8.5	ACQ/PEAKXD Acquisition Mode

8.5.1 Description

8.5.2 Effects of FUV Detector Y Walk

8.5.1 Description

An ACQ/PEAKXD sequence is used to improve centering in the cross-dispersion (XD) direction. The steps executed in an ACQ/PEAKXD sequence are

1.	A short exposure of the Pt-Ne wavelength calibration lamp through the WCA aperture is obtained. The spectrum is collapsed along the dispersion direction, its centroid is calculated, and the center of the target aperture is computed.

2.	A target spectrum is recorded for the user-specified time using a sub-array tailored to each grating and central wavelength (excluding edge effects and airglow lines). The spectrum is collapsed along the dispersion direction.

3.	The target XD location is assumed to be the median (NUV) or mean (FUV) of the collapsed spectrum.

4.	The slew required to move the target spectrum in the XD direction to the center of the aperture is computed.

5.	The telescope is slewed by the calculated offset to center the target in the XD direction.

The user must specify the aperture (PSA or BOA, typically the same as for the science exposure), the grating and central wavelength, and the exposure time. The use of MIRRORA or MIRRORB is not allowed. For NUV ACQ/PEAKXD acquisitions, the stripe (SHORT, MEDIUM, or LONG, corresponding to stripes A, B, or C) to be used in the computation may be specified; however, the default stripe B (MEDIUM) is recommended, as it achieves the best centering. For FUV ACQ/PEAKXD acquisitions, only data from segment A are used to compute the centroid; data from segment B are ignored. For G140L, segment B is turned off by default.

Note: Because light from the three spectral stripes will overlap, PEAKXD acquisitions will fail—and should thus be avoided—for extended sources observed with the NUV detector.

8.5.2 Effects of FUV Detector Y Walk

As described in Section 4.1.6, the COS FUV detector exhibits gain sag. This has become significant at the original lifetime position of the spectrum on the detector. On July 23, 2012 the location of the spectrum on the detector was moved to the second lifetime position, which has not exhibited gain sag. Over time, as the total number of incident photons on this new position grows, gain sag will appear at this new position.

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.

The FUV Segment B is more affected by Y walk and gain sag than Segment A, mainly due to the intense geocoronal Lyman α emission that hits the detector during G130M observations. To combat the effects of Y walk we have modified the FUV ACQ/PEAKXD algorithm to only use Segment A data as this has been shown to give better cross-dispersion centering.