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  1. #1
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    Default f/12 scope, Does this affect aFOV



    I've been trying to do some research on this. Does a f/12 scope have a less aFOV than say a f/6? I thought aFOV had to do with the EP's being used. What's the sense of buying a 82° aFOV EP if your scope can't handle it?

  2. #2
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    For two telescopes having different focal ratios. For any given size eyepiece, the telescope with the lower focal ratio will have the larger field of view.
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  4. #3
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    Quote Originally Posted by DennisS42 View Post
    I've been trying to do some research on this. Does a f/12 scope have a less aFOV than say a f/6? I thought aFOV had to do with the EP's being used. What's the sense of buying a 82° aFOV EP if your scope can't handle it?
    Any scope will handle any design eyepiece, within a range of focal lengths. You just get different results.

    If your eyepiece gives an aFOV of 82 degrees, then 82 degrees is what you get. How much sky you get in that 82 degrees apparent field depends on the magnification, which in turn depends on the combination of focal length of the scope with the focal length of the eyepiece. Focal ratio doesn't com into it at all.

    Let's say you have two scopes that both have 1000mm focal lengths. One is 4"" (f/10) and one is 8" (f/5). You pop in a 20mm eyepiece that has an 82 degree aFOV. The magnificafion in both scopes is 1000 / 20, or 50x. The true field of view in both scopes is 82/50 or 1.64 degrees.

    If you want to compare two scopes of the same aperture, take a 4". Let's say we have an f/10 4" (FL = 1000mm) and an f/5 4" (FL = 500mm). Use the same 20mm, 82 degree aFOV eyepiece. The first scope has a magnification of 1000 / 20 or 50x. The true field of view is 82 / 50 or 1.64 degrees. The second scope has a magnification of 500 / 20 or 25x. The true FOV is 82 / 25 or 3.28 degrees.

    You are correct that aFOV is a property of the eyepiece only. Any scope will "handle" the aFOV. They will just give different results, depending on their focal length.

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  6. #4
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    OK then, I just put the scopes in stellarium 4" f/12 and 4" f/6. I see the mathematics and how they work. Has anyone used a .5 focal reducer and do they work? For visual not photo......

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    Yes, they work just like a barlow in reverse. I have one and have used it on my LX90-8. The shorter focal length is more sensitive to coma however. If that matters there are reducers which also flatten the field. I don't notice a problem with F/10 reduced to F/5 for visual use.

    My camera, however, does see more coma due to time integration.

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    I often use a 6.3 focal reducer for viewing. It makes a big difference on objects that cover a lot of sky such as M45, etc.
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  9. #7
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    Thanks for the info guys. I guess I didn't realize how expensive a reducer was. Over a hundred and up. There's got to be something less expensive that works just as well. Am I missing a web site or something?

 

 

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