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From: George Huxtable (no email)
Date: Wed May 03 2006 - 12:48:31 EDT
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."
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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.
George.
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contact George Huxtable at
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.
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