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    Re: Index error by a star-star distance
    From: Bill B
    Date: 2008 Oct 03, 02:02 -0400

    Frank Reed wrote
    > BUT NOTE THIS: most of the people that I have discussed this with over the
    > years get disappointing results from star-star distances, usually two or
    > three times worse than they get from lunar distances. So why not just use
    > lunar distances?? Several people, including me, believe that star-star
    > distances suffer from the distorted diffraction patterns around star images.
    > Stars never look like little dots with well-defined centers. Instead they
    > are spikey things that in many ways resemble a child's drawing of a star.
    > And each person, depending on the imperfections of their eyes, sees a
    > different pattern of spikes in the asymmetrical image of the star.
    Different strokes for different folks (non-USA translation--one man's meat
    is another man's poison).  I have had miserable results (plus/minus 1' on
    average) trying to determine IE with one star, for many of the reasons
    mentioned above.
    I have had very good results using the sun or a natural horizon for IE
    checks, although there is a predictable difference between the two thanks to
    astigmatism in my shooting eye. For me, eyeglasses are a larger nuisance
    than the bare-eye problem unless it is pitch-black with no ambient light. An
    additional wild card, your lens grinder can dial out a portion of
    astigmatism correction at your request if you find it bothersome.
    Moon IE checks do not work well for me.
    I have very good results calibrating along the arc with star-to-star.
    Frank, IMHO, flagged one caution in an article about distortion of a point
    source as the eye becomes night adapted. The other challenge, as I
    discovered using Frank's laser technique for finding sextant IE, is that
    (contrary to my understanding of Frank's opinion that the scope created a
    star pattern), is the mirror/glass juncture of a traditional horizon mirror
    is quite cable of diffracting the laser beam (with no scope mounted).
    My thinking may seem contradictory, but with star-to-star one has a luxury
    not available with one-star IE checks, that being swinging the two stars
    through each other (other off-list conversations confirm this opinion). An
    advantage of using stars of different magnitudes is dropping in a shade that
    changes the color of the brighter star.
    Given my above opinion on glass/mirror diffraction, it is important when
    doing star-to-star to center them horizontally in the vertical "overlap
    zone" to keep their shapes relatively consistent. It may be matching a green
    vs. a white spikey dumbbell shape against anther, but it works for me.
    As to lunars, the same problem I have with sun IE vs natural horizon IE
    occurs.  With a vertical, I can make a personal correction for the
    astigmatism between two disks and a line/disk.  Once I go horizontal, the
    whole IE perspective changes (compounded by the problems I seem to have with
    a moon edge). I was starting to zero in on a personal correction for lunars
    given errors were consistently in one direction and of roughly the same
    magnitude, but other time eaters and weather stalled that project out for
    the past year.
    In the overview, I believe a beginner can obtain 1' to 2' accuracy with a
    good instrument and practice.  To achieve the lower side of sub-minute
    results requires experience, a good grasp of personal error, and an
    understanding of what works for *you* and what doesn't.
    Bill B.
    Navigation List archive: www.fer3.com/arc
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