COS Instrument Handbook for Cycle 25
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Cosmic Origins SpectrographInstrument Handbook for Cycle 25 > 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 method for centering depends on the Lifetime Position (LP) for FUV target acquisitions. Cycle 25 sequences will execute at LP4.
The steps executed for NUV and FUV LP1-LP3 ACQ/PEAKXD sequences 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 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 or MIRRORB is not allowed. The number of steps, called NUM-POS, may be 3, 5, 7, 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, STEP-SIZE=1.3, 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 and CENTER=FLUX-WT-FLR. The special parameter CENTER=DEFAULT sets CENTER=FLUX-WT if NUM-POS=3 and CENTER=FLUX-WT-FLR if NUM-POS=5, 7, 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.
We recommend a minimum S/N of 40 for all dispersed-light target acquisition exposures, except for FUV LP1-LP3 ACQ/PEAKXD exposures, which require only S/N = 25 in detector Segment A.
Note: 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.
Note: Cycle 25 FUV ACQ/PEAKXDs on extended sources are possible, but require a NUM-POS and STEP-SIZE combination tuned to the extent of the source.
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 are alleviated by periodic lifetime moves, which place the FUV spectrum on unsagged regions of the detector (see Section 2.1).
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 FUV LP1-LP3 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 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.

1
There are special restrictions with the G230L grating, which positions first-order light on the detector only for certain stripes. The MEDIUM stripe is required for the 2635 central wavelength, the SHORT stripe is required for the 3360 wavelength, and either SHORT or MEDIUM may be used for the 2950 and 3000 wavelengths. The LONG stripe may not be used with G230L at all.


Cosmic Origins SpectrographInstrument Handbook for Cycle 25 > Chapter 8: Target Acquisitions > 8.5 ACQ/PEAKXD Acquisition Mode

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