How to Process Planetary Images
A premier planetary photographer shares his secrets for capturing the finest details on our neighboring worlds.
Over the past five years, a tremendous resurgence in amateur planetary astronomy has taken place. This is due in large part to the simple, inexpensive webcam, which has enabled amateurs armed with modest telescopes to produce images of the planets that rival those captured with large professional instruments (S&T: October 2005, page 115). The secret of the webcam’s success is that it can produce images with very short exposure times, perhaps one-tenth of the durations required with conventional cooled CCD cameras. This means that webcams can “beat the seeing” capture sharp images during fleeting moments of atmospheric tranquility.
Unfortunately, there’s a downside. Webcam frames are noisy when compared to those from cooled astronomical CCD cameras. But because signal increases directly with exposure, whereas noise increases only as the square root of exposure, you can stack lots of frames to produce an image with a much higher signal-to-noise ratio than that of any individual frame.
It typically takes hundreds or thousands of webcam images stacked together to achieve an acceptable result. And not every frame you record will be worth keeping, so before you stack images you need to weed out the blurry ones. Sorting and stacking thousands of images manually would be about as much fun as undergoing root-canal surgery! Thankfully, there are computer programs available today that will do this job automatically.
The primary goal of planetary imaging is to tease out as much fine detail as possible without introducing spurious artifacts. Careful image processing using the tools of the “digital darkroom” is every bit as important as acquiring high-quality webcam frames in the first place if you want to produce highly detailed and scientifically useful images. I frequently employ several different programs to process my images, because no single program yet contains all the tools I find necessary.