Quote:
Originally Posted by gregbradley
Hi Marcus,
How do you open the files in PS if you don't scale the image in CCDstack?
Do you use FITs liberator?
Also I think using DDP in CCDstack - especially the auto setting, tends to clip the histogram a bit.
Greg.
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Yep - Fits Liberator.
Quote:
Originally Posted by gregbradley
I import my LUM as FITS (logarithmic) into PS. Same with Ha but yes, I use a 10nm filter and my subs are usually 30 mins at the longer FL. I generally apply 2 to 4 stretching curves layers (depending on signal level). I then apply noise reduction to a masked copy of the unstretched layer. For my RGB I may apply more stretching layers and sometimes do noise reduction the same way. I typically also blur it a bit (but not always). Layer masking is a must of course.
Hi Fred (nay Marcus),
How do you do your noise reduction with a layer mask?
Greg.
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Premise - don't stretch noise.
Of course using a layer mask is so you minimise the destruction of detail.
My recipe is pretty straightforward:
Duplicate the unstretched layer
Stretch it to taste (I use multiple curves adjustment layers)
Create a "flatten visible" layer by holding down the Alt key and selecting the menu option.
Copy this to the clipboard and delete that layer.
Zoom in on a dark area of the image to see the noise. Good to be looking at some higher S/N areas too to see the effect of the noise reduction on detail
Apply noise reduction to the duplicate unstretched layer so that all the noise is gone from the low S/N (dark) areas and most detail is retained.
Create a layer mask (hide all) to the noise reduced unstretched layer. The effect of the noise reduction will disappear
Paste the stretched image you copied earlier into the mask.
Invert the mask image. Now the whiter areas of the mask correspond to the darker (noiser) areas of the image so the noise reduction is only applied to those darker areas.
Depending on noise levels, image contrast and detail in the image, you may need to tweak the layer mask with levels or curves to make it more contrasty to maximise the filtering on the dark areas and minimise it on the lighter good S/N areas. Some blurring of the mask may also be wise
Blend the noise reduced level with the original if required.
I would've thought this is a pretty standard technique - so doesn't everyone do this?
Of course there's no substitute for good data. The effectiveness of any noise reduction technique will diminish as S/N decreases.
I find that doing this on the unstretched layer gives more latitude to fiddle the noise filtering tool in PS to best effect.
Cheers, Marcus