8.5 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 NICMOS data is more complicated than for 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 and at the same detector temperature. For ACCUM observations we encourage observers to select exposure times that correspond to existing delta-times of the pre-defined MULTIACCUM sequences (Table 8.2). These available delta-times are also listed in Table 8.3 below.

Table 8.3: Recommended Exposure Times with Dark Current Calibration.

Time (seconds) (NREAD=1)
0.203
0.303
0.389
0.998
1.994
3.994
7.994
15.994
32.000
63.998
127.994
256.000
383.999
511.998

STScI is not planning to obtain dedicated ACCUM dark observations, but dark reference files for ACCUM observations can be constructed from MULTIACCUM darks with identical delta-times.