8.2 Multiple-Accumulate Mode

One of the concepts inherent in the operation of the NICMOS arrays is their non-destructive readout capability.

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 (MULTIACCUM). The observer uses this capability by specifying one of the pre-defined MULTIACCUM sequences, SAMP-SEQ (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.

In MULTIACCUM 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 and ACCUM readout modes.)

In most cases, MULTIACCUM mode provides the highest quality scientific data. The benefits of obtaining observations in MULTIACCUM mode fall into two areas.

•

The dynamic range of the observation is greatly increased. Rather than being limited by the charge capacity of a NICMOS pixel (a few × 105 electrons), an observation’s dynamic range is in principle limited by the product of the pixel capacity and the ratio of the longest and shortest exposures (8590.0 and 0.203 seconds).

•	An image can be reconstructed by processing the stack of readouts to cope with the effects of cosmic rays and saturation.

MULTIACCUM provides the best choice for deep integrations or integrations on fields with objects of quite different brightness.

MOST observers should use MULTIACCUM for their observations. Starting with Cycle 14, the four MIF MULTIACCUM timing sequences (MIF512, MIF1024, MIF2048, and MIF3072) have been replaced with four new SPARS timing sequences (SPARS4, SPARS16, SPARS32, and SPARS128). The new SPARS sequences can be most useful for fields with faint sources; i.e. nebulae, star clusters and galaxies.

Figure 8.1: Example MULTIACCUM with NSAMP = 4.