The problem is that no matter how clean the instrument is when it's launched, there's still a residual layer of crud everywhere. Some of this is water, while other types of crud include oils, grease, hydrocarbons, and any other type of sludge you can imagine (But in tiny, tiny amounts!). This stuff has a tendency to gravitate towards the coldest part of the instrument, which happens to be the CCD.
Eventually we have enough of this contamination on the CCD that it begins to affect image quality. The way to fix this is to do what's known as a CCD bake-out: the CCD heaters are turned on and the temperature is taken up to around +20C for a day or so. This drives the ice and sludge off the CCD and the resulting images are usually better. SXT does this nominally four times a year. Why isn't once enough? Because the boiled off contaminants generally don't leave the instrument, they just gravitate to some other place on the instrument. Over time, they again collect on the CCD, and it's time for another bake-out.
|The plot to the right shows a typical bakeout as done this week: The temperature goes up initially because the cooler is turned off, then "soars" again as the heater is turned on - actually, the bakeout temperature is only about +20 degrees C, which won't roast your turkey very fast.|
These are regular 0.668-sec exposures before, during, and after the bakeout; the image in the center shows the presence of large dark current throught. But one can still see features - how well? We'll do an elementary correction for dark signal on the central image:
This works surprisingly well, it turns out! The image above has not been corrected for stray light, nor interpolated properly for dark noise, so it is much more primitive than our usual standard image reduction. One can see artifacts, and one can see dark spikes, but one can also see faint features on the Sun! So if something dreadful happened to Yohkoh and (a) the cooler failed, while (b) the heater got stuck "on", we would still get data even on the quiet Sun. Flares, of course, are much brighter and easier to deal with, but they present their own problems when the CCD is warm.
If there is any positive fan mail about a technical nugget of this type, we'll do another on the interesting behavior of the flare mode during this week's bakeout.
B. Handy (firstname.lastname@example.org),
H. Hudson (email@example.com)
July 23, 1999