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    Re: star-to-star distances
    From: Fred Hebard
    Date: 2004 Sep 30, 18:07 -0400

    On Sep 30, 2004, at 9:26 AM, Alexandre Eremenko wrote:
    > Dear Fred,
    > Here is one more observation I made yesterday:
    > Sept 29 GMT 21:33 Sun, Davis art horizon (with water,
    > perfect weather) , Galileo scope:
    > 8 observations in approx 1 minute intervals. Altitude approx 12 deg.
    > Average deviation (of corrected alt from the computed one) 0.2'
    > 0.0', 0.4', 0.0', 0.4', 0.1', 0.0', -0.2, -0.1'.
    Here are my results from 6 observations in a Davis Artificial Horizon
    with oil over corn syrup, made Sept 30, 2004, 21:15 GMT, of the upper
    limb of the sun, using a 4x40 scope.  The altitude was about 21.5
    degrees.   These deviations of Ho from Hc had a mean and standard
    deviation of 0.00 +/- 0.11' of arc.  It's only recently that my
    standard deviations have started coming in consistently under 0.2' of
    arc, most of the time.  Now if I can get the mean to not be
    significantly different from zero more often!
    Raw data: 0.20, -0.01, -0.12, 0.02, -0.03, -0.03
    One trick with altitude shots away from the meridian is to set the arc
    slightly different from the final contact point, then wait for the
    object to settle on the contact point.  You can wait with stop watch in
    hand and click it at the moment of contact.  It also might help a bit
    to record to the tenth of a second, which I haven't done thus far.
    >> One quibble, the inverting scope probably is not 7x30.
    > Yes, 7 is the magnification, and 30 (milimeters) is its diameter.
    > This is what the manual says, and I also measured the diameter
    > of the objective lens with a ruler.
    > Is not 30 mm a sort of standard diameter for a modern 6x scope?
    > (I understand the modern ones are prizmatic, and the old
    > 6x and 7x scopes look much thinner on the pictures, but nevertheless,
    > my scope is 7x30 and non-prizmatic).
    I saw a picture of one on Ebay, the end was fat, as you indicate,
    unlike the old English scopes, which were about 15mm.  My understanding
    is that 30mm is good with a 6x scope and 35mm for a 7x scope, going up
    to 10x50, which is standard for nautical binoculars, so the two numbers
    are always in a 1:5 ratio.  The Japanese scopes are 7x35.  This may be
    a bit bulky for a sextant for altitude shots, but might be better for
    The advantage of the prisms is a shorter tube length, although there is
    some light loss with them, and a gain in weight.  And, of course, the
    image is upright.  The 7x30 inverting scope may be able to get away
    with the smaller objective because of less light loss.
    > Yes, it is 4x40, that is 40mm diameter, like the standard
    > Cassens-Plath scope. They say these scopes are interchangable
    > between Cassens-Plath and SNO-T. I would be very curious to compare
    > the quality of the Russian made one with Cassens-Plath.
    Perhaps we can meet sometime at Purdue when I go to see my daughter;
    I'll bring my sextant.

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