8.6 ACQ PEAKD Acquisition Mode

ACQ/PEAKD exposures are used to improve centering in the along-dispersion (AD) direction after an ACQ/PEAKXD. ACQ/PEAKD works much like ACQ/SEARCH except that, instead of a spiral, the spacecraft is moved linearly along the AD 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 AD 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, here 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=5, STEP-SIZE=0.9, and CENTER=FLUX-WT-FLR, as this combination is the least sensitive to high or variable background rates and covers a large area on the sky. This combination is the only PEAKD configuration that is routinely tested. Observers who wish to use NUM-POS=3 are advised to use STEP-SIZE=1.3 and CENTER=FLUX-WT. 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/PEAKD acquisitions, either Segment A or B may be used when available, but use of SEGMENT=DEFAULT is recommended. All G140L ACQ/PEAKD acquisitions and those with G130M cenwaves restricted by the COS 2025 rules use only Segment A data.