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Originally posted by brokenheadphonez
In order to (i) keep read noise down (suppress the "snow") and (ii) prevent cosmic ray hits from permanently raising the read noise level by damaging the detector, the EIT CCD is usually operated at a temperature of about -67 C. This temperature is achieved by passive cooling: the CCD chip is thermally contacted to a titanium "cold finger" (at far left in the image above) that is attached to a radiator plate that is pointed at a piece of sky perpendicular to the earth-Sun line. Unfortunately, there's a small amount of "slush," probably a mixture of water vapor and hydrocarbons, that avoided the initial bakeout (just after launch) of the instrument. The back end of the EIT telescope, unfortunately, is a difficult place from which to escape, because of the plate holding the final, thin aluminum filter just in front of the CCD, and a labyrinthine venting system (designed to prevent stray light). At -67 C, even with the low partial pressure in space, the slush condenses on the CCD and the cold finger --- they're the coldest parts of the back end of the instrument. The slush absorbs some EUV, and so reduces the thoughput of the instrument. In addition, overexposure to EUV (say, from bright flares or --- before the onboard software was fixed to prevent this --- accidentally long exposures) can produce electron traps in the CCD material, which reduce the detector's throughput (how many electrons it produces for a given number of photons striking a pixel). Thus, we need to warm up --- "bake out" --- the detector to evaporate the slush (if only temporarily) and anneal out the traps in order to maintain the performance of the instrument.