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    Re: Backlash
    From: Bill B
    Date: 2005 Nov 20, 20:46 -0500

    > The problem of sidewise shake hadn't occurred to me! The sun-diameter
    > experiment, with the sextant upright all the time, was poorly designed for
    > catching such an effect.
    I think there must be lateral, as well forward and backward thrust when the
    worm gear is turned, so even in an upright position it will occur.
    Another interesting error I believe I have nailed down comes from the scope.
    From the article Frank pointed out a few months ago, our night focus can be
    different than our day focus.  I put tape by on the main barrel and focus
    ring of the scope, and put witness marks where optimal day focus occurred
    (aided by sunspots) and optimal focus on a star occurred after adapting to
    the night sky.  And sure enough, in test after test, the night witness marks
    (approx. 1/4" apart from day to night) almost lined up at night time after
    time, as did the day marks.
    So what?  Well, I would adjust side error to perfection, and after a day
    shot that night it was off.  Refocus, close again.  What I was able to see,
    and Alex confirmed I believe, is that in both the Astra and low-power SNO-T
    scopes changing the focus changed the side error.  In essence, the reflected
    body was rotating around the direct image (or vice versa).  We can only
    conclude it would change index error as well.  Now I have semi-permanent
    tape arrows affixed to the scope for day and night focus.
    A bit off topic, but the article also indicated that as the eye dark adapts
    and the pupil opens ( 2mm to 7mm) what was a pinpoint becomes a much larger
    "disco mirror ball"  (my words).  Now for star-to-star distances with bright
    stars I observer for several minutes, then expose my eyes to a well-lighted
    white wall, then shoot for a few more minutes. It really seems to help me.

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