13.3 Mode = ACQ/PEAKXD
Config = COS/FUV

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.

1. The optimum cross-dispersion position of the science aperture on the detector is located by flashing the calibration lamp and measuring the mean cross-dispersion location of the calibration spectrum.
2. The target spectrum is recorded for the specified period of time, and its mean cross-dispersion position is also computed.
3. Then, HST is repositioned so that the target location coincides with the optimum cross-dispersion position for the target spectrum within the aperture.

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.

13.3.1 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.

13.3.2 Spectral Element

Enter a spectral element from Table 13.3 for the COS/FUV configuration. This will generally be the same as the one used for the subsequent TIME-TAG or ACCUM science observation.

13.3.3 Wavelength

Enter the value of the central wavelength in Angstroms. Table 13.3 gives the allowed values of the central wavelength for each grating.

13.3.4 Optional Parameters

SEGMENT
= BOTH (default)
= A (default for G140L, =1105 Å), B

This is the same as in ACQ/SEARCH Mode; see Section 13.2.4.

13.3.5 Number of Iterations

The Number_Of_Iterations must be 1.

13.3.6 Time Per Exposure

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 Mode exposure are given in the COS Instrument Handbook.

13.3.7 Special Requirements

The special requirement POSition TARGet <X-value>,<Y-value> is not permitted on ACQ/PEAKXD exposures.