Banding in images only when using flats, ideas?

[Trippelforge]

I need to post the example photo's but it seems like my son didn't shoot them over like he was supposed to. I still figured I might as well ask as you guys probably get what I mean. When I stacked over the weekend (240x30 darks) the image came out with some dust spots and a few other artifacts. Outside of that it seemed fine otherwise. I took a set of flats (30), and when I added them into the process it created horizontal banding across the entire image. One thing that I did screw up on is taking them at a bit higher ISO. As I was trying to center on the histogram and had to set it a bit higher to move it over. When I believe I was supposed to do so via exposure time. 

So that might be the simple answer, but I wanted to check if anyone had anymore insight on possible causes. 

Side question on flats; I am obviously new to using them, and was curious HOW people are taking them with fading light. IF I do have to increase exposure time I notice a drop in the histogram, each shot moving a tiny bit over. However I can't take it earlier due to needed to check focus on a star. And I can't take it after as I don't shoot until sunrise. 

Thanks for the help!

[Stefan73]

I think you need an artificial source of light... I've got an app on a tablet which just lights it up with a white light as my telescope isn't too wide.  Otherwise you can get light pads which are A3 size or bigger or there are dedicated flat field generators.

Are you using bias or dark flats too?  As you say for lights everything should be the same except for the exposure time.

As you mention ISO are you using a DSLR?  Try losing the darks and just have bias and lights.  I never noticed much difference adding darks with my 550D.

 

[alacant]

1 hour ago, Trippelforge said:

240x30 darks

Dark frames of any sort on a dslr are bad news. Lose them.
Instead, calibrate simply by subtracting the offset. I think on a 5300 the bias is around 600 adu

1 hour ago, Trippelforge said:

HOW people are taking them

Use a light panel over the aperture of the telescope. Set the exposure to aperture priority and use sheets of paper to adjust the exposure to anything over 2s.

Cheers and HTH

**apologies to @Stefan73. Simultaneous post.

Edited February 13, 2023 by alacant