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How to combine images from Red, Blue and Yellow filter?

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  • Mark Striebeck
    Hi, I am trying to use Stromgren filters for narrowband imaging of galaxies and stars. I am using a Ha filter, a Stromgren Yellow and a Stromgren Blue filter.
    Message 1 of 3 , Feb 2, 2015
      Hi,

      I am trying to use Stromgren filters for narrowband imaging of galaxies and stars. I am using a Ha filter, a Stromgren Yellow and a Stromgren Blue filter. I will use the Ha filter for Red.

      I was thinking that I first create a synthetic green image from the Blue and the Yellow filter. And then combine the Red, Blue and (synthetic) Green filters as usual. And then I have to determine the weights of the different colors (using a G2V star). This isn't my idea, but from Rob Hawley.

      Does this sound like the right approach? If yes, how would I create the synthetic green image? (I guess, I can't just subtract the yellow from the Blue...)

      Thanks
            MarkS
    • Ron Wodaski
      My first instinct would be simply to combine them as Ha/Y/B and see what you think. I don’t see that you have enough information to do a proper color
      Message 2 of 3 , Feb 2, 2015
        My first instinct would be simply to combine them as Ha/Y/B and see what you think. I don’t see that you have enough information to do a proper color calculation to get a true green. Granted, you wind up combining one narrow and two broad filters, so exposures may be very hard to balance, and star diameters are likely to be quite different because of that as well. 

        Yellow sits between green and red. I would guess that your peak wavelength is going to be around 550nm. The half-width is quite narrow, so maybe you won’t have terrible star-width problems on a combine. 550 is actually fairly green, so it’s more of a yellowish=green filter. Here are some plots and a color spectrum:



        They are uniquely composed to do stellar classification, so you will have to see what you can do with them for imaging; some trial and error may not be a bad idea. With the blue centered at about 466nm, there is  a pretty big gap between b and y filters due to how narrow they are. You might look at which elements lie within the half-width of the filters you do have, and consider them as potential narrow-bands. :-)

        Ron Wodaski



        On Feb 2, 2015, at 4:06 AM, Mark Striebeck mark.striebeck@... [ccd-newastro] <ccd-newastro@yahoogroups.com> wrote:


        Hi,

        I am trying to use Stromgren filters for narrowband imaging of galaxies and stars. I am using a Ha filter, a Stromgren Yellow and a Stromgren Blue filter. I will use the Ha filter for Red.

        I was thinking that I first create a synthetic green image from the Blue and the Yellow filter. And then combine the Red, Blue and (synthetic) Green filters as usual. And then I have to determine the weights of the different colors (using a G2V star). This isn't my idea, but from Rob Hawley.

        Does this sound like the right approach? If yes, how would I create the synthetic green image? (I guess, I can't just subtract the yellow from the Blue...)

        Thanks
              MarkS


      • Mark Striebeck
        Hi Ron, Thanks for the reply. So far I have only looked at one processed image close (double cluster) and just using the Y filter for Green does seem to
        Message 3 of 3 , Feb 2, 2015
          Hi Ron,

          Thanks for the reply. So far I have only looked at one processed image close (double cluster) and just using the Y filter for Green does seem to produce an OK image.

          The Ha filter is actually a 32nm filter to make it similar to the Stromgren filters.

                MarkS

          On Mon, Feb 2, 2015 at 3:29 PM, Ron Wodaski rwodaski@... [ccd-newastro] <ccd-newastro@yahoogroups.com> wrote:
           

          My first instinct would be simply to combine them as Ha/Y/B and see what you think. I don’t see that you have enough information to do a proper color calculation to get a true green. Granted, you wind up combining one narrow and two broad filters, so exposures may be very hard to balance, and star diameters are likely to be quite different because of that as well. 


          Yellow sits between green and red. I would guess that your peak wavelength is going to be around 550nm. The half-width is quite narrow, so maybe you won’t have terrible star-width problems on a combine. 550 is actually fairly green, so it’s more of a yellowish=green filter. Here are some plots and a color spectrum:



          They are uniquely composed to do stellar classification, so you will have to see what you can do with them for imaging; some trial and error may not be a bad idea. With the blue centered at about 466nm, there is  a pretty big gap between b and y filters due to how narrow they are. You might look at which elements lie within the half-width of the filters you do have, and consider them as potential narrow-bands. :-)

          Ron Wodaski



          On Feb 2, 2015, at 4:06 AM, Mark Striebeck mark.striebeck@... [ccd-newastro] <ccd-newastro@yahoogroups.com> wrote:

           

          Hi,

          I am trying to use Stromgren filters for narrowband imaging of galaxies and stars. I am using a Ha filter, a Stromgren Yellow and a Stromgren Blue filter. I will use the Ha filter for Red.

          I was thinking that I first create a synthetic green image from the Blue and the Yellow filter. And then combine the Red, Blue and (synthetic) Green filters as usual. And then I have to determine the weights of the different colors (using a G2V star). This isn't my idea, but from Rob Hawley.

          Does this sound like the right approach? If yes, how would I create the synthetic green image? (I guess, I can't just subtract the yellow from the Blue...)

          Thanks
                MarkS



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