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HST Cycle 25 Phase II Proposal Instructions > Chapter 12: Cosmic OriginsSpectrograph (COS) > 12.7 Mode = ACQ/PEAKXDConfig = COS/NUV

12.7
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.
2.
3.
If the target coordinates are not accurate to 0.4", an ACQ/PEAKXD should 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.
12.7.1 Aperture or FOV
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:
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.
12.7.2 Spectral Element
Enter one of the gratings from Table 12.5 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.
12.7.3 Wavelength
Enter the value of the central wavelength in Angstroms. Table 12.5 gives the allowed values of the central wavelength for each grating. Due to increased NUV detector background and mechanism position uncertainties, certain NUV central wavelength settings have proven to be unreliable in the NUV ACQ/PEAKXD target acquisition phase. The NUV central wavelengths settings listed in bold in Table 12.5, COS Spectral Elements and Central Wavelengths have been proven reliable for ACQ/PEAKXD using STRIPE=DEF. NUV ACQ/PEAKXD exposures should use one of these settings. See the COS Instrument Handbook for further details. Users who wish to use other central wavelengths, or non-default STRIPE settings, should contact the COS team via the Help Desk.
12.7.4 Optional Parameters
STRIPE
= DEF (default), SHORT, MEDIUM, LONG
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. To determine the mapping between the selected spectral wavelength and the wavelength coverage in each stripe, please apply the corrections noted in the March 2015 STAN article to the information tabulated in the COS Instrument Handbook.
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. STRIPE=DEF 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 the DEF 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.
12.7.5 Number of Iterations
The Number_Of_Iterations must be 1 in this Mode.
12.7.6 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 exposure are given in the COS Instrument Handbook.
12.7.7 Special Requirements
The special requirement "POSition TARGet <X-value>,<Y-value>" is not permitted for ACQ/PEAKXD exposures.

HST Cycle 25 Phase II Proposal Instructions > Chapter 12: Cosmic OriginsSpectrograph (COS) > 12.7 Mode = ACQ/PEAKXDConfig = COS/NUV

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