The time taken to read through a quadrant on the array sets a fundamental limit on the fastest electron collection rate which can be achieved by resetting all the pixels. An inherent consequence of the methods of operating the NICMOS array detectors in the
MuLTIACCUM and
ACCUM modes is, therefore, that there is a minimum possible exposure time, 0.203 seconds (~ 0.6 seconds for
ACCUM), set by the time required to read the array. Although the detector arrays are multiplexed by division into four quadrants, each pixel in a 128 в 128 pixel quadrant must be sampled in some order (note that there is no transfer of charge as is done in a CCD). For a very bright object, the time between the reset of a pixel and its final read is sufficiently long that the pixel saturates.
The solution adopted to this problem for NICMOS is the provision of a bright object mode which enables targets to be observed which are ~200 times brighter than is possible in the other modes
without saturating. In
BRIGHTOBJ mode, an
ACCUM sequence of operations is performed on
one pixel in each quadrant at a time. That is, the pixel is reset, read, integrated, and read again with the difference between the final and initial readouts being stored as the measured signal and the interval between the reads being the exposure time. This process is repeated sequentially for all pixels in each quadrant. Users can think of this as integrating on a single pixel at a time. The smallest integration time which can be used is 1.024 milliseconds, the longest 0.261 seconds.
Figure C.1 illustrates the operation of bright object mode. Initially, the entire detector is reset. Then the first pixel (solid shading) in each quadrant is read. After the requested integration time, the first pixel in each quadrant is read again. Then the second pixel in each quadrant is reset, then read, integrated, and read again. The process continues until all 16,384 pixels in each quadrant have been read twice, separated by the integration time. The image down linked is made up of the difference between the two reads of each pixel.
The time required to take a BRIGHTOBJ mode exposure can be rather long. Since photons are only collected in one pixel per quadrant at a time, the time associated with obtaining the frame is
where
EXPTIME is the integration time per pixel (i.e. the observation time is approximately (128
2) в the exposure time). For example, if an integration time of 0.1 seconds is used to observe a bright target then the actual time required to complete the observation would be around 27 minutes! This means that, allowing for acquisition time, only two such exposures can be obtained in a single target visibility period. However, it is not always so serious. In the case of Jupiter, for example, the integration times required per pixel are only of the order of milliseconds and so the total integration time will only be around 20 seconds.
The longest exposure time which is possible in BRIGHTOBJ mode is 0.261 seconds, requiring 4278 seconds in total. Thus it is possible, in the worst case, for a single
BRIGHTOBJ mode exposure to use more than an orbit. In general, observers are strongly advised to consider the trade-off between relatively long
BRIGHTOBJ mode exposures (which take the longest time) and short
MULTIACCUM mode exposures (using a narrow filter).
The advantage of this mode of operation is the ability to acquire objects significantly brighter than the normal saturation limit of the detector.
