Vignetting is horrible

[psy1280]

I'm new to EAA and still getting my feet wet. And still trying not to get too discouraged.

I'm assuming this is a terrible vignetting problem, but I don't know what I can do about it. Flats are supposed to help/eliminate vignetting (my flat taken with SharpCap and a light-pad, had a histogram that went from 25-75%, which I thought was okay--I took 20 frames), but obviously, my vignetting is severe. Also, a side note, I think my stars need to be crisper, i.e., either more in focus or perhaps a collimation issue.

Equiptment:
I'm using my Evo 8" SCT with a f/6 focal reducer along with a ASI294 MC Pro + and optolong pro LP filter. I recently purchased a light-pad to take a flat before proceeding. My flat (using SharpCap) had a histogram that went from 25-75%, which I thought was okay and took 20 frames. My camera was cooled to about -15 degrees. This image with the HORRIBLE vignetting was a LIveStack image (250 gain with 30 second exposures).

If anyone can suggest a path forward, I would be most appreciative.

Joe

[admin]

Hi,

are you using darks as well as flats? If not then you really need to as you can get over or under correction when you try to apply flat correction's without a corresponding dark frame. You should also let SharpCap capture bias frames as part of the flat creation process which serves an equivalent purpose in making the flat frame. This is especially critical for deep sky imaging where the stretching that we apply to the final image magnifies any inaccuracy in flat frame application.

Cheers, Robin

[psy1280]

Thank you so much Robin.

That was exactly what I needed to hear, i.e., a path forward. I look forward to implementing your suggestions.

Joe

[goldtr8]

Are you using a 1 1/4 T adapter visual back for the camera? If yes you will not be able to solve the issue you need to get to a 2" diameter or you wont be able to fix this issue. I also have an 8 in SCT and I learned this very early on.

Also with the ZWO 294 camera it has been stated that when using SharpCap do not use bias frames enabled with the flats. I was having issues earlier this year until I found this out.

So dark's are very important and flats with bias turned off and also monochrome for the Flats. Since you have a cooled camera I would do a flat library

I believe that the bias issue was reported on the ZWO forums. I did not research any further once I tried it and my flat subtraction started to work again.

[donstim]

I have a 1-1/4 inch T adapter and visual back on my 8" Celestron SCT and with proper flats and darks, it completely addresses vignetting.