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 the HST is repositioned to maximize (peak up) the light intensity through the COS aperture.
The visual magnitude from the Target List, along with the exposure time, will be used to verify that the integrated target flux through the selected aperture is appropriate for target acquisition. Details on the target acquisition sequence and limiting magnitudes for the
PSA and
BOA can be found in the COS Instrument Handbook.
An ACQ/PEAKD 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
ACQ/SEARCH and
ACQ/PEAKXD exposures to ensure that the target is positioned in the aperture and well centered in the cross-dispersion direction.
There are no appreciably bright airglow lines in the NUV portion of the spectrum, but subarrays are still used to exclude portions of the detector which are not illuminated by the input spectrum or image; this improves accuracy by reducing the contribution from background counts. The subarray dimensions depend on the selected grating; see the COS Instrument Handbook for details. For this Mode, there is no need to specify the spectral stripe as in
ACQ/PEAKXD Mode because the subarrays include counts from all stripes to maximize the photon counting statistics.
The aperture used for the ACQ/PEAKD Mode exposure will generally be the same as the aperture used for the subsequent
TIME-TAG or
ACCUM science exposures. The following apertures are allowed:
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PSA Primary Science Aperture
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BOA Bright Object Aperture
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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 one of the gratings from Table 12.3 for the COS/NUV configuration. This will generally be the same as the one used for the subsequent
TIME-TAG or
ACCUM science observation. This Mode only uses dispersed light, so the mirror may not be used.
Enter the value of the central wavelength in Angstroms. Table 12.3 gives the allowed values of the central wavelength for each grating.
NUM-POS= 3, 5 (default), 7, 9 (points)
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. When the flux-weighted centroid centering method with the "flooring" option is used (the default), increasing the number of positions will improve the accuracy, but simulations show that value of a 5 does almost as well as 9. For more details consult the
COS Instrument Handbook.
This is a required parameter that specifies the size in arcseconds of each step in the linear scan. The optimal step size may depend on the number of positions and the centering method; see the COS Instrument Handbook for details. Note that both the
PSA and
BOA are 2.5 arcsec in diameter. Assuming the target is already positioned in the aperture, a scan that greatly exceeds the aperture diameter is not likely to be useful.
CENTER= DEF (default), FLUX-WT, FLUX-WT-FLR, BRIGHTEST
Specifies the method used for locating the target within the search pattern. Two methods are available: (1) a flux-weighted centroiding algorithm (
FLUX-WT or
FLUX-WT-FLR), or (2) a return to the brightest dwell point (
BRIGHTEST).
If FLUX-WT-FLR is used, the minimum number of counts measured from any of the dwell points 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.
See the COS Instrument Handbook for recommendations on when to use these different methods.
The Number_Of_Iterations must be 1 in this Mode.
Enter the total time of data collection 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.
