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George, you wrote:
"That's interesting, and rather a surprise to find such extreme
bad-behaviour. Was the index error similarly unstable, or did the index
error stay put while the calibration changed?"

The index error changed similarly, but it was not the same.

And you asked:
"Had the instrument been given any time to stabilise, in temperature?"

Sure. Ten to fifteen minutes.

And:
"Presumably, the instrument had been checked-over to ensure that no mirrors
or brackets were at all wobbly"

Yes. The instrument was brand new also. In any case, the sextant was lying
on its side and completely untouched during this test. What I was describing
was looking through a 10x scope without touching it at all. One could
visibly see the changes in just a few seconds. Very entertaining!

You wrote:
"I would never expect to measure angles to better than an arc-minute or so
with an Ebbco, however."

Sounds reasonable to me.

And:
"Frank's comments, applying to an unspecified "plastic sextant", tar all
such sextants with the same brush."

To all sextants everywhere, plastic or otherwise, wherever you may be on
Earth or in near-Earth space, I deeply apologize for any offense to you
(sextants, that is) that may have been caused by my unintentional "tarring".
:-)

And George, you concluded:
"It would be fairer, and more useful to others, for him to actually "name
names", and specify the type of plastic sextant which he found to be so
faulty."

I was describing an example of a type of error that could not be considered
correctable because it can not be reliably, repeatably measured. That's all.
I wasn't discussing plastic sextants generally.

Since you asked, the instrument in question was a brand new Davis Mark 15.
Even Davis considers this instrument to be less stable. Right on the side of
the box for the Mark 15, there is a little description of the Mark 25 which
says "constructed with stronger, more stable materials to make this the most
accurate of the Davis sextants".

It might be fun to do a detailed comparison of the accuracies of different
brands and models of plastic sextants. Any volunteers?

 -FER



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