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In reply to Asbjorn's comment-
> > And when I look through my earlier measurements, all sights from the
> > same "batch" (i.e those taken at about the same time and same set-up)
> > gives about the same error in offset.  E.g when I have an error of 5'
> > in a sight, all sights taken at that time have about the same error.
> > I have not yet understood why, and I have too little statistical
> > material to give any conclusions about this.  It might be coincidence.

Bill wrote-

>If the error is relatively consistent within a batch, perhaps index error is
>the culprit.  I have no first-hand experience with plastic sextants, but
>from what I have read IE appears to vary to much more within a batch, or
>from batch to batch, than does a metal sextant.

and Asbjorn replied-

>And it does.  But I check for this after each sight.

==============================

Comment from George.

I don't think of index error as an "error" at all, in that sense. Index
error degrades observations ONLY if it isn't allowed for, or if it happens
to change between an index zero-check and an altitude observation. It's a
simple check to make, and a simple matter to allow for. That should be part
of the routine of every sextant user, of expensive metal sextants as well
as simple plastic ones, whenever the instrument comes out of its box.

Indeed, the check is so quick and easy to make that if the user of a
plastic sextant is being ultra-cautious he can alternate index-checks
within a batch of observations. That's what I used to do, at first, with my
plastic Ebbco, until I became confident that it wasn't showing short-term
changes. Now I check the index zero (against the horizon, in daytime, if
it's sharp) when it comes out of the box, and again before it goes back,
and it never shows a discernable change (reading to the nearest minute,
which is all the instrument is good to).

If a sextant shows real instability, it's likely to relate to looseness
somewhere, probably in a mirror adjustment, which can be discovered by
observing whether the reading responds to gentle pressure on each corner of
a mirror. This can happen, even on a metal sextant, particularly if a
"sextant-worrier" has over-slackened the grub-screws so that
counter-pressure by the return springs has been lost.

It's important to minimise backlash by always making the final adjustment
of the index in the same direction, for index-zero observations as for the
altitude observations. Asbjorn described his technique, which as far as I
could tell required adjustment of the index error to zero (using the
grub-screws?) on each occasion, which appeared to me misguided. A better
procedure would be to leave those grub-screws alone, after an initial
setting, and just accept and allow for any subsequent index error: However
large that index zero-error happens to be, if it's allowed for then it
doesn't degrade the precision of the measurement AT ALL. If there's
significant backlash, then  Asbjorn's initial adjustment method, if I've
understood it right, could be badly affected, though even then I would be
surprised if it gave rise to a ten-minute error, which is what we're
looking for. However, his recent communication makes it clear that he
checks for index error after each sight, which is all that anyone could ask
for.

Bill admits that "I have no first-hand experience with plastic sextants",
yet goes on to add "but from what I have read IE appears to vary to much
more within a batch [of observations], or from batch to batch, than does a
metal sextant.". In that, he is simply relating popular prejudice against
the plastic sextant in general. In the hands of a careful user, there's no
reason at all for index error to get in the way of a good observation, to
the rather lower standards of accuracy that one accepts with a plastic
instrument.

I should add that, just like Bill, I have no experience with the Davis
model that Asbjorn was using, but Bill's comments tarred all plastic
sextants with the same brush; and unfairly so, in my view.

George.

===============================================================
Contact George at george@huxtable.u-net.com ,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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