Phase II Roadmap for Cycle 25
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HST Cycle 25 Phase II Proposal Instructions > Chapter 12: Cosmic OriginsSpectrograph (COS) > 12.3 Mode = ACQ/PEAKXDConfig = COS/FUV

12.3
ACQ/PEAKXD mode invokes a procedure in the COS onboard flight software that places the target near an optimal position within the selected aperture in the cross-dispersion direction on the detector.
 
For Cycle 25, the ACQ/PEAKXD centering algorithm has been enhanced in the COS flight software to employ the ACQ/PEAKD algorithm, but executed in the cross-dispersion direction. Three optional parameters (NUM-POS, STEP-SIZE, and CENTER) have been added to ACQ/PEAKXD in APT to support this enhancement. A linear scan is performed in the cross-dispersion direction, with the number of dwell points corresponding to the value of NUM-POS. The optional parameters NUM-POS, STEP-SIZE and CENTER have default values that are appropriate for centering a point source with target coordinates accurate to 0.4".
If the target coordinates are not accurate to 0.4", ACQ/PEAKXD Mode should normally be preceded by an ACQ/SEARCH to ensure that the target is positioned in the aperture. Details on the target acquisition sequence and limiting magnitudes for the PSA and BOA can be found in the COS Instrument Handbook.
Subarrays are used during target acquisition to avoid undesirable detector features (e.g., hot spots) and 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.
12.3.1 Aperture or FOV
The aperture used for the ACQ/PEAKXD 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.
12.3.2 Spectral Element
Enter a spectral element from Table 12.5 for the COS/FUV configuration. In order to minimize overheads, this will generally be the same as the one used for the subsequent ACQ/PEAKD and TIME-TAG or ACCUM science observation.
12.3.3 Wavelength
Enter the value of the central wavelength in Angstroms. Table 12.5, COS Spectral Elements and Central Wavelengths gives the allowed values of the central wavelength for each grating.
12.3.4 Optional Parameters
SEGMENT
= BOTH (default except for these cases: G140L
, λ=1105 and G130M, λ=1300,1309, 1318,1327 )
= A (default and only allowed value for these cases: G140L, λ=1105 and G130M, λ=1300, 1309, 1318, 1327 )
= B
For a description see Section 12.2.4.
NUM-POS
3 (default), 5, 7, & 9 (points)
Specifies the number of dwell positions (including the initial target position) in a ACQ/PEAKXD scan. The time required for the scan scales with the number of positions. NUM-POS values of 3 should be used with the flux-weighted centroid algorithm without the "flooring" option (CENTER=FLUX-WT), while values greater than 3 should always use the "flooring" centering option (CENTER=FLUX-WT-FLR). In most cases, NUM-POS equals 3 (the default) is recommended for FUV PEAKXD acquisitions.
 
STEP-SIZE
= DEF (default), 0.01 - 2.0 (arcsec)
This parameter specifies the size in arcseconds (") of each step in the linear scan. This parameter defaults to 1.30" when NUM-POS=3, 1.0" when NUM-POS=5, 0.80" when NUM-POS=7, and 0.65" when NUM-POS=9. These default values are tuned to point-sources with initial centering accuracies of 0.4" or better. 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 each of the 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 defaults are to use the flux-weighted centroid algorithm without the "flooring" option (FLUX-WT) when NUM-POS=3, and with the "flooring" option (FLUX-WT-FLR) when NUM-POS>3.
12.3.5 Number of Iterations
The Number_Of_Iterations must be 1.
12.3.6 Time Per Exposure
Enter the 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. The procedures to determine the exposure time for the ACQ/PEAKXD exposure are given in the COS Instrument Handbook.
Beginning with Cycle 25, FUV ACQ/PEAKXD exposures will use both FUV segments, so the combined count rate for both segments should be used to calculate the exposure times. The exceptions to this are when SEGMENT = A or B is specified as an optional parameter, or when acquiring with the G140L grating. In these cases, only the single segment count rate should be considered.
12.3.7 Special Requirements
The special requirement "POSition TARGet <X-value>,<Y-value>" is not permitted on ACQ/PEAKXD exposures.

HST Cycle 25 Phase II Proposal Instructions > Chapter 12: Cosmic OriginsSpectrograph (COS) > 12.3 Mode = ACQ/PEAKXDConfig = COS/FUV

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