stable PSF. • Such programs may now be able to make short exposures with less vibration using the exposure-level option BLADE=A in APT. • Since this option causes additional movement of the shutter mechanism, its use will be allowed only as an available mode when scientifically justified.