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/NUV configuration. If a grating is selected, this will generally be the same as the one used for the subsequent
TIME-TAG or
ACCUM science observation. If the
MIRRORA or
MIRRORB spectral element is selected, the search for the target will be done in undispersed light. Use of the
MIRRORB spectral element (possibly in conjunction with the
BOA aperture) will attenuate the flux and allow undispersed-light acquisitions to be done on targets that would otherwise trigger bright object checks.
Enter the value of the central wavelength in Angstroms. Table 12.3 gives the allowed values of the central wavelength for each grating.
Note: No wavelength should be specified if
MIRRORA or
MIRRORB is the spectral element.
Note: 2x2 dispersed light searches with the NUV channel should be avoided in most cases, as simulations show significantly degraded accuracy; a 2x2 search can even worsen the alignment of a target that is initially well centered. For more details on target acquisition strategies, please consult the
COS Instrument Handbook.
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.
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. This drastically reduces the centering accuracy for
SCAN-SIZE=2; therefore
FLUX-WT-FLR is not allowed if
SCAN-SIZE=2 (
FLUX-WT is recommended).
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 exposure time will be repeated at every dwell point in the search.