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    Re: Instumental error?
    From: Fred Hebard
    Date: 2005 Apr 21, 21:33 -0400

    On Apr 21, 2005, at 6:44 PM, Bill wrote:
    
    > 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.
    >
    > Fred
    >
    > 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.
    >
    > Bill
    >
    
    Bill,
    
    My only "seasoning" has been on dry land, so I don't consider myself
    "seasoned."  Also, I've only been at this since Sept of 2001.
    
    Those results were with a C+P standard with a 4x star scope.  I started
    with a $30 Davis sextant, moved to a Husun, then the C+P.  If I could
    have gotten the inverting scope for the Husun, I would have gotten that
    instead of the C+P.  In retrospect, I think a new Astra would have been
    the way to go.
    
    Like you, I've noticed that fatigue is an important factor.  If I'm
    going to be shooting altitudes of more than one body, I only take three
    cuts at any single body.  Fatigue is even more important for lunars
    since one often is holding the sextant up, an perhaps tilted, and
    trying to twiddle the knob to perfect the contact.  One can't wait for
    the contact as with altitude shots away from meridian passage.  Frank
    Reed's suggestion to look fairly quickly, then lower the scope to tweak
    the knob, before raising it again to check the contact may be useful
    here.
    
    Sun-moon lunars are still giving me fits, whereas I've had some nice
    ones with Jupiter.  For the sun and moon, I even had an assistant call
    out the time and I noted when I thought contact was perfect, while
    seated reasonably comortably; it was nowhere near the correct time.  I
    think it might be related to not centering the objects in the scope, as
    with sun semi-diameters.  To alleviate that problem, in Jan I bought an
    inverting Russian scope with cross hairs from Joel Jacobs, but am still
    getting used to it.  I find it hard to detect both images, even with
    sun semidiameters, and I have yet to figure out what's up in that
    regard.  At this point, I haven't used the Russian scope much. [Even
    though I live in Virginia, I must have been imagining the winter was
    Canadian].
    
    Another problem I've had with lunars is that I was using what I thought
    was a faithful representation of Young's method of clearing, as put
    forth by Geo. Huxtable.  It doesn't give the same results as Frank
    Reed's lunars page, usually giving cleared distances about 0.3' of arc
    away from Frank's.  I'll have to cross check these two with Bruce
    Stark's book.
    
    Fred
    
    
    

       
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