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WFC3 UVIS Shutter Blade

Kailash C. SahuSTScI

STScI Calibration WorkshopAugust 13, 2014

With thanks toSylvia Baggett and Megan Sosey

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WFC3 UVIS Shutter Blade

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

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WFC3 UVIS Shutter Blade

WFC3 UVIS shutter mechanism. The blade rotates half a turn for each exposure.

WFC3 UVIS uses a shutter blade with two sides, A and B.

Blade sides A and B are used alternately in consecutive exposures.

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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.

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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.

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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).

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Mass of Proxima Centauri through Astrometric Microlensing

0.5s for Proxima, ~100 sec for source. PSFs critical for the science.

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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.

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Exposure Time Distributions

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Exposure Time Distributions

Science exposures with Texp < 10 sec: 11% Texp < 30 sec: 21%

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Possible solution

• The fraction of short exposures is not 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 short exposures would result in improved accuracy for the science results.

• So it was decided to add an option to choose BLADE=A.

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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.

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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.