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Space Telescope Imaging Spectrograph

ACTIVITY TITLE: STIS External Focus Check



DESCRIPTION: Check STIS image quality using two tests. In the first test, use a dispersed light ACQ/PEAK with the G230LB grating to dither the 0.1x0.09 aperture across a point source to sample the PSF. Relative counts at different wavelengths give a measure of the PSF shape at different wavelengths. Also take a comparable ACCUM spectrum with the G230LB and a large aperture to provide a baseline flux for estimating the relative throughput of the 0.1x0.09 aperture. In the second test, image a point source onto the CCD through the F28x50OII filter. Compare the resulting PSF shape with the pre-repair PSF. Sufficient S/N in the O II image should be obtained to allow the use of phase retrieval techniques to estimate the focus position.

Sufficient point source targets should be selected to allow observations to be done any time during the BEA period. The default plan will be to use the targets selected for this purpose during SMOV3B, although different targets could be substituted. See proposal 8958 for details.


DEPENDENCIES: Should follow as soon as possible after the STIS to FGS Alignment Check STIS-09, but this program need not wait for downlink or analysis of that data.

DURATION: 3 Orbits (external). Note that using either of the two brightest targets defined for proposal 8958 will require only one orbit, but they are not always available in BEA.


ANALYSES & EXPECTED RESULTS: The variation of the count rate with the offset position during the peakup exposures will be used as a measure of the PSF shape. The throughput vs. wavelength will be checked by comparing the count rate through the small 0.1x0.09 and large (52x2 or 50CCD) apertures. The O II image PSF will be examined and phase retrieval techniques will be used to estimate the STIS focus position. If there is evidence for a significant misalignment of STIS with respect to the HST optics, then one or more visits of the contingency program STIS-12, “Corrector and Focus Alignment” would be triggered. Requires support of TEL branch personnel to analyze some data.


AUTHOR/telephone/email: Charles Proffitt/410-338-4938/

DATE: September 21, 2007