Persistence and After-Images in WFC3/IR Data

Knox Long, long@stsci.edu, Sylvia Baggett, sbaggett@stsci.edu, & Karen Levay, klevay@stsci.edu

 

Most infrared arrays show after-images, known as persistence, following exposures that significantly saturate the detector. The mercury-cadmium-telluride detectors on the Wide Field Camera 3 (WFC3) infrared channel (IR) are no exception. The after-images can last for several hours. As a result, the after-images can masquerade as stars or nebulae in exposures that follow IR observations of bright objects. Although after-glows that can affect the science quality of images are rare, observers need to account for them when analyzing WFC3/IR images. Here we describe the nature of persistence in the WFC3/IR detector, and present some strategies for minimizing its effects in the analysis of WFC3/IR images.

The after-image contained in the image in Figure 1 is exceptional. Had it not been recognized as an after-image and was published, it would have certainly been embarrassing. Had the observation been an important part of the observer’s program, a repeat observation of the field would have been needed. Persistence at this level is rare, however. In 98% of the WFC3/IR images taken over the six-month period from January through June 2010, persistence above 0.03 electrons s-1 per pixel affected less than 2% of the pixels.  For 95% of WFC3/IR images, less than 0.5% of pixels were affected at this level.

After-images arise from traps (dislocations) in the individual diodes (pixels) that compose the array. As a diode accumulates charge during an exposure, voltage levels within the diode change, exposing traps to charge—either as free electrons or holes. When the diode is reset after the exposure, some of this charge remains in the traps, which gradually release it over time. For a saturated pixel in WFC3/IR, the signal rate in the after-images is about 0.3 electrons s-1 per pixel 1000 seconds after the end of the exposure and about 0.08 electrons s-1 per pixel an hour after the exposure. By comparison, the dark current is typically 0.05 electrons s-1 per pixel.

As shown in Figure 2, we observe very little persistence in the WFC3/IR detector for signal levels that are less than half saturation (70,000 electrons per pixel). Persistence—if it exists at all—is confined to regions of the detector where the images of a star or a few stars were saturated in an earlier exposure. Usually, such saturated stars were not the primary targets in the earlier observation. Because random, bright stars can occur anywhere—and the telescope could have pointed anywhere recently—astronomers should be aware of the possibility of unsuspected after-images appearing in their science exposures.