8.1 Persistence in WFC3 IR
Image persistence, a phenomenon commonly observed in HgCdTe IR detectors, 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. An image of 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. This signal decays with time as a power law with a slope of about -1. Thus after 10,000 s, the persistence signal will be about 0.03 e¯/sec, just under 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. The shape of the curve reflects 1) the density of traps in different regions of the pixels and 2) the fact that once the detector is saturated the voltage levels within the diodes do not increases sharply 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, in seconds, since the end of the stimulus exposure and A and γ are functions of both exposure time and fluence level in the stimulus exposure. We refer to this model as the “A-γ” model (WFC3 ISR 2015-15).
Additionally, clear evidence of spatial variation in persistence across the IR detector has been measured (WFC3 ISR 2015-16). 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 is available from MAST (Version 3.0.1 of the persistence software).
Persistence of the magnitude (and impact) seen in Figure 8.1 is rare. In part, this is because the STScI contact scientists check all Phase II submissions to identify any programs that are likely to cause large amounts of persistence so that the mission schedulers can insert 2 WFC3/IR-free orbits before executing another WFC3/IR program. However, this process is only intended to identify the worst cases. A large fraction of WFC3/IR exposures have some saturated pixels and all of these pixels have the potential to generate persistence in a following IR observation. Inhibiting IR observations of 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's 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
- • What's New in This Revision
- 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
- • 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