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Subject: [Nml] index error adjustment...
From: Rick Emerson (rick@XXX.XXX)
Date: Tue Jan 26 1999 - 11:56:33 EST
Russell Sher writes:
> Hi - With all the talk on sextants, perhaps I can squeeze in a question
> about index error adjustment;
> I have an instrument which has two right-angle shaped brackets for assisting
> with index glass adjustment - With the instrument held horizontally, these
> calibrating brackets are placed on the arc so that both the reflected and
> true images are seen. The relative heights of these are adjusted into
> co-incidence with the adjusting screw to give perpendicularity. When I
> removed these brackets and eye-balled the arc directly, after performing the
> adjustment, I noticed that there was still some error - In other words,
> these two methods are possibly not as reliable as each other- OR is this a
> case of parallax error, or the way which I am viewing the arc? Celestaire
> (www.celestaire.com) market calibrating cylinders for the same purpose -
> does anyone have any experience with these? Any comments etc.
Without seeing what you're doing with the brackets, it's hard to
suggest a remedy - if one's needed. We can talk about this off-list
if you want.
As to Celestaire's cylinders, save your money and get a set of dice
from a toy store. Dice are machined to surprisingly close tolerances
(or the results aren't random as they should be). Made of plastic,
they don't rust and replacement costs are minimal. I keep a set in
the sextant box for use as calibration tools and, if all else fails in
reducing a sight, as decision-makers... [wink]
Rick
S/V One With The Wind, Baba 35
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