While the growing number of satellites above our heads is a concern, there are ways to mitigate their appearances in deep-sky astrophotography.
Unless you live in a digital cave, you’ve seen all the excitement — and no small amount of angst — about SpaceX's recent launch to kick off their Starlink satellite constellation. Both the International Astronomical Union (IAU) and the International Dark Sky Association (IDA) have issued statements about it, and amateur and professional astronomers alike are justifiably worried about the possible loss of the night sky. The satellites pose a threat to visual and radio astronomy, wide field astrophotography, and the pristine night sky, which is already very difficult to find anymore.
These are not trivial concerns, but in at least one respect, there's a way to deal with the problem. I'd like to tackle how to deal with satellite trails in your deep-sky astrophotos. Optics and cameras designed to pick up all those faint fuzzies in the sky will certainly pickup satellites fainter than the eye can see. Satellite trails don't actually ruin your deep-sky images, but it'll take you a few steps to remove them. But don't worry, you won't be spending hours in Photoshop tediously cloning them out!
At a star party once, a friend came by and told me he was using my new camera plug-in, and there might be a bug in it. All the images of Orion have artifacts in them. Little straight lines all over the place like bad columns, but in each image they were changing. “Those are satellites,” I told him. The Orion Nebula is smack-dab in the middle of where most geosynchronous satellites orbit. They're particularly visible in the early evening during late winter, when Orion is crossing the meridian at sundown. In the summertime, all those geosynchronous satellites are in full sunlight.
Sometimes it's next to impossible to avoid recording something unwanted in a frame. I get asked all the time if I’ve ever seen a UFO. Nope. Trust me, if UFO’s were buzzing us, we’d have solid photographic evidence of them by now. I’d have photographic evidence by now. No, I’ve never captured a UFO, but I've gotten lots of satellites and airplanes!
Oh airplanes, we HAVE airplanes! All those images of an airplane flying in front of the Moon? Great, but honestly it’s not that rare. Airplanes fly through my field of view all the time, and they are much bigger and more problematic than satellites by far. But do they ruin my exposure? Certainly not!
Remember when I talked about stacking? We don’t take a single hours-long exposure; instead, we take many shorter exposures and combine them into that final image. Anyone who throws away an exposure in their stack because it has a satellite or airplane trail going through it is throwing away perfectly good data.
How to Remove Satellite Tracks
Almost every modern astronomical post-processing program has a rejection process (sometimes referred to as sigma-reject) to remove unwanted signals, though the exact sequence will depend on which program you use.
The way this process works is that, while averaging all of the pixels in a series of, say, 10 images, the program mathematically calculates which pixels fall far away from the mean value because they're much brighter (or much fainter) compared to the same pixels in other frames. The algorithm then discards those out-of-range pixel values so they don’t affect the final image. This process easily removes satellites, airplanes, and UFOs from your final stacked image.
Some programs, including PixInsight, even create a "rejection map" that displays only the outlier pixels that were not used. If you take a look at this image, you’ll see cosmic ray hits, a single slight tracking error or bump, the occasional airplane, and (increasingly) many satellite trails.
For deep-sky astrophotography, the number of satellites is about to skyrocket, and SpaceX is only one player that will be contributing to this. Thankfully, for this issue at least, nothing about how I practice deep-sky astrophotography needs to change.