WFC3 Data Handbook v.4
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WFC3 Data Handbook > Chapter 8: Persistence in WFC3/IR > 8.1 Image Persistence

8.1
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 Figure 8.1. 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 Figure 8.1.
Figure 8.1: Persistence in an IR image.
Example of an IR image, ia21h2e9q, in which persistence due to earlier visits is very obvious. The amplitude of the persistence for the brightest pixels in the diagonal trails of stars is about 0.1 electrons/sec. The image is plotted on a linear scale from 0.7 to 0.9 electrons/sec.
The pixels in the WFC3 IR array and other HgCdTe IR detectors are operated as reversed biased diodes. Resets increase the reverse bias. Electron-hole pairs created when light falls on the detector reduce the bias. Persistence is understood to arise from imperfections, traps, within the detector pixels that are exposed to free charge as the bias is reduced. The number of traps exposed is determined by the amount of charge accumulated in the diode. A small percentage of order, 1% of the free charge ( WFC3 ISR 2013-07) is captured by the traps and released later, creating the afterimages known as persistence. Resets prevent more charge from being trapped, because they remove free charge from the location of the imperfections, but do not affect the charge that has already been trapped. A detailed theory of persistence has been presented by Smith et al., ( 2008a, 2008b and WFC3 ISR 2013-07).
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 Figure 8.2, 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 Figure 8.2 shows the power law decay of the persistence at different fluence levels.
Figure 8.2: Persistence as a function of stimulus and time.
Left. Persistence as a function of stimulus in a series of dark frames after an observation of the globular cluster Omega Cen. Each color represents one dark image, hence the color sequence indicates a time sequence in the darks series (blue is the first dark, green the second, and so on).
Right. The persistence decay as a function of time. Each color represents the persistence decay-with time for pixels grouped by different fluence values in the stimulus exposure ( Long et al 2012)..
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 Section 8.2) is thoroughly described in WFC3 ISR 2015-15, and is parametrized as:
 
 
where t 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” model.
Additionally, clear evidence of spatial variation in persistence across the IR detector has been measured. One quadrant (upper left) has a higher persistence amplitude than the other three. The shapes of the power law exponents also appear to differ between quadrants. Using a correction flat provides a factor of two reduction in the peak to peak uncertainties. This flat is incorporated into the persistence prediction software and available from MAST (Version 3.0.1 of the persistence software). A full description is in WFC3 ISR 2015-16.
Persistence of the magnitude (and importance) seen in Figure 8.1 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.

1
Fluence here is expressed in electrons. We use this nomenclature, however, with some “abuse-of-notation” regardless of whether or not the pixel full-well capacity is reached. Therefore when values larger than the typical ~80,000 e- full-well capacity for the WFC3/IR channel are reported throughout this Chapter, their meaning is not that of “detected” electrons, but rather that of “electrons that would have been detected for an infinite full-well capacity”. This number is basically proportional to the impinging photon flux multiplied by the exposure time.”


WFC3 Data Handbook > Chapter 8: Persistence in WFC3/IR > 8.1 Image Persistence

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