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RE: [ccd-newastro] Re: Help -- New to RGB

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  • Wodaski Yahoo
    I like to always take at least 8 darks and combine them using Russ Croman s RC-Astro Console. But a median combine if you have cosmic ray hits will be good
    Message 1 of 7 , May 2, 2004
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      I like to always take at least 8 darks and combine them using Russ Croman's
      RC-Astro Console. But a median combine if you have cosmic ray hits will be
      good enough for most.

      Noise adds in from every source, including dark and bias frames. So if you
      scale darks, you will have two additional noise sources: the net noise of
      the combined bias frames, and the noise from scaling (scaling will never be
      perfectly accurate, as the thermal current is never perfectly linear). Both
      are small, but noticeable. So the ideal is to take darks of the same
      duration. But I would test scaling to see if you are satisfied with it; the
      difference is usually small if you follow the rule below.

      Ron's Rule for darks: take at least as many dark frames as light frames and
      combine.

      Corollary #1: Take as many bias as darks if scaling.

      Corollary #2: The darks for scaling must be as long as the longest light
      frame. Due to the nonlinearity of dark current, I don't recommend taking
      extra-long darks when scaling.


      Ron Wodaski
      The New CCD Astronomy
      http://www.newastro.com/ipb

      -----Original Message-----
      From: ralldredge2001 [mailto:ralldredge@...]
      Sent: Sunday, May 02, 2004 11:12 AM
      To: ccd-newastro@yahoogroups.com
      Subject: [ccd-newastro] Re: Help -- New to RGB

      Thanks for your advice. It sounds like modifying exposure length is
      the way to go. Do you just take a humongous number of darks -- or
      is it possible to scale the darks using bias frames? And how well
      does this work?
      --- In ccd-newastro@yahoogroups.com, "Wodaski Yahoo" <yahoo@n...>
      wrote:
      > If you want to do it numerically, you would need to do it in MaxIm
      DL where
      > you can see the numbers. The problem is that it involves
      iteration. First
      > you clean up backgrounds, then scale the data, then you clean up
      the
      > backgrounds because the scaling affects the background levels as
      well as the
      > foreground, and so on. I find it simpler to do this visually in
      Photoshop.
      >
      > In my classes, I recommend against shooting color at the same
      exposure times
      > because of some problems that can occur. I'll try my best to
      explain without
      > the advantage of illustrations. <G>
      >
      > There are two things to aim for when imaging color: using exposure
      times
      > that will make sure of accurate color in the bright objects, and
      having
      > enough total image time for each color to provide similar noise
      levels in
      > the dimmest areas (dark zone).
      >
      > When you scale an image, you are scaling everything in the image -
      both the
      > signal and the noise. If you increase the signal, you must also
      increase the
      > noise (the signal to noise ratio remains constant when you scale
      the data).
      > If you scale, for example, the blue image, then the background of
      the blue
      > image will be noisier that when it started out.
      >
      > This is why I recommend against scaling; it creates problems for
      overall
      > color balance. Your stars and galaxy cores may have accurate color
      after
      > scaling, but the dimmer portions of the image suffer from color
      noise and
      > this prevents you from displaying the Dim data effectively.
      >
      > The normalization technique outlined in the first book will clean
      up
      > backgrounds. If you then scale, the backgrounds will be different
      again, and
      > must be renormalized. This can also be done numerically in MaxIm
      using the
      > same technique. But renormalizing may change the color balance,
      and require
      > another round of scaling....you get the idea. You continue until
      the changes
      > are small enough not to matter, iterating toward a good balance in
      > foreground and background. Thus simple ratios are difficult to
      achieve -
      > hence my recommendation to do the balancing in Photoshop.
      >
      > It's not in the first draft of the book, but consider displaying
      the
      > Histogram palette, and turn on display of the separate histograms
      for all
      > channels. That will give you the ability to get numbers - pass the
      mouse
      > over the Histogram palette for each color, and you will get a
      numeric
      > display of where you are in the histogram. This might help you
      integrate
      > both techniques at the same time. The next draft of the color
      balance
      > chapter will include details on this technique.
      >
      >
      > Ron Wodaski
      > The New CCD Astronomy
      > http://www.newastro.com/ipb
      >
      > -----Original Message-----
      > From: ralldredge2001 [mailto:ralldredge@p...]
      > Sent: Saturday, May 01, 2004 8:02 PM
      > To: ccd-newastro@yahoogroups.com
      > Subject: [ccd-newastro] Re: Help -- New to RGB
      >
      > Actually, I have bought your new book and have downloaded your
      > current chapters. Your chapter on color balance does give me the
      > basics ---- But, I was hoping for a more mathematical way to do to
      > first color combine, especially when a typical Don Goldman ratio
      is
      > something like .92, 1, 1.71. This means, I think, that I must add
      a
      > lot of blue. Adjusting the histograms using the mid-point slider
      by
      > eye can give me a pleasing picture. But it is clear to me that my
      > eye is not yours.
      >
      > --- In ccd-newastro@yahoogroups.com, "Wodaski Yahoo" <yahoo@n...>
      > wrote:
      > > You need my new book. There is a full chapter on color balancing
      in
      > > Photoshop. The book is incomplete, but you can pre-order it and
      > get access
      > > to the content online as I write it. Please see:
      > >
      > > http://www.newastro.com/ipb
      > >
      > > Ron Wodaski
      > > The New CCD Astronomy
      > > http://www.newastro.com/ipb
      > >
      > > -----Original Message-----
      > > From: ralldredge2001 [mailto:ralldredge@p...]
      > > Sent: Saturday, May 01, 2004 11:18 AM
      > > To: ccd-newastro@yahoogroups.com
      > > Subject: [ccd-newastro] Help -- New to RGB
      > >
      > > I am new to RGB and have just taken some images. I assumed that
      > > equal exposure times would be the way to go, as I could combine
      > them
      > > in CCDSoft and would only need one set of darks.
      > >
      > > Sure enough I could combine them in CCDSoft, but they were
      > galaxies
      > > and the core of all of them is burnt out when using any of the
      > > histogram settings which seem available through the color
      combine
      > > command. Frankly trying to maintain color balance while using
      > some
      > > set of curves in CCDSoft exceeded my abilities.
      > >
      > > What I really want to do is use Photoshop CS. I now can color
      > > combine using 16 bits, and do curves and levels. However, I
      > cannot
      > > figure out how to add the RGB weighting. I can wing it using
      the
      > > color balance commands, but it seems a shame that Don Goldman's
      > RGB
      > > combine program will give me ratios to two or three decimal
      places
      > > and I just have to guess in Photoshop. Am I missing something?
      > Is
      > > there a way to do this?
      > >
      > >
      > >
      > >
      > >
      > > Yahoo! Groups Links
      > >
      > >
      > >
      > >
      > >
      > >
      > >
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      >
      >
      >
      >
      >
      > Yahoo! Groups Links
      >
      >
      >
      >
      >
      >
      >
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