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18755Re: Imaging train question -- Bruce

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  • Larry
    Oct 1, 2008
      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!

      --- In William-Optics@yahoogroups.com, "Bruce D" <noisejammer@...>
      > 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
      > filter's properties are not the same over the optical aperture.
      > In an extreme case, light from the edge of the aperture might
      > the filter at an angle that completely displaced the bands. The
      > would be to degrade the image quality by allowing (say) light
      > in and keeping (say) Ha light out.
      > Filter designers compensate for modest focal ratios - I'm guessing
      > but I would design for optimal performance at around f/7-f/10.
      > 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-
      > my narrow band filters converted the scope into something like an
      > effective 6" aperture.
      > Clearest
      > Bruce, Toronto
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