sequence is used to improve centering in the cross-dispersion (XD) direction. We recommend a minimum S/N of 40 for all dispersed-light target acquisition exposures.
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. 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.1
: For extended sources observed with the NUV detector, light from the three spectral stripes will overlap. In this case ACQ/PEAKXD
acquisitions will fail and should be avoided.
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, the LP1-LP3 FUV ACQ/PEAKXD
algorithm was modified to use only Segment A data.
At LP4 (beginning in Cycle 25), the FUV ACQ/PEAKXD
sequence differs from the algorithm at previous LP. It uses only the total number of counts and is not affected by Y walk. For FUV ACQ/PEAKXD
acquisitions, either segment A or B may be used when available, but use of SEGMENT=DEFAULT
is recommended. All G140L ACQ/PEAKXD
acquisitions and those with G130M cenwaves restricted by the COS 2025 rules use only segment A.
The LP4 algorithm 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
: FUV ACQ/PEAKXD
s on extended sources are possible, but they require a NUM-POS and STEP-SIZE
combination tuned to the extent of the source.