Noise with STF stretch after DBE...

[footbag]

I'm running into the same issue that I was before.  I've been told a lot of it has to do with my LP, but I still need to find a way to mitigate it. 

My problem is that when I do a DBE (or HDRMWT) and then use the STF tool, I get a noisy mess.  Initially, I was thinking that I needed to be more delicate with the DBE.  I'm not so sure that's the correct approach.  I can manually set the levels of the STF tool.  This brings the image back to somewhat normal looking.  In other words, STF is overstretching after DBE and some other processes. 

But, I don't know if that means I cannot use the STF tool to generate the Histogram.  Maybe that doesn't matter. 

I have attached an image of what the STF does. 

My two questions are is it normal and how do I deal with it?

Thanks,
Adam

[Carlos Milovic]

Yes, this is normal. STF is meant for image analysis, not to get pleasant results. If you like what you see, and would like to apply it as an histogram transform, excellent! but this is not the purpose of the tool.
Having said that, you should check the DBE model/samples. Stay away from the galaxy, and double check that stars are not affecting the statistics, introducing weird oscillations or halos. A few well placed sampled do a much better work that a crowed image with poor samples.

[pfile]

yes, this does not answer your actual question but i think if you carefully inspect the background model extracted by DBE, it's going to be far too lumpy. check your samples and as carlos says, move them off of any stars. while the brightest part of the star is rejected by the algorithm, the 'halo' around the star can be picked up and the tool thinks it's part of a gradient. same goes for the outer arms of the galaxy. what you can do is re-place the samples on your DBE'd image - it's a lot easier to see where there is galaxy signal in that image - and then save the process icon and with the original image in the foreground, close DBE and then double-click the process icon. that will transfer the samples to the original image, and you can try again with confidence that you haven't picked part of the galaxy by accident.

i think i said this on CN but when you are shooting in LP skies, your background is composed of the unwanted signal of the skyglow. when you subtract a smooth model from that signal, you're pretty much left with the noise. you can't really see the noise until you subtract the background because it's a small percentage of the signal that's there. but remove the signal and suddenly the noise jumps right out at you.

when i shot m101 from my home, i had to put in 30 hours to get something that looked even remotely okay, and i still had to do a lot of NR on the background, and kind of clip it off.

rob

[footbag]

Thanks Rob.  You did say that about LP on CN.  I understand it, just needed to figure out a way to mitigate it.

It does seem like I was a bit too aggressive with my DBE points.  Fewer, but better seemed to improve my results a bit.  I won't get a chance to reprocess until later, but I think I'm getting somewhere. 

The background is a bit lumpy and some dust donughts didn't calibrate out.  I may be rethinking my flat approach.  It's possible they are introducing some noise as well. 

[cdesselles]

i think i said this on CN but when you are shooting in LP skies, your background is composed of the unwanted signal of the skyglow. when you subtract a smooth model from that signal, you're pretty much left with the noise. you can't really see the noise until you subtract the background because it's a small percentage of the signal that's there. but remove the signal and suddenly the noise jumps right out at you.


rob

so would it be OK to run DBE twice? once to remove the LP, then again on the resulting image to get the noise?

[pfile]

yes i frequently run DBE twice but DBE won't get rid of the background shot noise no matter what you do… it might get rid of large-scale color "noise" but that's not the same thing.

rob

[cdesselles]

yes i frequently run DBE twice but DBE won't get rid of the background shot noise no matter what you do… it might get rid of large-scale color "noise" but that's not the same thing.

rob

So how would you deal with the issue outlined above? 

[Carlos Milovic]

To remove noise you should use any of the available noise reduction tools, like TGVDenoise, ACDNR, MMT, ATWT, etc.