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.
ACQ/PEAKXD Mode should normally be preceded by an ACQ/SEARCH to ensure that the target is positioned in the aperture.
Information 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.
In ACQ/PEAKXD Mode, acquisition subarrays are limited to a single stripe. The subarray dimensions may also depend on the selected grating; see the COS Instrument Handbook for details.
The aperture used for the ACQ/PEAKXD 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:
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 one of the gratings from Table 13.3 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 13.3 gives the allowed values of the central wavelength for each grating.
Identifies the spectral stripe to use for the acquisition subarray. The shortest wavelengths of the spectrum are projected onto the stripe designated by SHORT, wavelengths in the middle of the range onto the stripe designated by MEDIUM, and the longest wavelengths onto the stripe designated by LONG. See the COS Instrument Handbook for the mapping between the selected spectral wavelength and the wavelength coverage in each stripe.
There are special restrictions with the G230L grating, which positions first-order light on the detector only for certain stripes. The MEDIUM stripe is required for the 2635 central wavelength, the SHORT stripe is required for the 3360 wavelength, and either SHORT or MEDIUM may be used for the 2950 and 3000 wavelengths. The LONG stripe may not be used with G230L at all.
Note: A value of DEF is normally the same as MEDIUM. The one exception is with the G230L grating and the 3360 wavelength, where only the SHORT stripe is allowed. The middle stripe (MEDIUM) is preferred because the flight software only stores the cross-dispersion offset between the location of the calibration spectrum and the optimal location of the science spectrum for that stripe. The true offset varies by +/- 2.5 pixels between stripes, so there will be a slight error in the calculated "optimal" position of the target spectrum if SHORT or LONG is used. This might cause a small drop in flux, but is not expected to affect the success of target acquisition.
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.
The procedures to determine the exposure time for the ACQ/PEAKXD exposure are given in the COS Instrument Handbook.
The special requirement POSition TARGet <X-value>,<Y-value> is not permitted for ACQ/PEAKXD exposures.
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