Single 1/15 second exposure of Jupiter through the 10-inch refracting telescope, scaled to 1/4 original size. 2006:07:05 21:49:28
Picking up where we left off yesterday, recall that I had a bunch of shots of Jupiter like the one shown here or a few days ago. The major problem with taking such photos is that, due to the magnification and quantity of light involved, you typically are faced with choosing between a long, blurry exposure or a short, grainy one. If I had a decent mount, better seeing, and an astronomical CCD this wouldn't be such a problem, but with these consumer camera-based solutions, that's the tradeoff.
For something like a planetary image, there's a way around that. Most cameras of this sort have a rapid-fire mode, which I used to take a series of 15 1/15 second exposures like the one at left. Now each one of those is dark and pretty grainy, but crucially, each one is a picture of the same planet, only with some different random-ish pixel noise and offset by random amounts due to jitter. Offsets are easy to take out by hand, although you could also write an image correlation algorithm to do the work if you want to do this a lot. Then it's just a matter of combining all those images in such a way that the noise averages out, leaving a clearer picture of the planet.
For now let's continue working in the Gimp. I can load up each image as a fresh layer in a single file. To line them up, what I find works best is to set one layer to divide the one below it; slewing the top layer around until the result is as close to 1 (white) as possible does the trick very sensitively. This would only work for a large, bright object like a planet, however.
The Gimp doesn't have a concept of averaging many layers, but you can fake it with transparency blending. The average of many layers is an image where each layer contributes equally. So from the bottom upwards, I set the opacity to the series, 1, 1/2, 1/3, 1/4, and so on, so at each level the resulting image is an equal combnation of those lower in the stack. The final result has enough color information that it's actually worthwhile to play with the transfer function to enhance Jupiter's banding, and the final result is below. Whee!
Stack of 15 1/15-second exposures of Jupiter through the 10-inch refractor, color enhanced to emphasize the cloud bands. There is a moon just coming out of eclipse at lower right, but it is totally invisible with this transfer function. I could alternately have added the layers to build up enough photons to clearly show Europa, at the expense of washing out the planet. There is a hint of a dark spot on Jupiter's lower left limb, which is right where Europa's shadow ought to be.