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Hubble Space Telescope
Observing Modes

Q.: Is there any reason to use ACCUM rather than MULTIACCUM for observations in thermal-background limited parts of the spectrum?
A.: For observations with short integration times and chopping due to a large thermal background, use of ACCUM can limit the number of readouts that must be stored and dumped to the solid-state recorder. This can result in a large reduction in overheads associated with clearing the buffer. Use of ACCUM in such circumstances may be advantageous.
Q.: Can I use MULTIACCUM on short exposure? Up to how many exposures? what is the shortest interval between successive reads for this mode?
A.: Yes You can use MULTIACCUM for short exposures. You can do up to 25 integrations with MULTIACCUM. The shortest interval between successive reads for MULTIACCUM is 0.215sec. All the (up to) 25 exposures and a 0th read - making a total of (up to) 26 exposures - are read out (the _raw file) and dumped to the ground in 1 image. This image has each individual exposure in it so the size of the image will depend on how many exposures were taken. As NICMOS non-destructively reads out the array after each exposure, each exposure is the total sum of the counts up to that point. So the last exposure will encompass the entire integration time of the observation.