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    Re: Vernier sextant
    From: Alexandre Eremenko
    Date: 2007 Mar 10, 21:16 -0800

    > I like seeing the raw numbers!
    > Fred
    Here are some preliminary tests of the old vernier
    C. Plath. After collimation adjustment.
    March 10. IC from Sun, average of 3:
    IC=-50", 4SD=64'42" True value 64'.4=64'24"
    March 11, IC from star (Rigel) -1'10".
    GMT 2 a.m. Star distances (corrected for IC=-1'10")
    Betelgeuse-Rigel: 18d35'50", error 14"
    Betelgeuse-Sirius:27d05'30", error -6"
    Betelgeuse-Sirius:27d05'30", error -6"
    Betelgeuse-Sirius:27d05'40", error +4"
    Aldebaran-Sirius: 46d01'50", error -2"
    Aldebaran-Sirius: 46d01'55", error +3"
    Aldebaran-Sirius: 46d01'50", error -2"
    The scope used was 12x, inverting.
    Here are the results with SNO-T, 6x inverting scope:
    March 11, IC from star (Rigel) -0.5
    GMT 3 a.m. Star distances corrected for IC=-0.5'
    Betelgeuse-Rigel: 18d35'.5, error +0.2=12"
    Betelgeuse-Rigel: 18d34'.8, error -0.5=-30"
    Betelgeuse-Rigel: 18d35'.4, error +0.1=06"
    Betelgeuse-Sirius:27d05'.6, error -0.1=-6"
    Betelgeuse-Sirius:27d05'.8, error  0.2=6"
    Betelgeuse-Sirius:27d05'.6, error -0.2=-6"
    Aldebaran-Sirius: 46d01'.2, error +0.5=30"
    Aldebaran-Sirius: 46d01'.6, error +0.4=24"
    Aldebaran-Sirius: 46d01'.6, error +0.4=24"
    So, as I expected, a vernier sextant seems more
    accurate, but of course it is MUCH harder to use.
    The mirrors (and filters!) are very small. It is hard to catch
    the star, unless you preset the distance. The telescopes
    have very tiny field of view, it is hard to keep the two
    stars in the field. The picture is good only at the center
    of the field, and only if your eye is at the center of the eyepiece.
    I suppose that on a small boat I could use no telescopes, only the
    zero-tube. Only the 12x inverting scope can be used for IC with
    Sun. For all other scopes the shades are too small and let the
    Sun shine straight to your eye.
    In SNO inverting scope you see the sky like in a wide screen cinema:-)
    No need to preset the distance, you can just scan the sky.
    The scale of the C. Plath is very hard to read, especially at night.
    With SNO, reading a scale to 0.1' takes few seconds,
    even without light, while on the
    vernier sextant, more than a minute. You have to look VERY carefully
    to read the scale to 10" (approx 0.2'). And it takes time.
    Roughly speaking, a series of 5 observations of the same distance
    takes 3 minutes with SNO and 10-15 minutes with the vernier C. Plath,
    mainly because of the slow reading of the vernier scale.
    Finally SNO is light, while the old sextant is heavy, and my hand
    especially with a 12 x scope.
    Shortly speaking, it is a very different experience with these two
    different types of sextant.
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