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Read Times and Dark Current Calibration in
ACCUM Mode

Because of the effects of shading, and the possibility that the underlying dark current may vary with time since reset, the removal of dark current (for calibration purposes, we implicitly assume shading and amplifier glow is a part of the time variable "dark current") from data is more complicated than for many other instruments. The most accurate way to remove the dark current from any observation is a measurement of the dark current with an identical integration time. This would also apply to every individual read-out in a MULTIACCUM observation. Of course, this would be prohibitively expensive in on-orbit calibration time. Instead, we have adopted two strategies. First, for ACCUM observations we will make dark current calibration observations for a set of 8 exposure times for a single initial and final read (listed below) and 14 exposure times for 9 initial and final reads. As for WFPC2, only a certain set of exposure times are allowed in ACCUM mode (173 times are specified currently, ranging from 0.57 seconds to 3600 seconds), and in Phase II proposals any user selected exposure time will be rounded down to the nearest available option from the 173 available times. If you select one of the times listed below (this list will be published with the Phase II Proposal Instructions), then the calibration database used by the calibration pipeline software will contain dark current calibration files which are an exact match to your exposure times. On the other hand, if you must use any other one of the 173 allowed exposure times in ACCUM mode, the calibration pipeline will interpolate between the various files in the database to determine the dark current for your exposure time. Based on our current understanding, we expect this to be a very accurate technique, and to be limited by the signal to noise in the dark observations rather than by the interpolation accuracy. For MULTIACCUM mode, dark calibrations will be made to match each of the 16 SAMP-SEQs. Finally, we note that SLOW readout mode is not supported and no calibration of this mode will be provided by STScI.

Table 8.3: Preliminary Exposure Times with Dark Current Calibration
Time (seconds) (NREAD=1)	Time (seconds) (NREAD=9)
1.071	9.939
2.038	12.075
4.963	15.157
9.715	18.339
25.714	22.533
58.142	30.274
110.246	44.969
229.680	62.702
	88.335
	114.806
	163.642
	234.240
	365.156
	485.148

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stevens@stsci.edu

Read Times and Dark Current Calibration in ACCUM Mode

Table 8.3: Preliminary Exposure Times with Dark Current Calibration

Time (seconds) (NREAD=1)

Time (seconds) (NREAD=9)

Read Times and Dark Current Calibration in
ACCUM Mode