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    Re: Wires, back sights and collimation
    From: Frank Reed CT
    Date: 2004 Nov 30, 23:50 EST
    Alex E wrote:
    "Chauvenet, on p. 113 of volume II gives the precise formula,
    as well as the following approximate one:
    error=-i^2 sin(1') tan(A/2),
    where the error is in minutes, and i is the distance of a wire
    from the center line in minutes."
     
    Today we would write this as "error = i^2 * tan(A/2)". The sin(1) thing is just a conversion from radians to dms.
     
    In general this error is tiny and unimportant as long as the telescope is properly collimated. As long as contact is made with some portion of the Moon's disk crossing the centerline of the field of view (which is easy and normal in practice and implies that i is less than 16 minutes of arc), even at 90 degrees distance the error is less than a tenth of a minute of arc. At 120 degrees, it's only a little more than a tenth of a minute of arc. Nothing to fuss over.
    Frank R
    [ ] Mystic, Connecticut
    [X] Chicago, Illinois
       
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