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    Re: Backlash
    From: Paul Bryans
    Date: 2005 Nov 11, 10:25 -0000

    Easy one as I am instructing a class and gave a sextat use demo last night
    (moon just visible through cloud).
    Friberger "Yacht" sextant no 100222 bought new in 1976 and no backlash
    measurable, the spring seems to hold the worm drive firmly in place.
    However I suspect backlash may be different at different points of the arc
    since it must be a function of wear or manufacturing quality.
    Paul Bryans
    ----- Original Message -----
    From: "Alexandre E Eremenko" 
    Sent: Friday, November 11, 2005 1:18 AM
    Subject: Backlash
    > Dear list members,
    > Could you make the following experiment for me:
    > determine the backlash of your sextant.
    > This is very simple.
    > 1. Choose a weak star. (As weak as you can see with your naked eye).
    > 2. Point your sextant at it (with 0 setting) and align the reflected
    > image  of the star with the straight image of the same star,
    > as you do in
    > determining your index error
    > (or IC).
    > The two images will not be perfectly coinciding if your
    > horizon mirror is not perfectly parallel to the index mirror,
    > this is a "side error", and it does not matter in this experiment.
    > 3. When the two images of the star coincide (or are "on the same
    > horizontal level"
    > if you have a slight parallelism error),
    > 4. Rotate your screw CLOCKWISE a little bit
    > to break the alignment, and then ANTI-CLOCKWISE until the lighnment is
    > perfect.
    > 5. Read the scale and record.
    > 6. Rotate your screw ANTI-CLOCKWISE a little to break the alignemt,
    > then then CLOCKWISE until the alighnemt is perfect, and
    > 7. Read and record the the reading.
    > 8. E-mail these two readings to me or to the list,
    > together with the brand, and year made (and any other detail you wish to
    > add about your sextant).
    > Thanks in advance.
    > Alex.

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