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A few comments from the peanut gallery and you can take this whatever way
you want:

In my experience (and I have a fair amount of it), determining IE,
especially when one is reckoning in 10ths of a minute, can sometimes be very
subjective. I suspect that when it gets down to those levels of minutiae, it
would be rare if two or three people could agree on whether or not the error
is 0.1 or 0.2'

As Lu suggests -- and I am inclined to agree with him -- a tenth of a minute
or two is nothing to worry about.

Slag away at me gentlemen but this is my experience on this matter. I have
long since given up on getting wound around the axle if the IE at the end of
a round of sites is one or two tenths different that what it was at the
start. I am convinced that it is a function of the human eye and brain; to a
degree.

On another front, I have a C.Plath which is supposed to be the very best and
the IE changes all of the time. It is very much subject to temperature
fluctuations, such that taking a sight in the shade will yield a different
result that one taken in the sun. In my part of the world, where the
temperature dips to -30 C in the winter, the temperature fluctuation is
pronounced. Summer or winter, I always allow my sextant to sit outside for
at least ten to 15 minutes to allow it to adjust to the ambient temperature,
even if the temperature inside my house or boat is the same as it is
outside.

I owned a brand new Cassens and Plath several years ago and experienced the
exact same phenomenon. Others may beg to differ but I suspect that it is
impossible for a metal sextant to remain absolutely stable. After all,
metal, by its very nature, is a superb conductor of heat. I would be very
surprised if they did not expand and contract in accordance with temperature
fluctuations.

My two bits worth.


Robert


----- Original Message -----
From: "Lu Abel" 
To: 
Sent: Monday, May 08, 2006 1:42 PM
Subject: Re: Question about Davis Mk 25 sextant beam converger


> Let's make that very clear:  One should check IE every time a sextant is
> used, not for each sight taken at that time of use.  I usually check
> (better, *determine*) IE both before and after taking sights. Invariably
> IE is the same, even on a Davis plastic sextant.  But if it ever were
> different, that would be a flag that IE might have drifted throughout my
> run of sights.  Then I'd have to look at how much it had changed (a
> tenth of a minute or two is not a lot to worry about, a few minutes
> definitely is) and decide if that drift put my observations in doubt (on
> the other hand, even a position thrown off by IE drifting a minute or
> two might be a whole lot better than a DR!).
>
> I think all this comes under the topic of "getting to know your
> sextant."   Love it, cuddle it, hold it, take sights with it, check it.
> Soon you'll have a feeling for it's peculiarities and behavior well
> beyond any words of wisdom from this list.
>
> Lu Abel
>
> Fred Hebard wrote:
>> Right,
>>
>> One needs to check IE each time the sextant is used.  But one doesn't
>> need to ADJUST IE to zero each time.
>>
>> Fred
>>
>> On May 8, 2006, at 10:45 AM, Red wrote:
>>
>>> Greg-
>>>  Even with a better sextant, it is "proper" practice to check for
>>> IE every time you use the sextant. I've seen IE hold at zero for a
>>> zero, even with changing temperatures, but all it takes is one
>>> bump, one slip, and then your sights are down the drain. Far more
>>> "professional" to check it every time so you know it hasn't
>>> changed, instead of assuming so.
>>>  The plastic sextants are supposed to be less stable with
>>> temperature change, so that taking one from a warm indoor home out
>>> into cold air, or taking one and leaving it in hot sunlight, etc.,
>>> all can cause thermal changes in the frame, which bring changes in
>>> the IE.
>>>
>>
>>

   

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