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What is plate solving and what does it have to do with Astrophotography?
Simply put, plate solving is when a computer looks at an astro photo and figures out what's in the photo. There. Now you know. But why is it called "Plate Solving" and why is it so critical to astrophotography? First, a little history.
Back in The Day, photographic plates were made of glass with lots of chemicals on them. Those chemicals were exposed to light and then processed in other chemicals to make glass plates that had either positive or negative images on them. It was the precursor to film. You can read more about it here, because this isn't an actual history article. Suffice it to say that these plates were used to take the first photographs of astronomical objects.
Back then, people used to have to look at those photographic plates, do some careful measurements with calipers and stuff, and then do a lot of math. In the end, they would be able to compare the "plate" to a "solution" of what they were expecting that the telescope was looking at when the image was taken. This "solved" the plate and it was marked as aiming at a particualr set of celestial coordinates, which might have been a little off from where the telescope thought it was pointing at the time. This was called Plate Solving.
Today, we do all of that with computers and massive surveys we've taken of the skies so we know pretty much where all the stars are that will show up on a photograph. Plate solving literally takes seconds now, and can be done on your home computer. This is where software such as ASTAP comes in. It can solve an image so fast that it is routinely used as part of the general astrophotography workflow while capturing images.
Old time plate solving was so that the plate could be cataloged and future researchers would know what was on it. Modern plate solving is so that a computer knows where the telscope was looking when it took the picture. This has two major uses: