Flats mess

This looks like the flats are not matching the lights basically. Are you correcting the flats with a proper darkflat or bias (bias being 0.5 seconds of exposure, not much shorter)? What is the offset of both the lights and the flats?

Thanks for the reply. 

As I said, the camera is an ASI 2600 MC P. I used "highest dynamic range" which sets the offset to 50. The lights and the flats were shot at that setting. With that said, I used 30,000 ADU as the target and with my present T shirt thickness, the flat exposure was about 6 seconds. Maybe I need to re-shoot at 2 seconds (take a layer out of the t shirts).

I haven't done the bias frames yet as I didn't want to take the train apart before I figured out the issue with the flats.

Thanks again

Lynn

Well, 6 seconds can be fine if that produces proper flats, target seems good though I would aim for higher, just making sure not to clip on the right-hand side. You do need to calibrate the flats with a bias or dark-flat though for it to work nicely.

High again,

So, I reshot the flats at 2.1 seconds and 25,000 ADU. Then I took the train apart and using SGP wizard, I shot 100 bias frames. I dumped the lights (two sessions) the flats and the bias frames into APP and let it process. Came out great, the big halo was gone. I'm not sure if the shorter flats or the bias frames fixed it, but I suspect it was correctly shot bias frames that made the difference.

From your comment above, are you saying that the flat ADU should be higher? If so, what would be a good target?

Thanks

I think it more likely was the bias yes. Glad it worked!

Regarding flats, yes it's better to go for an ADU which is more to the right of the histogram (just make sure it doesn't clip). So 60-70% may work better, especially when you're taking flats at gain 0, for maximum dynamic range. This will ensure that there's way less chance of an issue at the lower end of the signal.