Help requested with DBE and a nasty gradient

[frugal]

I have been trying to process some of the shots I took last weekend, and I am having a real problem getting rid of the gradient in the image. I think that the gradient is a very thin ice haze that became less apparent as I got higher up towards the zenith, it certainly wasn't light pollution, we were 50km away from the next human being

I have tried a number of different attempts at using various combinations of DBE and ABE. I have followed a number of tutorials; i have tried to vary the tolerances; I have tried different placements of sample points (perfect grids across the whole image, just on the light areas, just on the transitions etc). No matter what I do I still get a strong yellow/orange glow in the bottom right corner. I have tried multiple iterations of DBE to try and counter the yellow glow, but to no avail.

The best I have managed so far is this: 

Untouched image at the top and my best attempt at DBE at the bottom. Can anyone give me any hints on how to improve the background?

Also, is it just me, or does the noise from the DSLR become a lot more apparent after using DBE/ABE? Is it just an artifact of a larger stretch when the gradient has been removed? Or is it actually impacting the underlying data in a harmful way?

[Daniel-K]

a bit more carefull placement of the X's and it will get rid of it all for you, have you followed harrys tuts? also gradient exteminter in PS gets rid of most it

[frugal]

So where is the best place to put the X's, is it better to put them primarily on the areas you want to get rid of, or is it better to put them evenly over the whole image except for bits you explicitly do not want to touch (nebula, galaxies etc).

The tutorials all discuss the need to place the samples to get a good result, but they do not seem to go into any depth as to tactics for the actual placement. If you have an image like this one where you need to set the tolerances quite high to get the bottom right to show as good samples, are you better off doing multiple passes of DBE, concentrating solely on that area to try and even it out a bit in order for subsequent passes to work with lower tolerances?

[ollypenrice]

Remember what DBE is trying to do. It's trying to model the gradient in your image without accidentally modelling the image itself. For this reason a small number of well placed markers beats a large number, which will start to pick up the details. You need to give it a 'broad brush' notion of the form of the gradient.

You have an image with lots of background sky so this should be perfectly possible. I'd aim for maybe 5 markers on the long side by 4 on the short. (Be sure to have cropped off all edge artefacts before opening in DBE. This is vital or it may 'read' them.) Clearly the corners need to be marked as do the edges, then I'd place the rest symmetrically. In placing them be sure to avoid stars or nebulosity or they will tell the programme that the sky is bright at that point. Set a fairly high tolerance and look at the model first. Does it look like the gradient? If so try it for real. If you need to raise the tolerances, do so.

Now for a heresy for which I might be burned by the PI Inquisitors; if you can't get rid of that bad corner with a reasonable dose of high tolerance, save the best you can get. Then go Berserck with the tolerance and bludgeon it into submission. Save that. It will probably have harmed other parts of the image.

Then go into Photoshop (will this get past the word filter???  ) and paste the first image with residual gradient onto the bludgeoned one. Use the colour select tool to select the gradient affected background sky and erase it. Flatten and (maybe!) rejoice.

Olly

[Physicist13]

PS !!!! how very dare you!!!!

[IanL]

It may be obvious, and you may have tried it already, but once you have a satisfactory DBE model that matches the background well, try applying it twice in succession.  I often find the first attempt will leave a residual gradient, but a second application cleans it up.  (Conversely, sometimes you end up with a colour cast appearing in the already cleaned up areas that is the inverse of the remaining gradient - you may get lucky or you may not).

Failing that, if you want to avoid being drummed out of the eternal cult of PI you could just crop the image - plenty of the interesting stuff in the top left 3/4 of the frame.

[frugal]

I ended up doing a combination of things. I played with Tolerance, Shadows, Smoothness and placements. Trying to apply DBE multiple times ended up with a horrible purple cast to the corner instead I got it a lot better, but I still needed to crop out a fair bit of the bottom right hand corner.