4.8.2 Darktime

As of this writing, the "DARKTIME" keyword in the WFPC2 image headers does not reflect correctly the actual time during which the CCD collects dark current. Instead, DARKTIME is merely set equal to EXPTIME (the exposure time) in the data headers, and this value is used for calibration. The error is small, and usually unimportant, but could be significant for programs aimed at measuring the absolute level of the sky background. The actual darktime in seconds is given by

where t is the requested exposure time in seconds, and n is the number of the CCD (PC1=1, WF2=2, etc.), and int() indicates the next lower integer. A duration of 16.4s is required to clear the CCDs before the exposure begins, and 13.6s is needed to read each CCD after the exposure. External exposures of 180s or longer made with the serial clocks off (CLOCKS=NO; the default setting) suffer an additional 60s of darktime (restart=1). This delay is associated with restarting the serial clocks for readout in exposures where the spacecraft AP-17 processor provides shutter control with loss-of-lock checking. Exposures made with the serial clocks on (CLOCKS=YES) avoid this extra 60s (restart=0).

We note that bias frames contain approximately seconds of dark current. No attempt is made to subtract this from the bias images when creating calibration files for use in the calibration pipeline. This effect is unimportant for most observations, but could be significant if one averaged many undithered deep exposures of the same field, or if one is interested in measuring the absolute level of the sky background. If the dark current were constant in time, this could be corrected by merely changing the value of DARKTIME used during calibration. However, the hotpixels vary on monthly timescales, so this simple correction is only partially successful.

The timing of dark calibration frames is slightly different from that of external science exposures. Dark calibration frames always have restart=0 in Equation 4.1.

The dark calibration reference file in the pipeline is revised weekly to track variations in the hot pixels. The current method of generating these files is to combine the bright hot pixels from typically five on-orbit dark frames taken over the space of about one week, with the low-level dark current from the average of 120 on-orbit dark frames spanning a much longer time period. This method gives an optimal combination of low noise and accurate tracking of hot pixels. Care is also taken that the same super-bias reference files is used for both science data and generation of the dark reference file, as this tends to reduce the noise in long exposures. (Early dark reference files used a much simpler method, and were typically combinations of about ten dark frames taken over two weeks.)