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RE: [APML] PixInsight questions ( was:Two New Images)
>I have one more question and it pertains to using DBE. I have a very difficult >time distinguishing what is and what is not faint nebulosity (i.e. nebulosity >just above the background noise). Will DBE remove the really faint stuff if >you do not remove an auto sample point embedded within it?
I'm affraid that you are right. If you put a sample above real features, like faint nebulosities, they will be removed from the image if you substract the model. When dividin, it is a bit more complex, but I'm sure that it will be very difficult to recover that info later.
>For me, it's really difficult to distinguish the faint nebulosity from the >background noise. Does anybody have any special techniques?
Well, I ussually put the samples manually, in those places I'm sure that are not nebulosoties. You don't need as many samples... and with the symetries you can help things a lot.
If the object is very large, or the image is really deep, so you don't know if there is something or not, there are two ways to determine the proper locations: first, and the most obvious, see a pic from somebody else, and try to locate pure background sky in there. If you can't find a pic good enought, there is a "divide and conquer" method described by Wei-Hao...
Here is what you have to do: put manual samples whre you think that aren't nebulosities, and generate the model. Before pressing "Done" (cancelling the DBE instance), create a new process icon with it. Now, apply the model. Work with the image in a very rough way, trying only to bring the faint stuff out. Now open the DBE instance saved in the process icon and inspect the image. Refine the locations of the samples, trying to avoid the faint stuff, now uncovered. Save the DBE instance, and then go back to the original image and apply the new process icon. Now you'll have a better model. You can try to make it as an iterative method to make things better.
Now, regarding the brightness of the background, make sure that the "Rescale" boxes are always unchecked (when dividing), the middtones balance and normalization (the background's median after the gama)values are correct. If everything is fine, as I said before, the outern regions of the image will become lighter, while in the middle it will become darker.
Regards,
Carlos Milovic F.
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