sequence is used to improve centering in the cross-dispersion (XD) direction. The method for centering depends on the Lifetime Position (LP) for FUV target acquisitions. Cycle 25 sequences will execute at LP4.
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 for most settings, 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.1
For LP4 (Cycle 25) the ACQ/PEAKXD
sequence works much like ACQ/SEARCH
except that, instead of a spiral, the spacecraft is moved linearly along the XD axis between exposures. An array containing the total counts at each dwell point is constructed. Its centroid is computed, and the telescope is moved to center the target in the aperture in the XD direction. The user must specify the aperture, grating, central wavelength, and the exposure time at each dwell point. The use of MIRRORA
is not allowed. The number of steps, called NUM-POS
, may be 3
, or 9
. The STEP-SIZE
is given in arcseconds. There are three options for the centering algorithm, CENTER=FLUX-WT, FLUX-WT-FLR
, and BRIGHTEST
, and they work just as described in Section 8.3
. For most applications, we recommend the use of NUM-POS=3
, and CENTER=FLUX-WT
, as this combination (which is the default) is the fastest pattern that centers targets to within the requirements. Observers who wish to use NUM-POS=5
are advised to use STEP-SIZE=0.9
. The special parameter CENTER=DEFAULT
, or 9
. For FUV ACQ/PEAKXD
acquisitions, either segment A or B may be used, but use of the SEGMENT=DEFAULT
(both, except for G140L) is recommended. All G140L ACQ/PEAKXD
acquisitions use only segment A data.
: Because light from the three spectral stripes will overlap, ACQ/PEAKXD
acquisitions will fail and should thus be avoided for extended sources observed with the NUV detector.
: Cycle 25 FUV ACQ/PEAKXD
s on extended sources are possible, but require a NUM-POS and STEP-SIZE
combination tuned to the extent of the source.
As described in Section 4.1.7
, the COS FUV detector exhibits gain sag. Gain sag effects are alleviated by periodic lifetime moves, which place the FUV spectrum on unsagged regions of the detector (see Section 2.1
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 LP1-LP3 FUV ACQ/PEAKXD
algorithm to only use Segment A data as this has been shown to give better cross-dispersion centering. The LP4 (Cycle 25) ACQ/PEAKXD
algorithm only uses the total number of counts so is not affected by Y walk. By default, the LP4 (Cycle 25) ACQ/PEAKXD
uses both detector segments.