The University of Alabama campus, with its observatory housing a 10-inch refractor from J.W. Fecker installed in 1950, was within the narrow path for a grazing occultation of Saturn by the Moon on the evening of 11 November 1997. We captured the event with a small Sony black-and-white surveillance camera at the prime focus of the telescope. This time-lapse sequence consists of images extracted at 30-second intervals, as we watch the Moon slide in front of one half of Saturn but never quite covering the whole planet or ring system. The video strikingly shows how much higher in surface brightness the nearly-full Moon is than Saturn's cloudtops or rings; their higher albedo is more than compensated for by the hundredfold dimmer sunlight at Saturn's distance.
Technical details: Individual frames were grabbed from the original video tape by Chris Dillard, a student in my class on astronomical observations. I imported them into IRAF so they could have some of the scattered moonlight removed (by a median filter applied only to a specified intensity range) and have Saturn aligned to remove telescope tracking errors. This started with a cross-correlation of the central part of the frames, with manual tweaking to improve the results when the Moon's limb confused the correlation results. The results were displayed and made into GIF files with ximtool and animated using whirlgif. (Specifically, for my own edification, whirlgif -v -loop 1000 -time 30 -o testsat.gif ~/sat*.gif). All this post-processing was done on my trusty SGI O2 machine.
Last changes: 9/2001 © 2001