Flats for post processing

[gmessyhome]

Hi folks, i have a question on flats for post processing. I took a load of flats through a Celestron edge HD8, using ZWOASI533mc-pro. Using the option of a white T-Shirts. I seem to have got what one can describe as out of focus circles. I tried to focus the circles out however after go both ways on the focuser i could`nt get rid of the circles. has anyone else come across this problem. The mirror both primary and secondary are as clean as a whistle. It could very well be the t-shirt is not the best option for the camera/telescope combo i`m using.

I know the backfocus of the ASI is 55 and made sure it was at this distance. Kinda open to suggestions

Ive probably missed a simply trick.

Edited January 28, 2021 by gmessyhome

[happy-kat]

Hi, focus for flats is the same as the focus for the lights.

 

[Merlin66]

John,

The flats should be taken with exactly the same optical arrangement you use for    normal imaging. Don’t change focus or anything else.

The exposure should be the only variable.

The doughnuts you see are supposed to be there. They are the “shadows” caused by dust in the optical train and exactly what the flat is designed to remove.

[gmessyhome]

Thanks. After doing some more digging  on web. What you have mentioned is what i should have done. I`ll remember not to change anything next time.