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    Re: Backlash
    From: Alexandre Eremenko
    Date: 2005 Nov 20, 19:12 -0500

    Dear Bill,
    The index arm of my SNO-T also shakes laterally a bit.
    But I do not see any effect of this when I look through the scope.
    On the other hand, when I measure the index correction
    with stars of with the Sun, I find substantial backlash:
    the result depends on whether I rotate clockwise or anticlockwise.
    Same happens with star distances.
    The thing which drives me crazy is that it does not ALWAYS happen.
    And the amount is different.
    Sometimes, especially with Sun, I see no backlash.
    The best results are obtained when I measure the IC say
    5 times clockwise and 5 times anticlockwise and then average.
    And do the same with star distances.
    I will shortly post a sample of my results.
    My main question is whether these (sometimes poor) results
    reflect the inferior quality of SNO-T, or it is just
    my poor performance, or maybe deteriorating sight.
    Comparison tests with Bill's Astra III did not give conclusive results.
    Despite all care taken (same observer, same scope and the tests were taken
    on the same stars and same evening).
    On Sun, 20 Nov 2005, Bill wrote:
    > Alex,
    > If I control the movement of the index arm laterally I cannot measure any
    > significant backlash.  (Astra III B Deluxe, tradition horizon mirror,
    > approx. 1 year old--worm gear spring loaded.)
    > What is causing me concern is a slight lateral play in the index arm at the
    > arc.  The front of the arm unit has slight left/right motion.  It can be
    > felt by moving the arm left/right or as a slight click when changing from
    > clockwise to anti clockwise. One motion will, given the angle of the notches
    > on the arc, force the arm in and the other direction will force the arm out.
    > I can see it as separation or overlap when I touch a the Sun's limbs and
    > press in our out.
    > Using Sun limbs, and small power lines (sextant tripod mounted for the power
    > lines to eliminated possible affects of side error) it believe what appears
    > as backlash (although I believe it to be lateral arm motion) is in the 0.2'
    > to 0.3' range.
    > Bill

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