Persistent gradient in corners

[BMartin]

I would welcome some wisdom here!  Attached is a shot of M45 with a persistent glow in the upper corners.  I have tried a number of passes with DBE but still can't get rid of the glow.  Any tips or tricks?  Image was taken with a Canon 1100D DSLR.

Many thanks!
 

[pfile]

you should be able to get most of this with DBE, but is this image cropped from a larger image and did you use flats when calibrating the subs?

rob

[BMartin]

Hi Rob

Yep - it is cropped but only after I had calibrated the entire image.  After cropping I then applied DBE several times.

Brian

[pfile]

i guess what i am asking is if there are matching gradients in the bottom two corners (which have been cropped off). this can be the result of bad flattening, so if that's the case it's better to sort that out rather than try to fix the problem with DBE.

rob

[BMartin]

I suspect that it is flat problems - there is pretty severe vignetting in the system and it seems that my flats did not fully account for that.  If I stay away from the edges DBE seems to work OK.  PI is great but you can't make a silk purse from a sow's ear!  Thanks for your comments.

Brian

[Nocturnal]

Hi,

DBE isn't really meant to deal with flattening issues unless you let it do multiplicative (divisive) correction. Rob is right that the real solution is to make your flattening work properly. Most likely your darks don't match your lights or your flat-darks don't match your flats.

  Sander

[avarakin]

You can try to increase the tolerance from default 0.5 to 2.0, it should help.

Alex