NICMOS Instrument Handbook for Cycle 11

Trade-offs Between ACCUM and
MULTIACCUM

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Given that there is so much more information present in a MULTIACCUM dataset than in an ACCUM dataset, it may seem obvious that MULTIACCUM should always be the preferred readout mode. In practice, the trade-off is not always so straightforward.

Because of the fixed read-out patterns available for use in MULTIACCUM mode (the SAMP-SEQs), in order to make an exposure of total integration time a minute or two, it is necessary in most modes to make a significant number of readouts. This leads to a significant volume of data to process. Additionally, the readouts are initially stored in a buffer in the NICMOS flight computer. A maximum of 94 readouts can be stored in this buffer, after which the content of the buffer must be dumped to the Solid State Recorder. A full dump of 94 reads takes about three minutes. The data dumps occur in parallel with the beginning of another set of exposures in most, but not all, circumstances. Thus, during the preparation of the Phase II proposals, some observers with very short (1-2 minutes) exposures may consider the trade-offs between ACCUM and MULTIACCUM.

There are a variety of disadvantages to ACCUM mode. First, the ability present in a MULTIACCUM exposure to filter out CR hits which occur during the exposure is lost. We find for NICMOS that typically between 2 and 4 pixels are hit per second per camera by CRs: most of these are low energy and so can be filtered out of a MULTIACCUM exposure by the calibration pipeline software. In ACCUM mode the process of CR removal requires separate exposures, and has to be done in post-processing. Second, the ability to detect pixel saturation, which again is done automatically for MULTIACCUM observations by the calibration software, can in some circumstances be lost in ACCUM mode. The reason for this is that the time elapsed between the first read for each pixel and the reset immediately prior to the read is approximately 0.2 seconds. During this time, pixels exposed to a bright target will accumulate significant signal, which is then present in the first read. When this is subtracted on-board in ACCUM mode, all the charge accumulated in the time between reset and read will be subtracted. If the pixel has saturated during the exposure, the difference between initial and final reads will be less than the expected saturation value for the pixel, and thus it may be impossible to recognize that the pixel is saturated. Thus in the case of bright targets, erroneous signal levels may be recorded in ACCUM mode. Third, in ACCUM mode, even if pixel saturation is detected, it is not possible to repair the data obtained in the saturated pixel. In MULTIACCUM mode, pixels which have saturated can be repaired by using the results of previous, unsaturated reads during the same exposure.

In conclusion, in cases where a multitude of short duration exposures must be made per orbit, and data volume is therefore a problem, ACCUM may possibly (but not necessarily) be a good choice. In all other cases it is likely that MULTIACCUM will yield the best results, and we recommend that all observers attempt to use MULTIACCUM.