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. 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, but use of the
SEGMENT=DEFAULT (both, except for G140L) is recommended.
