WFC3 UVIS Shutter Vibration-Induced Image Blur

During image quality testing in the third and final thermal-vacuum test of the integrated WFC3 instrument, an anomaly was discovered whereby the width and peak signal fraction of short exposure PSF images were observed to modulate for alternate exposures in a series. This behavior was traced to vibration of the UVIS shutter when its servo-control loop is enabled, which is believed to induce small (mas level) pointing jitter in the optics, most likely the pick-off mirror and the corrector mechanism (M1) mirror. We present model computations of the effects of this vibration on several PSF parameters for a range of exposure times, wavelengths and (currently uncertain) degree of severity of the vibration expected on orbit. Only very short (<10s) exposures are likely to be significantly affected; this may be partially mitigated by obtaining such exposures using the better-performing side of the shutter.