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Greg,

You're not understanding how it works.  Closer than a few miles, if
you have the index error zeroed on a star and you converge a nearbye
streetlight using the micrometer dial, you can use the resulting
angle to compute the distance between the index and horizon mirrors,
assuming you know the distance between the sextant and the
streetlight.  The distance between the index and horizon mirrors
becomes insignificant with objects far away.

By the way, don't be constantly readjusting the index error to zero.
Just measure it (on a distant object, such as the horizon or the sun,
moon or stars) and apply the correction.  Constantly readjusting the
error to zero will wear out the adjustment threads.  As long as the
error is within a minute or three, it's fine.

Fred

On May 3, 2006, at 12:05 PM, 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.
>
> --
> GregR
>

   

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