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Re: Q: Fixing background noise

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  • eja24601
    Thanks for the added tip, Randy! Neil, too! :) Eric ... with ... LRGB ... last ... the ... too ... the ... up ... use 1 . ... you ... your ... with ...
    Message 1 of 8 , Jun 1, 2006
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      Thanks for the added tip, Randy! Neil, too! :)

      Eric

      --- In ccd-newastro@yahoogroups.com, "Randy Nulman" <rj.nulman@...>
      wrote:
      >
      > Another thought after the gradients are removed (via Russ' program
      > or other techniques) when there is still some background noise...
      >
      > Use the "color range" tool and set the fuzziness so that the stars
      > and object are selected. Then, choose "inverse" so that only the
      > background is selected. Now you can do a gaussian blur to deal
      with
      > any remaining noise issues in the background, along with a slight
      > color desaturation to remove any remaining color mottling from the
      > background.
      >
      > This is the technique that I learned from Rob Gendler, and works
      > quite well as an addition to the tools offered by Russ (or if you
      > chose to just deal with the major gradients via PS techniques)
      >
      > Hope this helps,
      >
      > Randy Nulman
      >
      >
      >
      >
      > --- In ccd-newastro@yahoogroups.com, "Neil Fleming"
      > <neilfleming@> wrote:
      > >
      > > I do my best to balance and flatten the background gradients
      > during
      > > processing in CCDStack. I then do the technique below on the
      LRGB
      > after
      > > importing to Photoshop. Generally, I find it easiest to do that
      > way. That
      > > being said, even in Photoshop we have the ability to process out
      > color
      > > gradients and noise separately from that associated with the
      > Luminance.
      > >
      > > ...Neil
      > >
      > > >From: "eja24601" <eja24601@>
      > > >Reply-To: ccd-newastro@yahoogroups.com
      > > >To: ccd-newastro@yahoogroups.com
      > > >Subject: [ccd-newastro] Re: Q: Fixing background noise
      > > >Date: Thu, 01 Jun 2006 16:35:35 -0000
      > > >
      > > >Thanks a bunch, Neil! This is something to try for. Hopefully as
      > > >early as tonight, if I'm in the mood. I started from scratch
      last
      > > >night, and I have a more-stretched version sitting on my hard
      > drive
      > > >with all the ugly after-gradient swirls even more visible. This
      > > >should be fun to try your technique on.
      > > >
      > > >One question: do you perform this gradient removal on the
      > luminance
      > > >separately, or in the final LRGB? In my case, the gradient in
      the
      > > >color data was relatively easy to tame (a big surprise!), so I
      > only
      > > >applied the difference technique to the luminance data. I was
      > hoping
      > > >this would keep the background in the final image from getting
      too
      > > >dark, but that didn't work.
      > > >
      > > >Eric
      > > >
      > > >--- In ccd-newastro@yahoogroups.com, "Neil Fleming"
      > > ><neilfleming@> wrote:
      > > > >
      > > > > Eric, I've learned that same lesson. <g> So, instead of
      > creating
      > > >a
      > > > > difference layer that you use to subtract out the gradient,
      > here's
      > > >what you
      > > > > do:
      > > > >
      > > > > 1) Duplicate your image into a separate document, then
      > flatten it.
      > > > > 2) Use the clone tool to remove the brighter stars, and do
      the
      > > >best you can
      > > > > on removing the galaxy or nebula.
      > > > > 3) Use the "Noise/Median" filter set from 20-40 pixels in
      > order
      > > >to get a
      > > > > smooth replication of your background gradient. One lesson
      > I've
      > > >learned at
      > > > > this step is that the Noise/Median filter does a poor job at
      > the
      > > >edges - it
      > > > > doesn't treat them the same way as the middle part of the
      > image.
      > > >(You may
      > > > > have to add a temporary curves layer to boost the histogram
      up
      > to
      > > >see this.)
      > > > > So you may have to use the Smudge tool set fairly wide in
      > order
      > > >to "push"
      > > > > the correction out to the edges. You may also have to do a
      > > >blur/gaussian to
      > > > > smooth out the results.
      > > > >
      > > > > The goal of all of this is to create a separate document that
      > > >replicates the
      > > > > gradient in your image as closely as possible.
      > > > >
      > > > > 4) Remove the temporary curves layer in your gradient
      > document,
      > > >if you did
      > > > > one. You want a single layer.
      > > > > 5) Go back to your original image and make the main image
      > layer
      > > >active.
      > > > > 6) Go under "Image/Apply Image" and make the "source" image
      > the
      > > >one you
      > > > > created the gradient in.
      > > > > 7) Under "blending", select "subtract". For "scale"
      use '1'.
      > > >For offset,
      > > > > use something like '30'.
      > > > >
      > > > > This is the beauty of this approach. The 'offset' allows
      you
      > to
      > > >adjust the
      > > > > pedastal as you apply the gradient correction! Take a good
      > look
      > > >at the
      > > > > histogram of the result. (Don't forget to click on the
      > > >yellow 'yield' sign
      > > > > in order to ensure that the histogram is not cached.) If
      your
      > > >image is
      > > > > still too dark, use the appropirate history selection to go
      > back
      > > >to before
      > > > > the application of the gradient, and do it again - with a
      > larger
      > > >offset this
      > > > > time. If you resulting image was too light, try it again
      with
      > a
      > > >lower value
      > > > > for the offset.
      > > > >
      > > > > I have found this method to be the absolute best in gradient
      > > >corrections -
      > > > > bar none.
      > > > >
      > > > > Take a look at this sample of this approach:
      > > > > http://autostarsuite.net/photos/hoser/picture5123.aspx
      > > > >
      > > > > ...Neil
      > > > >
      > > > > >From: "eja24601" <eja24601@>
      > > > > >Reply-To: ccd-newastro@yahoogroups.com
      > > > > >To: ccd-newastro@yahoogroups.com
      > > > > >Subject: [ccd-newastro] Q: Fixing background noise
      > > > > >Date: Thu, 01 Jun 2006 12:48:59 -0000
      > > > > >
      > > > > >I have a couple of recent images taken from my light-
      polluted
      > > > > >backyard. In both images, there is a lot of background
      noise
      > that
      > > > > >manifests itself as a pattern of irregular, convoluted
      > variations
      > > >in
      > > > > >brightness. This is *after* gradient removal.
      > > > > >
      > > > > >In order to make the image halfway decent, my current
      > approach to
      > > > > >getting rid of this noise is the subtractive background
      mask:
      > > >remove
      > > > > >stars with Dust and Scratches, paint over the objects of
      > interest
      > > > > >with a suitable color picked from the background, blur the
      > > >remaining
      > > > > >image, and apply this to the original with a Difference
      blend
      > (all
      > > > > >in PS).
      > > > > >
      > > > > >This approach results in a very dark, not so natural-looking
      > > > > >background. It's nice and even, almost esthetic, but
      > technically
      > > > > >crude. I liken it to "Dancing with the Stars": the judges
      may
      > like
      > > > > >the performance, but criticize the technique.
      > > > > >
      > > > > >An example can be seen of my recently-posted M101 image
      > posted a
      > > >day
      > > > > >or two ago: http://members.nuvox.net/~on.aunt/DSO/M101.jpg
      > > > > >
      > > > > >I'm in search of a better way to get rid of my background
      > > > > >irregularities without clipping or darkening the image too
      > > >terribly.
      > > > > >Any tips/tricks?
      > > > > >
      > > > > >Thanks,
      > > > > >Eric
      > > > > >
      > > > > >
      > > > > >
      > > > > >
      > > > >
      > > >
      > > >
      > > >
      > > >
      > > >
      > >
      >
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