WFC3: Optical Alignment and Performance

Details WFC3: Optical Alignment and Performance WFC3: Optical Alignment and Performance

Jan 2010

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010aas...21546322d&link_type=abstract

American Astronomical Society, AAS Meeting #215, #463.22; Bulletin of the American Astronomical Society, Vol. 42, p.498

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Scientific paper

The UVIS and IR channels were aligned to the OTA (Optical Telescope Assembly) using exposures of the open cluster NGC 188. For each channel, an iterative series of focus sweeps and rasters of tip/tilt corrector mechanism settings were performed. Phase retrieval estimation of the low order aberration content and encircled energy measurements of many stars distributed over the field were used to determine the corrector adjustments required to optimize image quality. Assessment of PSF (point spread function) over the field in several filters uses phase retrieval, encircled energy, sharpness, and FWHM. Shallow and deep exposures of the isolated star GD153 are used to evaluate the PSF wings at 5 locations on each detector.
Preliminary analysis indicates that the observed metrics are a good match to the model predictions: encircled energy and azimuthally averaged PSFs agree well with model predictions except for the UV (F275W) exposures. The over-prediction of encircled energy from radius 0.2 to 1 arcsec in the UV is being investigated.
UVIS shutter vibrations smear the PSF in exposures shorter than 10 seconds, more so for one shutter position than for the other. We have investigated this effect in sequences of repeat exposures of GD153, which also clearly demonstrate the effect of OTA breathing on the PSF as the focus moves in and out of the optimal range. Photometric apertures should be chosen to allow for the effects of this vibration and breathing.

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