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    Re: Lunar Distances with Frank's sextant
    From: Alexandre Eremenko
    Date: 2006 Oct 31, 17:41 -0500

    
    Here are some results of observations
    with Frank's sextant he gave me to play with
    in exchange for my SNO-T.
    
    1. On the sextant. It seems to be a Tamaya clone
    (made by the International Nautical). Recently I saw on e-bay
    an identical sextant made by SIMEX.
    It has two scopes but I only used the 7 x 30 prizmatic.
    An unusual feature is that the sextant has ONLY polarised
    filters. This has one serious disadvantage: you cannot
    look at the Sun through the horizon glass.
    It is just not dark enough.
    So the usual procedure for IC check, as well as some Lunars
    cannot be performed with this sextant.
    Otherwise, I find it very good.
    
    On the first night I found a substantial side error
    and adjusted it.
    
    The index error varies a little from night to night,
    from +0.3 to -0.2 during a week of observations.
    Unfortunatelky I cannot use the Sun test with this
    sextant, and star test is less reliable.
    
    2. Here are some Lunars with this sextant from my balcony.
    
    Oct 28, LT=15:45 Sun-Moon at 76 degrees, strong wind,
    IC=+0.3 (determined the night before
    from stars), reduced with Frank's calculator.
    Errors: 0.3, 0.4, -0.2, -0.1, Average +0.1
    
    Oct 28, LT=16:00 Sun-Moon at 76 degrees, strong wind,
    IC=+0.3
    Errors: -0.3, 0.0, -0.1, 0.0, -0.2, Average -0.1.
    
    3. Just for fun, I measured the same lunar with my
    POCKET sextant (scale reading to 1')
    Oct 28, LT=16:16:54 (average of 5),
    measured distance 76d43' (average of 5), error
    -0.2', Believe it or not!
    
    4. Back to Frank's sextant:
    Oct 28, LT=16:30:
    16:24:21    76d45'75    Error 0.0
    16:26:02    76d46'5     Error 0.2
    16:27:34    76d47'.0    Error 0.1
    
    Oct 29, LT=17:20 IC=0.0 (from earlier star observations)
    Sun-Moon, no wind.
    Errors: -0.1, 0.1, 0.2, -0.4, -0.1, 0.4, 0.0. Average=0.
    
    Oct 29, LT=19.45, IC==-0.2 Moon-Altair:
    Errors: 0.0, 0.2, 0.0, -0.1.
    
    Ost 29, LT=19.30, IC=-0.2 Moon-Fomalhaut:
    Errors: 0.0, 0.0, -0.1, 0.2.
    
    All this looks perfect.
    Unfortunately, this is only one part of the story.
    I experience the same difficulties with Frank's
    sextant that I earlier reported with my sextant.
    And I have no explanation.
    Some series are VERY bad.
    Here is an example:
    
    Oct 29, LT=16:20 Sun-Moon IC=0
    Errors: 0.4, -0.1, -0.4, 1.5, 0.9, 0.4, 2.8, 1.3, 2.8.
    Average error: 1.0, sigma=1.2'
    Notice that one hour later I did almost perfect Lunar
    (see above).
    
    Some other series look good before you reduce them
    (small sigma=0.1 or so) but turn out to be hugely biased.
    (by 1' sometimes).
    
    It is even worse with star-star distances.
    MOST of them are bad. Some pairs of stars are consistently
    bad. For example Altair-Deneb and Deneb-Vega (the worst.
    also the worst with SNO).
    Example.
    
    Oct 29, LT=20:20. Altair-Deneb.
    Computed: 38d00'.1
    Observed: 38d00'8, 38d00'8, 38d01'0.
    Errors: +0.7, +0.7, +0.9.
    
    OK, I say, let me set on the sextant the computed distance
    38d00.1, and THEN look...
    I see a hudge gap between the stars!!
    This shows that this is unlikely to be a sight problem...
    
    But then WHAT is this???
    
    It is better with some other pairs of stars,
    like Alkaid-Altair, Alkaid-Deneb and Alkaid-Vega.
    I got consistent good results on two nights.
    
    On the other hand, the triangle Deneb-Altair-Vega
    is consistently bad (more than 0.4 errors),
    and this is also consistent.
    
    Alex.
    
    
    
    
    
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