Image Persistence
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| DARK exposure 32 seconds after bright illumination |
DARK exposure 512 seconds after bright illumination |
DIAGNOSTIC
The persistence is the excess dark current observed immediately after the
detectors have been saturated with bright light. Persistence tests have been
performed with NICMOS during SLTV and SMOV. The results and analysis of these
tests are presented in the following Instrument Science Reports and Posters:
The examples are images of dark current data obtained after the detectors
have been flooded by a long exposure of 512 seconds where the selected
star, Oph S1, is observed with the F160W filter and an over-exposure of
300 to 500 times linear full well are expected.
These images show the persistence to be present in data taken after
the bright illumination for at least 60-120 seconds, followed by
a slow decay where small persistence signatures are still visible
after 10 minutes of dark exposures.
CURE:
Care should be taken to not saturate the detectors, and observations of
bright sources should be dithered by large amounts (e.g. between quadrants)
to minimize problems with persistence. Vaguely extended bright patches may indicate a persistent image, and preceding exposures should be checked for
objects in this area. Note that a persistent image will not move when an exposure is dithered.
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