There are other aspects of Yohkoh operations that generally escape the eye of the casual armchair observer. One aspect of operations that might not escape the casual observer is illustrated in the following movie:
This is an example of what happens with the Star Tracker on Yohkoh loses Canopus, our trusty Guide Star. (The Star Tracker is explained below.) In this case, it's apparent that we lost that aspect of the pointing system which kept track of the the roll angle. The vertical bouncing later in the movie arose when the tohbans (spacecraft operators) sent commands to bring Yohkoh back into nominal pointing, with North pointing Up. Some of the images in the above movie show improper subtraction of stray light in the telescope arising from these non-normal pointings. (A tutorial on "stray light" removal appeared here last year).
Sun-pointing satellites such as Yohkoh, TRACE, and SOHO all require some method of keeping pointed at the sun to pretty tight tolerances. Various methods are employed, ranging from all-sky star trackers ("Any bit of the sky in, exact pointing coordinates out") to primitive photosensitive diodes that are only really able to tell that the sun is shining on them, so hence it must be somewhere In front of us. (Think of walking outside, closing your eyes, and figuring out where the sun is, based on how bright things look shining through your eyelids. That's about the level of precision we get here.)
Yohkoh uses two stars and three instruments to define its orientation in space. The Sun is the first obvious guide star. The second star may not be so obvious: Canopus. With these two stars, we are able to (a) able to keep the Sun centered in Yohkoh's sights, and (b) able to keep the Sun from spinning around on us, leading to the (possibly?) mistaken impression that the heart of our solar system has decided for a change to revolve about its axis for a while.
This system used to work well. But the HXA sensors (the linear CCDs) have degraded over time: the sensitivity has dropped, and the signal-to-noise level has worsened. (This is a consequence of putting a CCD detector in space. Radiation damage has probably taken its toll on both of these sensors after 10 years on-orbit.) The noise level was such that it became difficult to determine the location of the limbs, and often the returned pointing was incorrect.
The solution, again implemented by our Pointing Expert Jean-Pierre Wuelser, was to collect the entire row of CCD data for each of the HXA limb scans - one-dimensional images of the Sun - and fit a curve to the result. This method overcomes the worsening signal-to-noise problem in the sensors and gets the HXA sensors, and Yohkoh pointing, back on track.
Back on track, that is, except for solar eclipses which occur on regular schedules but sometimes surprise us anyway. Solar eclipses as viewed from a satellite whizzing around in low Earth orbit are kind of different... , , or search on "eclipse" in the Nuggets home page. During an eclipse of course the Sun's apparent shape changes erratically, so routine pointing procedures may (and sometimes do) go awry temporarily.
To compensate for this, a time-averaged signal from the HXA sensor is used as a baseline and the error signal from the IRU sensors is used to calculate the drift and jitter since the time of the HXA measurement. The HXA unit provides pointing information with about 2 arc second resolution on a time scale of 1-8 seconds, dependent on operating mode. Meanwhile, the IRU unit provides 0.08 arc second resolution on a time scale of 0.25-2 seconds. By combining these two datasets in ways beyond the scope of this nugget, we get fine-scale pointing information for Yohkoh, at a level generally considered sufficient for a detector with 2.5 arc second resolution.
Even this level of pointing isn't always good enough. The current algorithm doesn't use all of the IRU datapoints, so it's possible for the pointing between sequential images to be off by maybe 0.20 arc seconds, substantially less than an SXT full-resolution pixel. Usually this isn't a big deal. But it becomes a lot more important when someone decides to do temperature analysis with SXT images! Temperature maps of soft X-ray images involve differences and ratios of actual data, always a tricky business prone to systematic errors. Further information (for experts only) may be found in the literature: (Acta Astronomica, v.46, pp.15-28, 1996).
October 27, 2000Brian Handy <firstname.lastname@example.org>