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Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 8: Target Acquisitions > 8.5 ACQ/PEAKXD Acquisition Mode

8.5
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.
2.
3.
4.
5.
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.
We recommend a minimum S/N of 40 for all dispersed-light target acquisition exposures, except for FUV ACQ/PEAKXD exposures, which require only S/N = 25 in detector Segment A.
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 4.1.7, the COS FUV detector exhibits gain sag. Gain sag effects at the original lifetime position led to the need to move the location of the FUV spectrum to the second lifetime position on July 23, 2013. The second lifetime position is at a pristine area of the detector. However, as the gain continues to sag at lifetime position 2 we expect that the location of the FUV spectrum will need to be moved again, to a third lifetime position. This will probably happen in the Fall of 2014.
One consequence of the gain sag 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.

Cosmic Origins SpectrographInstrument Handbookfor Cycle 22 > Chapter 8: Target Acquisitions > 8.5 ACQ/PEAKXD Acquisition Mode

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