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Hubble Space Telescope
Changes to PEAKXD Acquisitions with COS

As described in a recent STAN*, the COS FUV detector is beginning to exhibit gain sag. One consequence of this effect is the mis-registration of photon events in the cross-dispersion (XD) or Y direction, commonly referred to as Y walk. While Y walk does not adversely affect science data, it can reduce the accuracy of target acquisitions obtained in dispersed light. If the target is centered in the aperture, but the Y walk shifts its spectrum in the XD direction, then the ACQ/PEAKXD algorithm will miscalculate its centroid and move the target away from the aperture center.

We have considered several schemes for modifying the ACQ/PEAKXD algorithm to correct for this effect. The simplest turns out to be the following:

This algorithm has two properties that make it particularly appealing:

For Cycle 19, version 19.2 of the COS ETC will compute the S/N for segments A and B separately for dispersed-light target acquisitions. In the mean time, here is a simple algorithm for estimating the S/N of a PEAKXD acquisition:

* http://www.stsci.edu/hst/cos/documents/newsletters/cos_stis_newsletters/stan1012.html