MULTIACCUM is a flexible mode that allows multiple non-destructive readouts of the array during integration spaced at user-specified intervals throughout the integration, with the results from each readout being recorded onboard and returned to the ground for analysis. See the
NICMOS Instrument Handbook for more information. This mode differs from the use of multiple readouts with
NREAD in
ACCUM Mode, because it actually produces multiple images whereas the multiple initial and final readouts in
ACCUM Mode are used to reduce read noise in the generation of a single image.
MULTIACCUM Mode may also be used with a single sample time to achieve integration times as short as 0.203 seconds (the shortest allowed in
ACCUM Mode is 0.598 seconds).
The Wavelength parameter is not required for NICMOS observations and should be left blank.
Specifies the NICMOS focus position to be used. If DEF is specified or this parameter is omitted, NICMOS will be focussed in the best position for the selected camera. A value of
1-2 selects a compromise focus position between the optimum foci for cameras 1 and 2. This will mainly be useful with coordinated parallels, but
CAMERA-FOCUS may be specified only on exposures using the primary NICMOS detector in the sequence as defined in
Section 6.3 on page 105.
Specifies the name of a predefined sequence of times from the start of the exposure at which the nondestructive readouts (samples) are performed. The number of samples (up to 25) taken for each exposure is controlled by the
NSAMP parameter (see below).
Table 12.2 gives the sample times (from the start of the exposure) for each sequence.
SAMP–SEQ is required.
Four different types of sequences are provided. The SCAMRR and
MCAMRR sequences are rapid sequences with linear steps, which obtain the densest temporal sampling.
SCAMRR is designed for use with a single camera and provides the densest sampling available, but may not be used with multiple cameras.
MCAMRR should be used if rapid sequencing is desired with two or three cameras (
NIC1,
NIC2,
NIC3) operating in parallel.
Sequences STEP1,
STEP2,
STEP8,
STEP16,
STEP32,
STEP64,
STEP128, and
STEP256 begin with logarithmic spacing up to the given number of seconds (1–256), and then continue with linear spacing for the remainder of the sequence, with adjacent steps separated by 1–256 seconds. These sequences all include three readouts during the first second to compensate for any nonlinear effects which may arise at the start of the exposure.
Sequences SPARS4, SPARS16, SPARS32, SPARS64, SPARS128 and
SPARS256 begin with two readouts during the first second, and then continue with sparse linear spacing for the remainder of the sequence, with adjacent steps separated by the given number of seconds (4, 16, 32, 64 128 or 256). These are similar to
STEP1 to
STEP256, except that the linear sampling begins immediately after the first two readouts rather than being preceded by a series of readouts with logarithmic spacing.
Specifies the number of samples in a predefined sequence that should actually be taken.
Table 12.3 defines 25 sample times for each sequence. If an
NSAMP value smaller than
25 is used, samples will be taken at only the first
NSAMP times from this table.
NSAMP must be specified.
The number of readouts will be NSAMP plus one for the initial readout, giving a maximum of 26 readouts (the initial readout plus a maximum of 25 samples) for a single execution of a
MULTIACCUM exposure. Each readout will be recorded and will appear in the final data set.
If Number_Of_Iterations > 1, the specified number of iterations will be taken at a single pointing. For a pattern (see
Chapter 8: Pointings and Patterns) the specified number of iterations will be taken at each point in the pattern.
Time_Per_Exposure must be
DEF in this Mode. The exposure time is unnecessary, because it is specified by
SAMP–SEQ and
NSAMP.
The exposure time sequences denoted by SAMP–SEQ are defined in the following table. These values are approximated for simplicity; actual exposure times may be reduced by up to 7 msec.
