Flats not correcting

[Forum_2017]

Can't figure out what's happening but for some reason my flats just wont correct my image. They were shot at 30000 ADU, along with 10 min dark frames. Below is the image after subtraction along with the master flat. You can see its not corrected the image at all' I'm using AA6 QHY OSC 10 ccd
Does anyone have any suggestions as to what is happening

[Forum_2017]

Did you take/use bias ?
They should be used for flats to work.

[Forum_2017]

Yep used Bias frames, even tried the old method of making a master flat then shifting the Bayer matrix. I just can't get it to work

[Forum_2017]

What is the exposure time of the flat?
If it's longer than 1-2 seconds you should use a real dark frame for it, a bias is not enough.

[Forum_2017]

Hi facavi the flats are 0.61 sec , 10 min darks , 0.01 sec bias and of course 10 min lights

[Forum_2017]

Using biases is a bad habit, but probably a bias 0.01 sec is almost equal to a dark of 0.61 sec, so the error must be another.
You wrote "subtraction" with the master field, probably you meant "division".
To find out the error you should post the images, or a detailed list of all the steps done to produce the final image (in this case don't use "masters" just try with a single image, single dark, single flat, single flat dark, without anything else).

[Forum_2017]

I don't get back home till Friday so will post then . I have tried without using a biase frames and also tried doing them using single images as well. I will post images of what I'm doing so please keep a look out for my post . Hopefully I'll post them Friday if not Saturday depending on how tired I am . Many thanks

[Forum_2017]

OK Facaci
here are my screen shots I've also tried with just one image of each and I get the same results

[Forum_2017]

Hi Mark,
the preprocessing seems correct. "Median" for darks and flats usually gives worse results but here it does not matter.
I guess that the flats were taken at -20 too.

I was not clear, I meant just one image, but proceeding by hand. So open a single image, open a dark, subtract the dark from the image, etc etc. This will probably reveal the problem. Check also: What is the result without flats? What does a flat look like?

If it still does not solve, post or send me by email one image/flat/dark/flatdark in FITS format.

[Forum_2017]

Hi facavi
I've sent you an email x4 with images attached. I've tried just stacking the lights but I still got the same results. Not sure how to do it manually hence why I've sent you the images
Cheers