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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Instumental error?
From: Bill B
Date: 2005 Apr 21, 17:44 -0500
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. 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