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Prism diagonals

Discussion in 'Eyepieces, Barlows, and Filters' started by Gabby76, Aug 16, 2023.

Prism diagonals

Started by Gabby76 on Aug 16, 2023 at 9:31 PM

3 Replies 467 Views 2 Likes

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  1. Gabby76

    Gabby76 Well-Known Member

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    Prism diagonals can be a handy accessory when you are in a situation that requires more drawtube travel.
    1.25" prisms have the shortest lightpath when compared to mirrors and a 2" prism lightpath falls between a 1.25"and 2" mirror. This can get you into focus without adapters or extension pieces.

    The issue with using a prism is that if the telescope is not made for prism use it can actually degrade the views.

    So how do you find out if a prism will work with your refractor? Keep reading and I will explain :)

    Once you are set up use a mirror diagonal and a high power eyepiece, something that will get at least 50x per inch of aperture. You may need to use a barlow. A 6mm with 2x barlow will give 59x with a 102 f/7

    Aim your telescope at a bright star and defocus inwards. Look at the first ring around the airy disk. If it has a red/ orange/ pink tint to it a prism will work with your telescope.
    If the ring remains white then a prism will not work with your telescope.

    Hopefully this quick test will save some people money.
     
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  2. sojourneyer

    sojourneyer Well-Known Member

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    That is very interesting Gabby. Thanks for sharing this information

    As an aside I read something about prism diagonals being better than a mirror diagonal for reflectors or vice versa.:(
     
  3. Ed D

    Ed D Well-Known Member

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    Thank you for the very useful information.

    I have always had both types of diagonals. I use mirrors for observing deep space and prisms for planetary and lunar observation. This in both my refractors and Maks.

    Ed
     
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  4. Gabby76

    Gabby76 Well-Known Member

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    Prisms and mirrors are both tools in the toolbox so to speak.
    I must say that I tend to use mine in the same way Ed.
     
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