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.
: 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 4.1.7
, 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, 2012. Continued operations at lifetime position 2 have resulted in the gradual sag of the gain at this new position. It is expected that COS FUV operations will move to a third lifetime position by the spring of 2015.
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.