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    Re: Instumental error?
    From: Bill B
    Date: 2005 Apr 21, 17:44 -0500

    Fred wrote:
    > Here are data for the string of sights when I finally felt I had
    > achieved proficiency on dry land.  These are altitude shots using an
    > artificial horizon, showing the mean Hc in decimal degrees and the mean
    > and standard deviation of Ho-Hc in minutes of arc.
    Thanks for the figures.  Very good results.  Encouraging to see even a
    seasoned observer has an off day where he can't hit his norm.
    What type of sextant and scope power did you use for your observations?
    I have only had my Astra for a 3 months, and the average for clear days
    during an Indiana winter is one out of nine, but do find using it is an art
    form.  In the beginning it exhibited 3'  backlash, and it appeared the frame
    was not very rigid when comparing vertical to horizontal to inverted
    measurements of the Sun.  Thank goodness the backlash is now not measurable
    and the frame has work hardened ;-)
    In tripod mounted index error checks using the Sun's limbs (I separated the
    discs in both cases) I found the standard deviations for my anticlockwise
    separations were always significantly better than my clockwise separations.
    It turned out the difference was caused by greater forefinger pressure in
    the anticlockwise tweaks and greater thumb pressure in the clockwise tweaks.
    Imagine that!  Also found that as the discs moved away from the center of
    the scope, apparent separation changed.
    My one-cut artificial horizon Sun observations consistently have intercepts
    in the 0 to 0.3 nm range (I consider anything under .15 to be good fortune
    as opposed to expertise.)
    My moon to star (Aldebaran) lunars have surprised me, with average errors of
    .02' to .12'.  (Again, anything under 0.15' I consider as luck). Standard
    deviation of errors have ranged from .40' to .64, so my technique still
    needs work.  Some nights I have tried I could not see straight, and gave up
    without recording a single observation after 15 minutes.
    Moon to Sun lunars are still giving me fits.  My averages from runs (with 6
    to 12 observation each) turn out well with the Sun and moon near the same
    elevation, errors generally ranging from 0.2' to 0.4' off, but the standard
    deviation of the individual errors in a run can be wretched, ranging from
    0.3' to 1.0', usually toward the high end of the range.
    When I look at my results, I often get worse over a period of 20-30 minutes.
    I tried just ducking out and taking one cut every 10 minutes or so (standing
    instead of seated), and 5 out of 6 shots showed errors of 0.2' or less.
    If I wait until the Sun drops to 25d or less and the moon is high, it is a
    much easier shot (Sun in the glass, moon in mirror).  Now my results are
    consistent from shot to shot, with standard deviation of errors below 0.2'.
    But in 4 trials of 6-or-more shots per run, my average errors were over by
    0.8' to 1.0'.  I might attribute 0.1' or so to the flattening of the Sun.
    Stat-to-star tests in the 70-90d range do not indicate instrument error of
    that magnitude, so I am stumped.  Any thoughts?
    Note:  All STDEV are n-1.

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