Flat too strong

[astronono]

Hello,

This is my first post.
I have an issue with preprocessing images, my flat seems to be too strong and is over correcting the image

I have checked that the offset has been substracted to flat before dividing to images, no idea where it is coming from.


Raw image

Masterflat

Image corrected


I still see rounds (over the small galaxy) which have been kind of overcorrected. Any ideas ?

FIT sources if needed
Raw Image  www.astronono.fr/images/PI/M100-005L.fit
Master flat   www.astronono.fr/images/PI/MASTERFLAT.fit
Master offset  www.astronono.fr/images/PI/MASTEROFFSET-BIN2.fit
Master dark www.astronono.fr/images/PI/MASTERDARK-M15-600S-BIN2.fit
Image corrected with bias/dark/flat  www.astronono.fr/images/PI/M100-005L_c.fit

[bitli]

Hello,
I have made a quick test, I think you'calibrate the dark twice (once explicitly, and once in the II of the lights)..  I get better result by selecting the master bias, dark and flat, but using NO calibrate option.  If you look at the histograms of the bias and dark, you will see that the dark is already much darker than the bias, so it was calibrated.

So was the master flat as you mentioned.

The optimize makes no visible difference in this case (factor around 1.01).

The result is not perfect, but well within the reach of a DBE I think, if you increase the tolerance and make sure to stay away of the galaxie with the sample.

-- bitli
 

[bitli]

In fact I had a slightly better result by adding 0.17 to the flat (found by experimentation) before using it with the image integration. At some point you get better rejection of the gradient but less rejection of some other irregularities. 
-- bitli

[astronono]

hello,

thanks for you lights, in fact, there is a problem with flats generated with skyflat assistant, I had two sets of flats and I've been able to reprocess using "legacy" flat method.



cheers,
bruno