8.1 Persistence in WFC3 IR
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.
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.
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.
1Fluence 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
- • Acknowledgments
- Chapter 1: WFC3 Instruments
- Chapter 2: WFC3 Data Structure
- Chapter 3: WFC3 Data Calibration
- Chapter 4: WFC3 Images: Distortion Correction and AstroDrizzle
- Chapter 5: WFC3-UVIS Sources of Error
- Chapter 6: WFC3 UVIS Charge Transfer Efficiency - CTE
Chapter 7: WFC3 IR Sources of Error
- • 7.1 WFC3 IR Error Source Overview
- • 7.2 Gain
- • 7.3 WFC3 IR Bias Correction
- • 7.4 WFC3 Dark Current and Banding
- • 7.5 Blobs
- • 7.6 Detector Nonlinearity Issues
- • 7.7 Count Rate Non-Linearity
- • 7.8 IR Flat Fields
- • 7.9 Pixel Defects and Bad Imaging Regions
- • 7.10 Time Variable Background Contamination
- • 7.11 IR Photometry Errors
- • 7.12 References
- Chapter 8: Persistence in WFC3 IR
- Chapter 9: WFC3 Data Analysis
- Chapter 10: WFC3 Spatial Scan Data