This mode invokes a procedure in the COS onboard flight software that searches for the target and locates it in the selected COS aperture for subsequent science exposures. A series of exposures are taken at different HST pointings in a spiral pattern from the initial pointing, and then the HST is repositioned to maximize the light intensity through the COS aperture. This mode is required for targets whose coordinates are not known to at least 0.4". Most COS observation sequences that use
ACQ/SEARCH Mode will also need to use other acquisition exposures (
ACQ/IMAGE,
ACQ/PEAKD, and/or
ACQ/PEAKXD) to achieve proper centering of the target in the science 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 apertures can be found in the
COS Instrument Handbook.
This method uses dispersed light from the object to be observed. Subarrays are used 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 target’s spectrum. The subarray dimensions depend on the selected grating; see the
COS Instrument Handbook for details.
The aperture used for the ACQ/SEARCH Mode exposure does not need to 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
|
Use of the BOA is recommended on 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. Target acquisition with the COS FUV detector is in dispersed light only.
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)
= B
Indicates which segment of the FUV detector to use for an acquisition. A value of
BOTH will activate both segments. Segments A and B record the long and short wavelength portions of the spectrum respectively. If
A is selected, only segment A of the detector will be activated for photon detection, and the spectrum will contain data from only that half of the detector. If
B is selected, only segment B of the detector will be activated and used to generate data. Use of a single segment may be warranted for sources that are too bright to observe safely over the entire detector. See the
COS Instrument Handbook for more information.
If you use the G140L grating at the 1105 Angstrom wavelength setting,
SEGMENT defaults to
A (see the following explanatory note). For all other central wavelength settings, the default is
BOTH.
Note: No subarrays are defined for segment
B when the
G140L grating is used, because the count rate after excluding areas contaminated by geocoronal airglow lines is expected to be too low to support target acquisition. Also, when
G140L is used at the 1105 Angstrom setting, the zeroth-order light from the target falls onto segment
B and becomes a bright object concern.
A required parameter which specifies the size of the square spiral search region in terms of the number of dwell points on a side (e.g. 2x2, 3x3, 4x4, or 5x5). A larger search pattern samples a larger area, but the time required scales as the square of
SCAN-SIZE. In some cases it may be advisable to use two
ACQ exposures, such as a 3x3 search followed by a 2x2 search; this strategy uses less time than a single 4x4 search. For more details on target acquisition strategies, consult the
COS Instrument Handbook.
STEP-SIZE= 1.767 (default); 0.2 - 2.0 (arcsec)
Specifies the size in arcseconds of each step in the spiral search. Both the PSA and
BOA are 2.5 arcsec in diameter. The recommended offset of 1.767 arcsec is the offset at which diagonal dwell points just overlap, and the maximum spacing that provides continuous coverage. Simulations show that offsets larger than 1.767 arcsec will introduce errors due to unsampled areas within the search pattern.
CENTER= FLUX-WT (default), FLUX-WT-FLR, BRIGHTEST
Specifies the method used for locating the target within the search pattern. Two methods are available: a flux-weighted centroiding algorithm (
FLUX-WT or
FLUX-WT-FLR), and 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 other dwell points before computing the centroid. The idea behind this threshold is to reduce the contribution of background counts. Note that this has the effect of eliminating the dwell point with the minimum counts from the centroid calculation, which can be a problem especially with small patterns. A value of
FLUX-WT-FLR is
not allowed with
SCAN-SIZE=2. See the
COS Instrument Handbook for recommendations on when to use these different methods.
The Number_Of_Iterations must be 1 in this Mode.
Enter the total time of data collection at each dwell point 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/SEARCH Mode exposure are given in the
COS Instrument Handbook. The
COS Exposure Time Calculator should be used to estimate the exposure time based on the nature of the source. The exposure time will be repeated at every dwell point in the search.
