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  1. #1's Avatar Guest

    Default Burgess Contrast Enhancing Diagonals?

    On Mar 19, 7:34 am, "Chris.B" <> wrote:

    Diagonals can be made from a reflective aluminum coating on a mirror
    surface, from a prism, or from a series of alternating hig/low index
    oxide layers. The latter is called a dielectric reflective coating. A
    low number of layers produces low reflectivity, but once you get above
    50 layers, the reflectivity is greater than 99.5% over the visual
    wavelength range and is very flat and even, with much lower scattered
    light compared to an aluminum coated diagonal. The reflectivity falls
    rapidly in the IR and UV to close to zero. Thus, a multi-layer coating
    is sort of a very selective filter, passing the visible waves while
    blocking the IR and UV. A number of companies sell them including
    Baader in Europe, AP, Televue and W.O. here in the US.

    If you limit the number of dielectric layers to a lower number and
    apply them in a certain way, you can reduce the width of the passband
    to include a smaller amount of the visible wavelength range. In that
    way you have a built-in violet/red filter, which can be used to
    suppress chromatic aberration of an achromat lens by selectively
    filtering out the unfocused light at the ends of the spectrum. The
    result would be increased contrast but at a penalty of reduced overall
    light throughput. You can also selectively filter out certain
    wavelengths similar to dielectric coated glass filters (Baader
    Planetarium Conrast Booster, Fringe Killer and Neodymium Skyglow
    filters come to mind).


  2. #2's Avatar Guest

    Default Burgess Contrast Enhancing Diagonals?

    On Mar 19, 5:26 pm, "Chris.Bee" <> wrote:

    I'm not sure what is being used in these diagonals, so i can't
    speculate how they would work with added filtration. Dielectric
    filters are inherently high contrast because light is not absorbed and
    possibly scattered inside the glass as with absorption filters. Light
    is rejected at the air-glass boundary and sent back out the front of
    the scope. Of course, if you add two dielectric systems on top of each
    other, you can get some interesting reflection halos around bright


  3. #3
    jerry warner's Avatar
    jerry warner Guest

    Default Burgess Contrast Enhancing Diagonals?

    "Chris.Bee" wrote:

    But if you need UV IR for spectroscopic reasons,
    well then .... go with silver?

  4. #4's Avatar Guest

    Default Burgess Contrast Enhancing Diagonals?

    On Mar 20, 12:29 am, jerry warner <> wrote:

    Go with nothing - go straight thru, no diagonal.




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