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.
Subarrays are used during target acquisition to avoid geocoronal airglow lines, which could bias the target acquisition calculations, as well as portions of the detector which are not illuminated by the input spectrum. The subarray dimensions depend on the selected grating and central wavelength; see the
COS Instrument Handbook for details.
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:
|
•
|
PSA Primary Science Aperture
|
|
•
|
BOA Bright Object Aperture
|
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.
SEGMENT= BOTH (default except for G140L
, λ=1105
е)
= A (default and only allowed value for G140L,
λ=1105
е)
= B
)
NUM-POS= 3 (default), 5, 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. The default value of 3 is recommended when the flux-weighted centroid centering method is used without the "flooring" option. With the other centering options, more positions will improve the accuracy. For more details consult the
COS Instrument Handbook.
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 (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.
The default is to use the flux-weighted centroid algorithm without the "flooring" option when
NUM-POS=3, and with the "flooring" option when
NUM-POS > 3. See the
COS Instrument Handbook for more details on these recommendations.
The Number_Of_Iterations must be 1 in this Mode.
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
