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    Re: Davis and Astra head to head
    From: Lu Abel
    Date: 2013 Mar 4, 08:50 -0800

    First of all, congratulations on 3,000 sights.   More than I've ever taken in my life, likely more than most ship navigators will take in their lifetimes these days.

    I don't think there's anyone on this list who would argue that there's any comparison between a Davis and an Astra.   I think, however, that there's a question of "good enough" 

    If I'm going to take sights off the pitching deck of a small boat in the middle of an ocean, do I need a sextant that is going to give hs's to tenth-of-a-minute accuracy, when it's doubtful that I can take a sight to anywhere near that accuracy?

    Also, is your Davis a Mark 15 or a Mark 25?  They seem to be made out of very different plastic and it would not at all surprise me to see a difference in variation in run if IE measurements between the two.

    Also, you describe measuring IE by alternately bringing the horizon up and bringing it down.  In my experience, IE can differ by a tenth or two on a Davis because of some lash in the micrometer.   My advice when I'm teaching celestial to anyone using a Davis is to measure IE both ways and then use whichever one matches the way the body is brought up/down. 

    Thanks for the comments on scope distortion.  The next time I have my hands on a Davis I'll look for that.

    Again, not trying to say that a Davis is anywhere near as good as an Astra.  At the same time, a good navigator understands the limits and weak spots of his instruments and tries to work around them.  I think there are some techniques to squeeze a bit more accuracy out of a Davis than you observed.


    From: Randall Morrow <randall.f.morrow{at}kp.org>
    To: luabel{at}ymail.com
    Sent: Monday, March 4, 2013 7:29 AM
    Subject: [NavList 22615] Davis and Astra head to head

    The topic of metal versus plastic comes up form time to time so I did a head to head comparison to satisfy my curiosity abotu how much diffrence there was. Though I am the only tester I have taken over 3,300 observations in the last 2-3 years so it is probably as good as any.
    Index corrections checks for the Davis were done the night before with a star, using 6 pairs of sights, each pair turning in opposite directions,and then averaged. The result was
    -8.4. The next day 3 sun sight IC checks showed and average of -8.0. The Astra IC was done witgh three sun sights that averaged +0.4. I have always found index correction sights to be inconsistent with the Davis so extra care was taken.
    Fifteen consectuive LL sun sights were done with the Davis using a mirror artificial horizon. The average intercept was 6.5 nm. The range was 3.6 minutes, from a low of 4.6' to a high of 8.2'. Every sight was "away".
    Only 12 sights were done with the Astra because I expected good results. Six were taken with the standard 3X scope with and average intercept of 0.4 nm. Another six with the 7X scope averaged 0.6 nm. The reange was 0.9', from 0.1' to 1.0'. There was only one "away". I was surprised that the 3X scope results wre pooere than the 7x, but thia is not typical.
    The preponderance of "away" sights is liekly a personal bias toward how I percieve tangency throgh the scopes. The Davis always appears to me to have optical distortion of the shape of the sun disc as you approach tangency.
    It appears the Davis is very consitent, though clearly not as accurate as the Astra. The reslults are certanly good enough to take you anywhere you might want to sail to. The metal frame was an order of magnitude better and though the stability of the metal frame and arc is a factor, I suspect that the quality of the optics may also be important. The more sights I take the more I am convinced that it is the observation, or the perception of what you see or think you see through the scope that is the dominant factor in accuracy.
    If anyone is interested I can post the sight data some other time. Randy
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