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Alex wrote:

> Could you make the following experiment for me:
> determine the backlash of your sextant.
> This is very simple.

Alex

As we both know, you hand is steadier and eye better than mine. For other
readers, my instrument is a year-old Astra IIIB deluxe, 3.5X scope, with
traditional split mirror.

I may be guilty of the politician/lawyer trick of deciding if I don't want
to answer the question at hand, answer a question of my own choosing.  (And
after the last 4 posts, why read on;-)

I find my ability to align a faint star to have a rather large standard
deviation (especially after the first few observations) unless a good bit of
side error is present.  I would reference the article Frank pointed out
(Sept. 05 Sky & Telescope) as a possible cause.

Referring to posts about the eye being able to beat the theoretical limits
of acuity when dealing with lines, I prefer to use small diameter power for
the test you proposed.

What I "think" I know from this is:

1.  My ability to detect touch and leave (wire appearing to be in alignment
and wire appearing to leave alignment) is at best plus/minus 0.1' in either
direction.  That I regard as *my* ability to "see" with my instrument. Note
these tests were tripod mounted so the sextant was plumb.  Note the focus
was not changed (a problem noted in off-list posts) during the tests, and
these were before the slight lateral play in the front of the index arm
became noticeable. Also note my impression (based on answers to my list
questions) for side-error or backlash tests do not seem to be distance
dependent--they align or they do not align.  (Alignment of a phone or power
line may be less than zero, but so what if if they agree?)

2.  When testing for touch and leave with drum rotation in either direction,
any difference possibly caused by backlash was below my threshold (0.1') of
seeing.  I can pretty much confirm that in determining IE from touching the
limbs of the Sun in both drum directions.

I am sorry that is not what you asked for, but I tend to play the odds and
go with methods that I can statistically prove give me repeatable results
(with the lowest standard deviation).

That being said, I could notice no significant change due to rotating the
drum in either direction.  Of course, there must be backlash in any gear
system, but it is below my ability to detect it--just noise from my vantage
point.  This was born out in recent Sun IE checks where, in one case, I
separated the images in both directions--using opposite drum rotation; and
in the other case I brought the images to touch (using the method in a
recent post) for one set of limb observations and separated the images using
the same drum rotation for the other set.  As my 4XSD averages were less
than 0.1' of each other with a standard deviation well below 0.1', I pretty
much feel I have hit the limits of this man/machine combination under ideal
conditions.

I imagine as the gears wear and develop more backlash (despite spring
tension), or if I were using a plastic unit, I might pay more attention. For
the moment nothing significant to report for practical use. Lunars and
star-to-star are limited more by my lack of ability and possible errors
along the arc than by backlash.

Bill

   

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