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Re: Imaging train question

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  • Bruce D
    Larry, strictly speaking, the sequence does not matter for a field flattener, however it _does_ matter if you use a focal reducer. Narrow band filters are
    Message 1 of 6 , Sep 30, 2008
      Larry, strictly speaking, the sequence does not matter for a field
      flattener, however it _does_ matter if you use a focal reducer.

      Narrow band filters are constructed from many layers of dielectric
      materials. The optical thickness of these layers sets the stop (and
      pass) bands. As you shorten the focal ratio, the optical thickness
      increases for light entering at the edge of the light cone so that the
      filter's properties are not the same over the optical aperture.

      In an extreme case, light from the edge of the aperture might encounter
      the filter at an angle that completely displaced the bands. The effect
      would be to degrade the image quality by allowing (say) light polution
      in and keeping (say) Ha light out.

      Filter designers compensate for modest focal ratios - I'm guessing here
      but I would design for optimal performance at around f/7-f/10. Assuming
      this is the case you could expect better performance from the filter if
      you insert it into the light path before the reducer.

      I tested this quite carefully when operating my LX200/12 at f/2-f/4...
      my narrow band filters converted the scope into something like an
      effective 6" aperture.

      Clearest
      Bruce, Toronto
    • Larry
      Bruce, Thanks for that very informative reply. It really is where my thinking is at, but perhaps for less well-thought out reasons, mainly to do with getting
      Message 2 of 6 , Oct 1, 2008
        Bruce,
        Thanks for that very informative reply. It really is where my
        thinking is at, but perhaps for less well-thought out reasons, mainly
        to do with getting the proper spacing between the reducer (WO 0.8)
        and the CCD, and also figuring that any light coming into that path
        might as well already be filtered. I did wonder if putting the IDAS
        and its holder in between the FR and CCD might just do double duty as
        a spacer, but was afraid that might introduce unneeded complications.
        Your reasoning confirms my gut reaction.
        Thanks again for the help, in a way even I can understand!
        Larry


        --- In William-Optics@yahoogroups.com, "Bruce D" <noisejammer@...>
        wrote:
        >
        > Larry, strictly speaking, the sequence does not matter for a field
        > flattener, however it _does_ matter if you use a focal reducer.
        >
        > Narrow band filters are constructed from many layers of dielectric
        > materials. The optical thickness of these layers sets the stop (and
        > pass) bands. As you shorten the focal ratio, the optical thickness
        > increases for light entering at the edge of the light cone so that
        the
        > filter's properties are not the same over the optical aperture.
        >
        > In an extreme case, light from the edge of the aperture might
        encounter
        > the filter at an angle that completely displaced the bands. The
        effect
        > would be to degrade the image quality by allowing (say) light
        polution
        > in and keeping (say) Ha light out.
        >
        > Filter designers compensate for modest focal ratios - I'm guessing
        here
        > but I would design for optimal performance at around f/7-f/10.
        Assuming
        > this is the case you could expect better performance from the
        filter if
        > you insert it into the light path before the reducer.
        >
        > I tested this quite carefully when operating my LX200/12 at f/2-
        f/4...
        > my narrow band filters converted the scope into something like an
        > effective 6" aperture.
        >
        > Clearest
        > Bruce, Toronto
        >
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