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    Re: How does the AstraIIIb split mirror work?
    From: Ken Muldrew
    Date: 2004 Apr 26, 13:03 -0600

    On 26 Apr 2004 at 19:21, George Huxtable wrote:
    > To me, it seems to have shown all the best features of how a
    > mailing-list discussion ought to be conducted. Listmembers proposed
    > various experiments, tried them out, argued about the results, argued
    > about the underlying principles, tossed various ideas to and fro, in a
    > spirit of courtesy and mutual respect, to reach a conclusion.
    > "Well, he would say that, wouldn't he?" I hear you say, because my
    > original suggestions were confirmed. But I think that all on this list
    > are open-minded enough to be prepared to change their stance, and
    > their minds, in the face of strong-enough evidence and argument that
    > points another way.
    > It's a real privilege to be a member of such a list as Nav-L. Thanks
    > to Ken for raising an interesting question.
    Jim Thompson raised the question initially but I quickly dove in with
    an exposition of what I thought was the correct answer. It is always
    a little frightening to expose one's ignorance in public in such a
    brash manner, but it is a very good way to learn what you know and
    what you don't know. Too, there is always the possibility that others
    of a less vocal bent share the same confusion. So I hope the
    listmembers will not resent my overly forceful presentation of
    arguments that were wrong. Certainly, the courtesy and forebearance
    of those who corrected me indicates that the experts on the list are
    only too happy to assist a beginner with discovering their ignorance.
    Like George, I thank the members of the list for this lively
    discussion. It has been a treat to learn something new and also to
    gain a better understanding of the principles by which a sextant
    As a post script let me add that I repeated my experiment with plate
    glass last night and once again saw two images of Jupiter. Since it
    wasn't cloudy, I also saw double images of stars. This time, though,
    the experiment was not confirming my prejudices but rather, it
    presented an anomaly. It's too easy to forego due diligence when your
    expectations are confirmed. This time, however, I was determined to
    repeat the experiment with a different piece of glass. I have a large
    piece of 1/4" plate glass that I use for sharpening chisels, so I
    brought that outside and tried once again. This time the celestial
    bodies gave a single image, so my one remaining obstacle to
    understanding was removed. I suppose the 1/8" glass that gives double
    reflections is slightly curved, so that despite the parallel faces,
    it gives double reflections.
    Ken Muldrew.

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