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From: Fred Hebard (no email)
Date: Mon May 08 2006 - 16:11:50 EDT
Greg,
I don't know why Davis would encourage such adjustments, but all the
"bibles" (Bowditch, etc) say not to. In my experience, after
adjusting the index error, it takes a while for the mirror to settle
down, although this change is more in the range of 0.4 arcminutes,
not 9.0 arcminutes. I have a metal sextant.
If you expose the plastic sextant to the sun for very long, it's
index error may change due to thermal expansion, even though there
has been little change in the air temperature. I read somewhere the
recommendation to take sights fairly quickly to avoid this.
Fred
On May 8, 2006, at 2:21 PM, Greg R. wrote:
> Fred:
>
>> One needs to check IE each time the sextant is used. But one doesn't
>> need to ADJUST IE to zero each time.
>
> I would agree in principle (and the explanations for not doing so
> certainly
> do make sense), but am curious why Davis would say that index error
> should
> be adjusted each time it's used? Maybe what they're actually doing is
> encouraging people to wear out the adjustments earlier so they'll
> sell more
> sextants?... ;-)
>
> BTW, does anyone know what kind of accuracy is possible with the
> plastic
> sextants? Is it possible to get a consistent zero (or very low)
> intercept to
> a known position, or with plastic construction are there just too many
> variables involved? I did notice that my index error varied over
> the weekend
> from around +5' to around -4' over just a couple of days.
>
> Temperatures ranged from mid-70s (my backyard) to mid-60s (beach),
> but I
> wouldn't think that would be enough of a gradient to cause that
> much change
> (or maybe that's typical for plastic?). The sextant wasn't dropped or
> handled roughly (and was always carried in its foam-surround case),
> so I
> don't think mechanical shock would be the cause.
>
> --
> GregR
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