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Subject: questions on sextants

I am interested in sextants precision.
I read the earlier discussions in this list on the errors
of altitude measurement in the sea, but I want
to seperate the question of sextant precision
(as an instrument)
from the rest of the factors like dip, refraction, difficulties
on a small boat etc.
In other words, how precisely can you measure an angle
with a sextant under the best possible conditions?

I could not find much on this question in the books.
Some authors hint that 0.1' is the best you can hope. Can this be really
be achieved? Can 0.2' be achieved?

Of course, this depends on the sextant:

Is it true (as some say) that XIX century sextants were
more precise (because they were designed for Lunar distances,
where super-high precision of the measurement is crucial)?
And that in the modern times they don't care so much about
sextant precision because other errors (dip and refraction)
are larger anyway?

Is it true that top of the line XX century sextants with brass frame like
Plath, Cassens-Plath or Kalvin-Hughes permit more precise measurements
than cheaper aluminium-frame sextants (like Astra, Freiberger and SNO-T)?

I am doing some experiments with my aluminium SNO-T
(have no other sextant for comparison), but these
experiments are non-conclusive yet, probably because
of my lack of agility/experience in the measurement.

I would also appreciate any suggestions on how can I really
test the instrumental error of my sextant.
The natural way to do this seems
to be star-to-star measurements. I read the two interesting
letters of April 6 by Frank on how to do this, but so far
I fail. (Unexpectedly, my star-to-star measurements turned
out to be much less precise than my Sun (art horizon) measurements! And
I have no idea why this is so.)
I will report the details of my experiments in another message,
if there is any interest to this "academic" question at all.

Alex.

   

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