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    Re: George's test
    From: Fred Hebard
    Date: 2004 Nov 13, 11:24 -0500

    On Nov 12, 2004, at 6:06 PM, Alexandre Eremenko wrote:
    > George Huxtable proposed the following test
    > for the sextant rigidity.
    > Measure some vertical angle which does not change
    > much with time. Then take your sextant upside down and
    > measure the same angle. (With a good sextant the result
    > should be the same).
    > I did this test today with my usual test for the index error.
    > Here are the results.
    > Sextant: SNO-T, made 1990, scope "inverting".
    > Time: about noon, Lat approx 40, AltSun approx 32d,
    > SD=16.2, (sun semidiameter), 4SD=64.8'
    > Sextant upright position:
    > Up     Low     Sum    IndEr
    > 32.5   32.5    65      0.0
    > 32.5   32.4    64.9    -0.05
    > 32.4   32.4    64.8    0.0
    > Sextant upside down:
    > Up     Low     Sum    IndEr
    > 32.4   32.4    64.8    0.0
    > 32.5   32.6    65.1    +0.05
    > 32.7   32.3    65      -0.2
    > Now, if we average all 6 observations (Low and UP) we
    > get 64.9 for upright position and 65.0 for upside down.
    > Which gives 0.025' for the "non-rigidity error".
    > Conclusions.
    > 1. My SNO-T is pretty rigid.
    > 2. Index error is probably 0.
    > 3. My random human error is about 0.3 for the Sun.
    > In fact, this 0.3 (human?) error causes a lot of trouble for me.
    > a) I almost always tend to overshoot. So the correction
    > (when it is substantial) is always negative.
    > b) The error varies from 0 to -0.6' in my observations, and
    > it is most frequently about -0.3' (if exists), but
    > c) for very long distances (more than 120 d) it is sometimes -0.6'
    > What is the resaon I cannot understand. Either it is
    > a sextant instrumental error, or really some human error.
    > All collimation tests I could do show 0.
    > Alex.
    I don't understand from where you are drawing the conclusion that you
    have a random human error of 0.3' for the sun.  Are you talking about
    the range of the measurements in your test or about the mean index
    error indicated by the test?
    Your numbers look very good, especially those with the sextant right
    side up; the range of measurements there is 0.1' of arc.  The upside
    down numbers are fairly rough, with a range of 0.3' of arc.  The
    semidiameter of the sun also is the correct value for the right side up
    measurements, but not upside down.  It would appear you are not very
    good at operating the sextant upside down, but quite proficient right
    side up.
    Measuring the sun's semidiameter with the sextant in various
    orientations does seem to be good practice for taking lunars.  In
    general, I think the exercise would be very helpful for  developing
    proficiency with the sextant.   From you numbers, it would appear you
    need more practice with the sextant upside down.  How about sideways or
    at a 45 degree angle?
    By the way, I get numbers similar to your right side up measurements on
    dry land, but, thus far, they are much worse on the water.

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