Image persistence is a phenomenon commonly observed in HgCdTe IR detectors. It is an afterglow of earlier images that in the case of the WFC3 IR detector is present when pixels are exposed to fluence1
levels greater than about 40,000 electrons. In cases where portions of the detector are heavily saturated in the initial image, the afterglow can be detectable at levels comparable to the background for several hours.
A very obvious example of persistence is shown in
. The image shows a high galactic latitude field; the observation was taken to search for the optical counterpart to a γ
-ray burst. However two visits from separate IR programs had preceded the observation of this field. The afterglow of the bright sources in these dithered observations is clearly visible as 5 point line patterns in
Different HgCdTe IR detectors have different persistence characteristics. In WFC3, a pixel exposed to an effective fluence level of 105
electrons produces a signal of about 0.3 e-/sec 1000 s after the exposure. The signal decays with time as a power law with a slope of about -1. Thus at 10,000 s, the flux will be about 0.03 e-/sec, compared to the dark current of 0.048 e-/sec (median). As shown in the left panel of
, the amount of persistence in the WFC3 IR detector depends strongly on the fluence of the earlier exposure. This shape of the curve reflects the density of traps in different regions of the pixels (and the fact that once the detector is saturated the voltage levels within the diodes do not change much with increasing fluence). The right panel of
shows the power law decay of the persistence at different fluence levels.
A further complexity of persistence modeling is due to the fact that persistence actually depends not just on the total fluence, but on the complete exposure history. The traps have finite trapping times (see
WFC3 ISR 2013-06
WFC3 ISR 2013-07
). A short exposure of a source that results in a fluence of 105
electrons produces less persistence than a longer exposure of a fainter source that reaches the same fluence level, because in this second case the traps have more time to capture free electrons before the diode gets reset. While tracking the complete history of each WFC3/IR pixel is a huge task, to a good level of approximation it is possible to model persistence only as a function of total fluence and total exposure time in the stimulus image. This model, which is currently used by the WFC3 team to predict persistence and derive the persistence products (see
) is thoroughly described in
WFC3 ISR 2015-15
, and is parametrized as:
is the time since the end of the stimulus exposure, in seconds, and A and γ
are function of both exposure time and fluence level in the stimulus exposure. We refer to this model as the “A-γ
Persistence of the magnitude (and importance) seen in
is rare. This is in part because contact scientists check phase II submissions to identify programs that are likely to cause large amounts of persistence and mission planners inhibit WFC3 IR observations for 2 orbits after such observations. However, this process is only intended to identify the worst cases of persistence and the process is not error free. A large proportion of the exposures taken with WFC3 have some saturated pixels and all of these pixels have the potential to generate persistence in the next observation in the schedule. Inhibiting IR observations after all exposures that could generate persistence would make it impossible to schedule the large numbers of IR observations that are carried out with HST, and in most cases, small amounts of persistence do not affect the science quality of the data, as long as observers and data analyzers take time to examine their IR images for persistence.