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George wrote:

"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

It was not my intent to tar all plastic sextants, but merely pass along the
feelings/observations of those more qualified then myself and with
first-hand experience--like Celestaire and David Burch.  Burch wrote in a
PDF on his site http://www.starpath.com: "I would also propose ? as a broad
generalization ? that using these procedures one should be able to obtain
accuracy?s of some 5 or 6 miles as a general rule with plastic sextants.
Maybe better in some cases, maybe a bit worse in others." Celestaire's
seminar handout claims 3 to 10 miles.  In your own words while speaking of
your Ebbco and IE checks, "...and it never shows a discernable change
(reading to the nearest minute,  which is all the instrument is good to)."
Given that is only a minute, not five or ten, it is still greater by an
order of magnitude than one would expect from a quality metal sextant. Your
results with the Ebbco are a testament both to the Ebbco and your abilities.
But if you are in the mood to shoot the messenger...  

George also wrote:

"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."

I find this interesting.  In my limited experience, adjusting for IE also
affects side error.  Again risking reprimand, what I take from the list is
that a little side error is not a big problem, and may be useful.
(Star-to-star or star-to-planet angular measurements?)  I am not clear
Asbjorn checked for side error after his index error adjustments.

I suspect side error introduced by adjusting for IE would not account a 5'
error.  The problem being I have no idea how to quantify side or
index-mirror perpendicularity errors.  My texts explain how to adjust those
errors out, and check for side error with a star or horizon, but little
more.

The texts I have read first have one adjust for index-mirror
perpendicularity.  Then there are back-and-forth adjustments for
horizon-mirror side error and index error until the index error is close and
the side error is eliminated.  One check for side error involves rocking the
sextant to see if the horizon remains aligned.  Being landlocked I use a
faint star.

The questions arise:

1.  How (other than the arc check for rear silvered and block check for
front-silvered mirrors) to check for index-mirror perpendicularity error in
the field?

2.  What errors will be introduced, and to what magnitude, if the index
mirror is not perpendicular?

3.  What errors will be introduced, and to what magnitude, if the horizon
mirror is not perpendicular?

It strikes me that by using a star to adjust out side error, it may appear
perfect, but still have problems in use if the index mirror is not exactly
perpendicularity.

Any thoughts or expertise you or the list could offer would be appreciated.

Bill

   

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