sequence is used to improve centering in the cross-dispersion (XD) direction. The steps executed in an ACQ/PEAKXD
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
is not allowed. For NUV ACQ/PEAKXD
acquisitions, the stripe (SHORT
, 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.
: 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.
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.
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.