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From: george huxtable (no email)
Date: Thu Sep 15 2005 - 18:41:23 EDT
At 21:11 15/09/2005, Bill wrote:
>George
>
>Don't really have the time, interest or energy to do a line by line debate
>and/or rebuttal of a discussion that has strayed so far in intent from the
>initial post/question.
He seems to have found enough energy to have a good shot at it, though. And
it was Bill who 'strayed' the discussion toward the general demerits of
plastic sextants.
>According to the person with first-hand knowledge, it was a Davis 15, and he
>used a Davis (water) artificial horizon with colored glass.
Neither Bill nor I have any first-hand experience of that sextant, so
neither of us is qualified to judge it.
>I consider the Astra
>IIIB and other metal units in that price-point niche to be an order of
>magnitude better than the plastic units in accuracy, precision, and
>flexibility.
Of course. Who's arguing with that?
>the Davis sextants suffer from considerable backlash (compared
>to a metal sextant). If Asbjorn did not take this into account when setting
>IE to zero or in use thereafter, that *may* account for a goodly portion of
>his error. Whether we choose to call it index error, or variable index
>error INDUCED by backlash is a reasonable distinction.
All sextants are subject to index error and backlash, to a greater or
lesser extent, Neither matters much, because both can be nulled by using
sensible procedures (Unless the instrument is so unstable that the index
error changes significantly between successive observations). If that
sensible procedure is not applied, that's a failure in the observer, not a
failure in the sextant.
>1. Let's compare apples to apples. Have you made observations with your
>Ebbco(s) and an artificial horizon? If so, what are your typical results?
Not seriously. I use it at sea.
>2. Have you read Burch's plastic sextant PDF, as well as sea/land data (Vic
>Maui Sextant Sights) or his IE/backlash with plastic sextants PDF?
I've had a look into Burch's Starpath website, which is quite an extensive
one, without yet finding the words that Bill quotes. But I have not
disputed that assessment as quoted by Bill, and said so, in an earlier
message..
However, I came across the following-
"But the all too frequent disparagement of plastic sextants is mostly
misplaced, and it distracts from their true value. All that one hears, for
example, about plastic sextants melting in the sun is much like the stories
we hear about sharks eating the taffrail logs." A sentiment I would agree with.
>3. If yes to #2, do you:
>
>a. Feel Burch is attempting to give an objective overview of the operational
>tolerances of Davis plastic units, and presenting methodology to achieve the
>best results with these units?
>
>b. Or, believe Burch is "tarring" plastic sextants. (If yes, as a sanity
>check, why does he sell and teach with them?)
It was Bill's tarring of plastic sextants that I was disputing, not
anything that Burch said.
> It is apparently not
>sufficient when the source does not support the first-hand observations of
>an exceptional individual with an instrument no longer readily available in
>the states--therefore no ability to verify or replicate *unpublished*
>results.
Does Bill refer to me?. I'm NOT a particularly skilled, or regular,
observer, and NOT an ocean navigator. My sextant observations have been
occasional ones, taken for fun. Any skill that I might claim is in
understanding the basics of how sextant observations work; which is not a
complicated matter. As for the Ebbco, it's now rebadged as a Plastimo, and
I would be surprised if Plastimo doesn't sell in the US, if Bill is keen to
try one for himself.
George.
===============================================================
Contact George at ,or by phone +44 1865 820222,
or from within UK 01865 820222.
Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13
5HX, UK.
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