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CCD Hot PixelsAs expected for a CCD in space, energetic particle impacts on the STIS CCD produce "hot pixels" with enhance dark current. For the STIS CCD, typically 2-3% of the pixels have dark currents more than 0.01 e-/s. Examples are shown in the three attached views of the "superdark" image h7p1439qo_drk. In the first view pixels hotter than ~ 0.5 e-/s are visible. In the second view pixels hotter than ~ 0.05 e-/s are visible. In the third view pixels hotter than ~ 0.005 e-/s are visible. The streaks visible in this last image are associated with the very hot pixels; the dark current in those pixels is sufficiently high that it adds significant charge do "upstream" rows during readout. Warming of the CCD by turning off the thermoelectric cooler is effective at "annealing" some portion of the hot pixels. Annealing takes place roughly once a month. After the first few STIS annealing cycles, the net growth rate of hot pixels is about 30% of the instantaneous growth rate; this fraction is expected to decrease as the hot pixel count asymptotically approaches a steady state. Weekly superdarks are delivered to the pipeline within ~three weeks of an observation and can be retrieved via StarView and used to recalibrate your CCD data. (Go to "HST Instrument Searches," choose "Reference," and choose the "Recommended" files for the data set you wish to calibrate). Additionally, daily darks are also taken. These are not routinely processed by the STScI and delivered to the pipeline, but you can retrieve them from the HST archive and use them to make a superdark tailored to your particular day of observation. A one day old superdark removes 95% of the hot pixels, while a 5 day old superdark typically removes 88% of the hot pixels. In the future we will make an easy script available for you to make your own daily darks; in the interim, for information on how to make daily superdarks, send inquiries to help@stsci.edu. To find STIS CCD darks stored in the archive, taken, for example between October 29 and November 1, you can use starview as follows: 1) Go to "General Search" screen 2) Enter the following:
Target Name: DARK
Instrument: STIS
Config: STIS/CCD
Start: 10/29/97 .. 11/1/97
(for example)
After extracting from the archive, check that the gains are the same.
If not, either throw out the ones you don't want, or correct them all
to the same gain.
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