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    Sextant telescopes. was: Re: The "big" sextant manufactures
    From: George Huxtable
    Date: 2007 Oct 27, 10:45 +0100

    John Karl writes-
    
    | Simple ray tracing (as I discuss in my book) shows why a lens using a
    | focal plane can image the entire field even though half of the
    | objective is blocked off.  This analysis is confirmed with all scopes
    | that I have seen that use an internal focal plane.
    |
    | But using the same simple ray tracing, I can't exactly see why the
    | Galilean scope (no eternal focal plane) doesn't display the whole
    | field also.  I think it's related to vignetting, depth of field, and
    | practical optical engineering, not something explained by simple ray
    | tracing.  Nonetheless, experimentally, every Galilean scope I've seen
    | shows only half the FOV when the other half is blocked off at the
    | objective.
    |
    | So I understand why prism scopes show the entire field, but not why
    | Galilean scopes don't.  In my book I explain the former and ignore the
    | latter -- you don't expect an author to confess his ignorance, do you?
    
    I like the honesty in those final words.
    
    John words- "I think it's related to vignetting, depth of field, and
    practical optical engineering, not something explained by simple ray
    tracing." seem somewhat obscurantist to me. I have suggested, just as a
    result of observation, and without any serious technical argument to back
    it, that it could simply be a function of the magnification, but he appears
    to ignore that notion. Why?
    
    To make some sort of test we need two differently-contrived scopes; a
    prismatic and a Galilean, with about the same level of magnification, to
    compare. Unfortunately, there's no overlap between those that I have here.
    The Galileans are only x3, the others x6 or more. Perhaps that's the general
    case.
    
    George.
    
    contact George Huxtable at george---.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
    
    
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