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In January 2022 we were visited by the relatively bright Comet Leonard. I had always wanted to image comets, but I was stymied because comets move relative to the background stars! If you take an exposure that is long enough to capture detail in the comet's tail, for example, then the comet will move while the exposure happens. My telescope mount is clever enough to adjust for the movement of the earth, so the stars appear stationary. But it is nowhere near clever enough to adjust for the movement of the comet.

Christian Sasse is a professional astronomer and astrophotographer par excellence, and he helped me learn how to solve this challenge in one of his astrophotography webinars. The trick is to take relatively short exposures, then combine them digitally. Normally, when you combine images in this way, you use the background stars to align the images. But for comets, the idea is to start with the background stars, then align the comet nucleus. This image is made up of five exposures in three channels (RGB). Since the images were taken consecutively, the software can calculate the speed of the comet nucleus and align the images such that the comet appears stationary and the stars actually drift!