In ACQ/PEAK Mode, a series of exposures is taken at several HST pointings offset along the detector axes, and the HST is repositioned to maximize (peak up) or minimize (peak down) the light intensity within a default or proposer-specified detector subarray.
AXIS1 and
AXIS2 are in the dispersion and cross-dispersion (or spatial) directions respectively. For more information, including when peakups are required, see the
STIS Instrument Handbook.
Mode ACQ/PEAK should always be preceded by an
ACQuisition.
The aperture used for the ACQ/PEAK exposure will generally be identical to the aperture used for the subsequent
ACCUM science exposure. For maximum centering accuracy, a peakup aperture smaller than the science exposure may be chosen. Peakups are required whenever an aperture dimension is less than or equal to 0.1 arcsec. Two peakups are recommended for the
0.1X0.03 aperture: one with the
0.2X0.09 aperture followed by another with the
0.1X0.03 aperture.
See Table 9.5 for the allowed
ACQ/PEAK apertures. Note that you cannot perform a peakup with an E2 aperture (i.e.
52X... apertures ending with the
E2 suffix), which are designed for use in conjunction with
CCD Fringe Flats. If you need an
ACQ/PEAK exposure prior to using an E2 aperture, perform the peakup using the
52X0.1E1 aperture. Then select the appropriate E2 aperture for the following science exposures.
When an ACQ/PEAK exposure using the occulting bar aperture
52X0.2F1 is specified, a peakdown rather than a peakup will automatically be performed.
A peakup will be performed otherwise. To peak up on the width of a slit with an occulting bar before peaking down on the bar itself, the occulting bar aperture name with
–R appended to it should be used. Such exposures will peak up on a reference point offset from the occulting bar along the slit’s length.
See Table 9.1 for a listing by operating mode of all Optional Parameters. Here are descriptions for those Optional Parameters for
ACQ/PEAK mode.
The default subarray with SIZEAXIS2=DEF depends on the aperture and spectral element. With the
MIRROR element, the default subarray will generally be 32x32
CCD pixels centered on the aperture. However, the
F1 fiducial slits use a 32x64 subarray. A subarray of 16x16 is also allowed when
MIRROR is used.
With a first-order CCD grating (G230LB,
G230MB,
G430L,
G430M,
G750L,
G750M), the default subarray will generally be 1022 x 32
CCD pixels. However, the
F1 fiducial slits use a 1022 x 64 subarray.These subarrays have been chosen to minimize cosmic ray perturbations on the acquisition process.
