Post on 30-Dec-2015
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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.