MIRROR is the only valid spectral element for this Mode.
See Table 9.1 for a listing by operating mode of all Optional Parameters. Here are descriptions for those Optional Parameters for
ACQ mode.
With the default value of POINT, an acquisition subarray of 100 x 100 CCD pixels (5x5 arcsec) and a checkbox of 3x3 CCD pixels are always used. The target position is the flux-weighted centroid of the brightest checkbox within the subarray.
With the DIFFUSE acquisition type, the acquisition subarray is a square array. The number of pixels on each side is 101 +
CHECKBOX (see below). The target position is either the geometric center or the flux-weighted centroid of the brightest checkbox, depending on the
DIFFUSE–CENTER parameter.
Specifies whether the target position of a extended source is to be located as the flux weighted centroid (FLUX–CENTROID) or the geometric center (
GEOMETRIC–CENTER) of the brightest checkbox.
DIFFUSE– CENTER is only permitted if
ACQTYPE=DIFFUSE.
Specifies the length of a side of a square pixel array over which the light intensity is summed to locate the brightest region of the acquisition. This parameter is required for extended targets (ACQTYPE=DIFFUSE). It is not allowed for point sources (
ACQTYPE=POINT), for which a fixed checkbox size based on the PSF will be used. The value of
CHECKBOX should equal the size of the brightest area of the source the observation is attempting to locate. The dimension of the acquisition subarray will be 101 pixels plus the
CHECKBOX parameter. The value of
CHECKBOX must be an odd number.
The exposure time for ACQ exposures on GO proposals must be less than 5 minutes for
ACQTYPE=POINT acquisitions, and for
ACQTYPE=DIFFUSE acquisitions must be less than 500/(101+
CHECKBOX) minutes.
