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    Re: Question about Davis Mk 25 sextant beam converger
    From: George Huxtable
    Date: 2006 May 3, 17:48 +0100

    Greg R wrote-
    "I bought a Mk 25 sextant a couple of weeks ago, and am wondering about the 
    accuracy of the beam converger (it's a full-horizon
    model vs. split-mirror) - from what I can tell so far, the alignment seems to 
    depend on the distance to the object. I can dial out
    all of the index error on a nearby object (say a nearby street light or a 
    rooftop), only to find it's different when using a
    celestial object (sun (with proper filtering on both mirrors, of course) or a star).
    Same thing in reverse when aligning it with a star or the sun, setting it up 
    that way throws nearby objects out of alignment (by
    several minutes of arc).
    Is there something I'm not understanding about how this works, or is this 
    sextant possibly defective? Thanks in advance for any
    input, I also e-mailed that question to Davis and I'll post the reply when I get it."
    No defect there; it's exactly what you would expect. It's the effect of local 
    parallax, the small angle of difference between the
    view of a nearby object via the horizon mirror, and the view seen via the 
    index mirror, from a few inches higher up. That's why, to
    check the index error, you have to use a DISTANT object, more than a mile 
    away, such as the horizon or an object in the sky or a
    distant landmark or power-line.
    I don't know how much that vertical separation between the two view-paths 
    actually is, in your sextant, but take it to be 3 inches.
    Than, looking at an object a mile away, those two sight-lines converge by just 
    0.14 arc-minutes, which is less than the accuracy of
    your sextant. If the object is a tenth of a mile away, that convergence error 
    will be 1.4 minutes; and so on. Don't worry about it;
    just avoid using or checking a sextant on close objects. It's not designed to do that job.
    You can make use of that convergence angle to use the sextant as a rangefinder, if you wish.
    contact George Huxtable at george@huxtable.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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