thanks for looking tom
I'm no expert at this type of imaging, but it seems plausible to me that the source of the dark area could be related to the recombination of the star image and the comet image. DSS does this automatically, so I don't know the exact methods it uses, but having attempted a few of these in PixInsight, you can tackle the issue in multiple ways, but essentially the comet has to be subtracted from the image aligned on the stars, and then this is added to a starless comet aligned image. For small comets this is pretty easy, but for a complex comet like NEOWISE, with such a large and bright tail, this is much trickier. There can be remnants left behind, and I can easily envision a scenario in which the final result has some strange areas directly around the comet. As it turns out, I was imaging on the same day as you, only about 25 minutes later, and only 100 miles away, at Mt. Laguna, CA. I posted a raw sub in one of my posts, linked below, and again, there is no shadow near the nucleus.
But regardless of what the dark area is, nice images of the comet!
Edited by Tom Glenn, 04 August 2020 - 09:42 PM.