During the exposure, all pixels are first reset
via three separate passes through the detector. The reset
is immediately followed by a fourth pass through the detector, which non-destructively reads
and stores the pixel values. This marks the beginning of the integration. The first array read will then be followed by one or more non-destructive readings of the detector. The last non-destructive readout marks the end of the integration. The total integration time is given by the difference in time between the first and the last array read.
The non-destructive nature of the NICMOS readout offers elaborate
methods of using the instrument, which aim at optimizing the scientific content of the results. In particular, it is possible to read-out images at intermediate stages of an integration and return both these and the final image to the ground
. This mode of operation is known as Multiple-Accumulate
). The observer uses this capability by specifying one of the pre-defined MULTIACCUM
(see next section) and the number of samples NSAMP
that corresponds to the desired integration time. The list of supported MULTIACCUM
sequences is given in the next section. These sequences are either linearly spaced or logarithmically spaced. Linearly spaced exposures may be useful for faint targets where cosmic ray filtering is important while logarithmically spaced exposures permit the observation of a wide dynamic range. The process is shown schematically in Figure 8.1
for the case of logarithmically spaced intervals with NSAMP=4
the detector reset is followed by a single read of the initial pixel values (zeroth read). Then a sequence of non-destructive array readouts are obtained at times specified by the selected sequence. Up to 25 readouts can be specified spanning a total integration time from 0.203 seconds to 8590.0 seconds. The last read of the detector array ends the exposure and thus the last NSAMP
will be selected to give the total exposure time. All of the readouts, including the initial readout, are stored and downlinked without any onboard processing. For N
readouts, this mode requires the storage and transmission (downlink) of N
+1 times as much data volume as for ACCUM
mode. (See Section 8.6
for trade-offs between MULTIACCUM
In most cases, MULTIACCUM mode provides the highest quality scientific data.
The benefits of obtaining observations in MULTIACCUM
mode fall into two areas.
provides the best choice for deep integrations or integrations on fields with objects of quite different brightness.