Hi all - Now that I have a cooled OSC camera and a filter wheel with IR cut, Ha and Dark filters I'm taking flats indoors the day after a session. I typically take flats with 2 layers of white T-shirt and an iPad on a white page. I point the scope at the ceiling with the iPad balanced on top of it.
Here's a screenshot of the stretched flat fits file created in the finder by the Mac Observatory app with crazy colors. I am surprised to see that there's an inverse vignette - the corners are brighter than the center. When I view the file inside Observatory, it is a more even gray but still lighter at the corners. When I apply this flat to the image I get dark vignetting at the corners.
I'm going to take the filter wheel apart and see if there are any light leaks around the filter (since it's an APS-C clip in an adapter ring). Any thoughts on what else could be causing this would be appreciated.