A bake-out: operating a camera in space

Science Nugget: July 23, 1999


The Yohkoh/SXT instrument suffers from a problem that affects many space-based imaging instruments: a cold camera. The detector (a CCD) has a background dark current that depends strongly on its temperature. We can correct the images for the dark current, which is pretty uniform. But the image quality is in general a lot better if the dark current level is small compared to the intensity of the recorded image. Hence, it's in our best interests to keep the CCD cold, and most CCDs of this sort have carefully designed cooling systems to keep them that way. The SXT CCD hovers around -20C.

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 bakeout

So, we do one bakeout every three months or so. Because we are not observing very well during this interval, interesting activity may escape us - in fact, this time, the Sun perversely chose to turn its activity up a notch, and flares occurred while the CCD was warm. This is (fortunately) fairly unusual.

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.

The results

The result of the bakeout looks like this:

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 (handy@isass1.solar.isas.ac.jp),
H. Hudson (hudson@isass1.solar.isas.ac.jp)
July 23, 1999