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Bill, you wrote:
"How can a "repeatable error" be "no error?" "No" and "repeatable" are
modifiers of "error." Semantics. If a calibrated 1 gram weight consistently
measures 1.5 grams on scale, there is an error, one way or the other."

But you see, that just isn't true in practice. Calibration is the process of
determining these "errors" so that they are no longer sources of real error
in measurements. The problem with your hypothetical one gram weight is
merely that it has been mislabeled. We can fix that. Similarly, every
sextant has an index correction which has to be measured and applied. It
presents no difficulty, and we hardly even think of it as an error.

And you added:
"The "bad news" and frustration is if you have checked mirror alignment,
eccentricity of the micrometer drum etc. and the advertising or
certification sheet claims accuracy within plus/minus 18" along the arc, and
the observations are 1!0 off at some point.  It can be compensated for, but
none-the-less a bummer."

Well, yes, but that's an attitude issue. It's an attitude that has become
rather "cultural" and common-place in the community of sextant users, and
it's an attitude that can be changed. Why put up with a correctable error?

And you wrote:
"Especially--as from what I can conclude from the list archives and what I
have seen--there is no pattern to the "blips."  88d OK, 90d off by 1', 92d
OK.  Testing at 10d intervals is interesting, but if the next tooth in the
gear is flawed the testing is all but useless for establishing a pattern of
"no" error."

I have not encountered a sextant where the arc error jumped in this fashion
(and I have not seen anything in the archives that I can recall that would
imply that sort of pattern). I'm not saying that they don't exist --only
that they are not a major concern. But let's suppose for the sake of
argument that we measure every ten degrees and discover that there is a
rapid change in arc error at some point, let's say between 70 and 80
degrees. We then have the option of closing in on that error. We can narrow
it down and maybe determine that it is localized at 73 degrees. A
traditional calibration table provides only a sparse sampling of angles on
the arc. Some detail might be hiding in the spaces between. Of course, if
you had a really pathologically bad arc with rapid changes all up and down
the arc, then it might just be too much trouble to work with the instrument.

And Bill you wrote,
"Good to see you posting again."

Thanks! Busy summer.

You concluded:
"After spending time in the halls of Purdue's Math/Science Bldg. with Alex
playing with your laser-calibration method I look forward to the posting of
your latest invention for calibration."

I may have to disappoint you on that. I should say right now that I may not
be describing the details of this apparatus here. It has some significant
commercial value, even though sextants are far less important than they used
to be in navigation. We will see...

Regarding those laser tests, you might get a kick out of these instructions
for building a very simple homemade laser collimator:
http://www.instructables.com/id/Home-Made-Collimator/

 -FER



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