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The detector in the Extreme ultraviolet Imaging Telescope (EIT) on the SOHO spacecraft is a backside-thinned charge-coupled detector (CCD). The EIT CCD (above) is similar to the frontside CCD's handheld video cameras, but with better read noise (the "snow" or "fuzz" you see in your home videos when there's little available light) and, thanks to the backside-thinning, it's sensitive to extreme ultraviolet (EUV) light. For several days now bakeout procedure has started and data from EIT (Data live monitor) will not be available for a while.

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" 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.

We bake out the CCD by turning on small electric heaters on the CCD camera. This raises the CCD temperature to ~ +16 C. We do so every ~ 3 months, during SOHO telemetry keyholes, for 2 - 4 weeks; we also run detector efficiency tests before and after the bakeout. If nothing else, it gives our crack operations team some much-needed time off now and then. See the bottom of the EIT bakeout history for an estimate of how long the current bakeout will last.

For nearly three months in the summer of 1998, while the SOHO spacecraft rolled out of control, its batteries discharged and its solar panels pointing uselessly edge-on to the Sun, the radiator which normally carries heat away from the EIT CCD to the cold of space was pointed right at the Sun. According to the best thermal models available, this raised the temperature of the EIT CCD to over +35 C. Tests after the recovery of the spacecraft in 1998 September showed that not only had the CCD regained much of its lost sensitivity (~ 60%), but the "slush" appeared to have gone away --- for good. The CCD experts on the EIT team suspect that the slush began to escape when the rear thin aluminum filter peeled slightly from its frame in 1998 February. Still needed to bake out from time to time to cure the electron traps, however.

Source: http://umbra.nascom.nasa.gov/eit/CCD_bakeout.html

 


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