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    Sextant Scope Parallelism ? ? ? (was Re: SNO-T Sextant)
    From: Frank Reed CT
    Date: 2004 Aug 12, 01:21 EDT
    Bruce S wrote:
    " I also wanted to be able to measure long lunar distances accurately. An inverting scope has wires in the field of view that help you keep the plane of observation parallel to the plane of the instrument. On page viii of the introduction to my Tables there's a small table showing how important this can be. "

    Checking for parallelism of the telescope (which is what those passages in old editions of Moore and Bowditch were discussing) does not require "wires in the field of view". The references to the wires are simply descriptive. The authors could as easily have said "right" and "left" but since sextant scopes commonly had those wires back then, it was easier to reference them. This check for parallelism can be done today just as easily as 200 years ago, but it's a little bit harder to describe verbally without those wires...

    You refer to this as a process designed to keep the plane of observation parallel to the plane of the instrument. I think this is a misunderstanding. The important factor is parallelism of the telescope (unless that is all you meant by plane of observation!). If it's not parallel, then you will get different angles, by a fraction of a minute of arc typically, depending on where in the field of view you bring the two images together.

    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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