CCD Performance

4.8 Dark Backgrounds

Low dark noise is one of the benefits of MPP, since inverted phase operation suppresses the dominant source of CCD dark noise production (Si-SiO2 surface states). The remaining source of dark noise, thermal generation in the silicon bulk, is determined by the quality of the silicon used in chip fabrication. The intrinsic dark rate of WFPC2 CCDs is <0.01 e- pixel-1 s-1 at temperatures below -80 degrees C.

Figure 4.5: Average Dark Rates vs. CCD Row.

The temperature set-points for the WFPC2 TEC coolers are: -88, -83, -77, -70, -50, -40, -30 and -20 degrees C. The corresponding approximate median dark rates are given in Table 4.2. For instrument health and safety reasons, GOs cannot command temperature changes.

4.8.1 Sources of Dark Current

The dark current appears to have two components: one from electronic sources in the CCD, and a second component whose strength correlates with the cosmic ray flux. The electronic dark current is ~0.001 e- s-1, consistent with the Thermal Vacuum Test data.

The second component of dark current appears only on-orbit, its strength drops towards the edges of each CCD, and it is both chip- and time-dependent. At the current operating temperature, this non-electronic component constitutes up to 80% of the total signal measured in the PC. The fraction and overall level are lower in the other chips, and lowest in WF2. This second component ranges from 0.001 e- s-1 (WF2) to 0.005 e- s-1 (PC). The edge drop of is shown in Figure 4.5, where the average of lines 200-600 for each chip (with hot pixels rejected) is plotted in e- s-1 as a function of column number. The drop near the edge is consistent with luminescence from the CCD windows, shadowed by a field stop mask just in front of the CCD.

Table 4.2: Dark Count Rates.

A further indication of the possible origin of this second component is the correlation between its amplitude and the cosmic ray activity in the same exposure, as shown in Figure 4.6. For example, the cosmic ray flux in the PC varies from 7x105 to 13x105 DN per 1000s, while the total dark signal in the PC varies concurrently between 0.0007 and 0.0010 DN s-1. Similar, though slightly smaller effects are seen in the WFC CCDs. These clues point to cosmic-ray induced scintillation of the MgF2 field-flattening windows as a likely source of the second dark current component. This might be caused by impurities in the MgF2 windows; if so, the window of WF2 must contain substantially less impurities. However, other explanations cannot be completely ruled out at this point.

Figure 4.6: Dark Signal vs. Cosmic Ray Flux.

For the great majority of WFPC2 observations, this effect is negligible. In fact, it is noticeable mainly because the true dark rate is very low at the -88 degrees C operating temperature. However, observations for which the dark current is an important limiting factor, either due to noise or background flatness, will require special handling to remove the signature of the dark current properly, as its amplitude depends on the time-variable cosmic ray flux.

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 13.6s is required both to clear the CCDs before the exposure begins, and 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. As of this writing (April 1996), 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. Work is currently underway to improve the way bias calibration files are computed.

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.

Dark calibration frames currently used in the calibration pipeline are averages of ten on-orbit dark frames taken over the space of about 2 weeks. The dark calibration file in the pipeline is revised ~every two weeks to track variations in the hotpixels. As of this writing there are plans to alter this method to both reduce the noise and provide better tracking of hotpixels. The new method would use the average of many (~100) dark frames taken over many months, and then hotpixels would be inserted into the calibration file to correct short-term variations. New files would be generated ~weekly, with only the hotpixels changing from week-to-week.

Figure 4.5: - Average Dark Rates vs. CCD Row.
4.8.1 - Sources of Dark Current
4.8.2 - Darktime