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. This procedure consists of several steps.
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 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/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:
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. In order to minimize overheads, this will generally be the same as the one used for the subsequent
TIME-TAG or
ACCUM science observation.
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)
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.
