4.8.3 Dark Current Evolution

The dark current in WFPC2 has had an interesting evolution over the lifetime of the instrument. Figure 4.7 shows the median dark current for the central 400 x 400 pixels of each CCD at gain 7, each taken just after WFPC2's monthly decontamination. Each data point represents the median of five raw 1800s dark frames (after rejection of cosmic rays and bias subtraction, normalized to units of DN/1000sec). As such, this plot reflects the uniform, low-level dark current near the center of each detector. During the first six years the dark current increased approximately linearly with time; the dark current increased by a factor of about 2 in the WFC CCDs and by a factor of ~1.3 in the PC. But after 1998 (MJD > 51200) the dark current leveled-off, and perhaps decreased somewhat.

Figure 4.7: Dark Evolution from 1994 to 2004.

As mentioned before, there are two primary sources of dark current -- a dominant component which is strongly correlated with the cosmic ray flux in the image (probably due to scintillation in the MgF2 CCD windows; see Figure 4.6), and a smaller thermal dark current in the CCD itself. The dark current increase seen during early years was smaller in the optically vignetted regions near the CCD edges, which suggests much of this increase is related to scintillation effects in the CCD windows. Moreover, the ratio between the dark current at the CCD edge and the CCD center has remained nearly constant throughout the mission (within a range of ~5%; see WFPC2 ISR 2001-05), even though the dark current itself doubled in WF2, WF3, and WF4. Hence, it seems an inescapable conclusion that most of the long-term evolution is related to scintillation effects and variations in cosmic ray flux.

Long-term changes in the cosmic ray flux are perhaps most easily attributed to the solar cycle. The leveling-off of the dark current ~1998 is coincident with the approaching solar maximum which has the effect of reducing the cosmic ray flux at HST's low Earth orbit. Ground-based cosmic ray detectors show a gradual flux increase from 1992 to 1998, followed by a sharper decrease through early 2004. It is possible that other effects might also play some role. For example, portions of the HST orbit near the South Atlantic Anomaly experience higher cosmic ray rates, and it is possible that changes in the HST scheduling system could produce long-term changes in cosmic ray flux and hence dark current. It is also conceivable that long-term changes in the instrument itself might indirectly influence the sensitivity to scintillation effects (e.g. long-term radiation damage might modify the luminescence of camera components).

The thermal dark current of the CCD may also undergo long-term change (i.e. from radiation damage, etc.), and contribute some minor variation. A small increase in the CCD cold junction temperature was seen early in the mission; however, the temperature change can account for only a very small portion of the increase in dark current.

Since the dark current is generally a minor contributor to the total noise in WFPC2 images, its long-term variation is unlikely to impact the quality of WFPC2 observations, except perhaps in special cases (faint sources observed through narrow-band or UV filters, especially in AREA mode).

We note that the variation in dark signal reported here affects all pixels, and thus is distinct from hot pixels which vary in a more cyclic fashion. The hot pixels are highly localized, and are almost certainly due to radiation-damaged sites on the CCD detectors. Their number and intensity increase continuously, but are significantly reduced during decontamination procedures where the CCDs are warmed to +22°C to clear the CCD windows of contaminants. These "decontaminations" were conducted monthly until June 2003, after which their frequency was reduced to 49-day intervals. Apparently the decontaminations anneal defects in the CCDs which produce hot pixels (see Section 4.11).