ACQ/PEAKD Mode invokes a procedure in the COS onboard flight software that takes a series of exposures at different HST pointings offset in the dispersion direction, and HST is repositioned to maximize (peak up) the light intensity through the COS aperture. Details on the target acquisition sequence and limiting magnitudes for the PSA and BOA can be found in the
COS Instrument Handbook.
ACQ/PEAKD Mode should always be preceded by an
ACQ/PEAKXD exposure. In addition, if the target coordinates are not accurate to 0.4", the
ACQ/PEAKD should be preceded by both an
ACQ/SEARCH and an
ACQ/PEAKXD exposure to ensure that the target is positioned in the aperture and well centered in the cross-dispersion direction.
The aperture used for the ACQ/PEAKD Mode exposure need not be the same as the aperture used for the subsequent
TIME-TAG or
ACCUM science exposures. The following apertures are allowed:
As with spectroscopic observations, use of the BOA is recommended on relatively bright targets to attenuate the flux and allow the acquisition to proceed without triggering bright object violations.
Enter a spectral element from Table 12.3 for the COS/FUV configuration. This will generally be the same as the one used for the subsequent
TIME-TAG or
ACCUM science observation. Target acquisition with the COS FUV detector is in dispersed light only.
Enter the value of the central wavelength in Angstroms. Table 12.3 gives the allowed values of the central wavelength for each grating.
Specifies the number of dwell positions (including the initial target position) in an ACQ/PEAKD scan. The time required for the scan scales with the number of positions. The default value of
5 is recommended in combination with the flux-weighted centroid centering method with the "flooring" option (
FLUX-WT-FLR). If you select
NUM-POS=3, we recommend using the flux-weighted centroid algorithm without the "flooring" option (
FLUX-WT). See the
COS Instrument Handbook for more details on these recommendations.
If FLUX-WT-FLR is used, the minimum number of counts measured from any of the dwell points (the "floor") will be subtracted from the number at other dwell points before computing the centroid. The idea behind this threshold is to reduce the contribution of background counts and extreme points at the edges of the scan.
Enter the total time of data collection at each dwell point as Time_Per_Exposure.
Time_Per_Exposure must be an integral multiple of 0.1 seconds. If it is not, its value will be rounded down to the next lower integral multiple of 0.1 sec, or set to 0.1 seconds if a smaller value is specified.
The procedures to determine the exposure time for the ACQ/PEAKD exposure are given in the
COS Instrument Handbook. The exposure time will be repeated at every dwell point in the search
