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    Re: Determining tangency for moon & sun sights
    From: Lu Abel
    Date: 2011 Nov 9, 20:24 -0800
    I think we have at least three separate factors working here (and thanks, Alan, for clarifying):

    1.  Quality of optics

    2.  Quality of AH

    3.  Overall quality of sextant.

    With respect to the latter, it's true that the Davis has lower accuracy than metal sextants, but the problem Randall described was one of image quality, so I don't think this is an issue.

    Let's get back to the problem he described.   It sounds as if he's getting a fuzzy image regardless of shades, etc.   That's not been my experience with a variety of Davis sextants (15, 20, and 25).   My own Mark 20 returns a sharp image of the sun; horizon clarity is more a function of weather conditions, local haze, etc, than anything with the Davis.   

    The one thing I do notice is that their full-horizon mirrors are a disappointment.   Trying to do something as simple as measuring Index Error is difficult if there's any haze on the horizon at all.   The reduction in clarity caused by reduced light transmission (and possibly optical distortion) makes it hard to get a clear, sharp horizon.   I experienced this personally taking my celestial class out for sights.  The view over San Francisco Bay was a bit hazy with low atmosphere-water contrast.   The class members with conventional split mirrors had no problem measuring IE and then bringing down the sun (UL and LL) for reasonably accurate shots.   But the one class member with a Davis Mark 25 with full-horizon mirror had a tough time getting a sharp horizon; I tried his Davis and very much experienced the problem.   

    But Randall says his Davis has a split mirror.

     Without having Randall's sextant in hand, I speculate that there's an optical problem, either in the sextant itself or in the artificial horizon.  I don't know if it's possible, but perhaps he can try another sextant?

    Lu Abel


    From: Alan <alan202@verizon.net>
    To: NavList@fer3.com
    Sent: Wednesday, November 9, 2011 4:46 PM
    Subject: [NavList] Re: Determining tangency for moon & sun sights

    Randall asked:
    Is the quality of the optics a factor in seeing the point of tangency?
    Can't prove it, but I suspect that the answer would be YES!!!
    Additionally, I also suspect that the quality of the mirror in your AH is also a factor.
    Also, while the Davis Sextant is certainly useable, metal frame sextants are a lot more expensive, the Davis isn't the most reliable of instruments.
    Alan
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