trouble with flats

[mkalika]

I've encountered a strange behavior and will appreciate your help.

I am trying to calibrate 18 M33 light frames [600sec X ISO1600] and getting radial gradient or vignetting when I apply master FLAT frame in ImageCalibration tool. If I uncheck FLAT frmaes in ImageCalibration, then all is fine and no radial glow or vignetting is visible.

Also, after applying STF to the original light frame, then no glow visible as well.

Below are few images to showcase what I mean.

What am I doing wrong please? Thank you!

[Alejandro Tombolini]

Hi mkalika, difficult to say with the screenshots, but your masterflat seems fine. May be is an effect that appears when applying ABE with samples over the galaxy. Try DBE with samples only in the background. 

Saludos, Alejandro.

[mkalika]

Thank you! I found what the issue is...

I used SGP for image acquisition and my images were set as FITS and CR2 format. Somehow, by mistake I mixed CR2 flats, darks and bias with CR2 light frames. Looks like CR2 & FIT combination is a root cause, because when I used all frame types as CR2 [I converted them to FITS in PI before pre-processing] then everything worked fine!

Here's what I've got so far:

[Alejandro Tombolini]

Perfect!, very nice result! The only suggestion would be that you try to correct the pink star using Repaired HSV Separation script at the beginning of the processing.  This may help.
Saludos, Alejandro.

[mkalika]

Thank you! I will look into it.

[mkalika]

Alejandro, thank you so much! I read the article you referred me, along with other material at http://pixinsight.com.ar. It is really gem! 

Here's what I came out with after reprocessing based on your recommendation: