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    Re: Index correction sun sights
    From: Greg R_
    Date: 2012 Feb 20, 12:56 -0800

    > http://fer3.com/arc/m2.aspx/Santa-Barbara-sights-PeterHakel-jul-2011-g16765
    
    Wish I'd known that you were in the area back then - I live about an hour
    from Santa Barbara and would have been glad to join you for some sextant
    practice.  :-)
    
    --
    GregR
    
    
    
    ----- Original Message -----
    From: P H
    To: NavList@fer3.com
    Sent: Monday, February 20, 2012 11:31 AM
    Subject: [NavList] Re: Index correction sun sights
    
    
    Same here with my Mark 15.  Last July I wrote:
    
    
    "The main difficulty I had was establishing the index error of the
    instrument. The index mirror held perpendicular to the frame very well but I
    spent a lot of time tinkering with the horizon mirror. I repeatedly measured
    and adjusted the index error by looking at the horizon, overlapping the two
    sun disks, and also using the "Solar IC procedure" from David Burch's book
    on plastic sextants. The IE tended to gravitate toward 8' to 9' off the arc
    but I would have to attach an "error bar" of a few arcminutes to that value
    from what I saw. Using this Solar IC method to recover the Sun's
    semidiameter also showed differences of the order of 1'. Between sights I
    turned my back to the Sun and shielded the sextant from direct sunlight."
    
    The full posting is here:
    
    
    
    http://fer3.com/arc/m2.aspx/Santa-Barbara-sights-PeterHakel-jul-2011-g16765
    
    
    
    
    
    Peter Hakel
    
    
    
    
    
    
    From: GregR 
    To: NavList 
    Sent: Monday, February 20, 2012 11:47 AM
    Subject: [NavList] Re: Index correction sun sights
    
    
    Your experience mirrors mine (pardon the pun...) with a Davis Mk 25, and
    I've also heard/read the same advice about not turning the adjustment screws
    on a plastic sextant very often. The solution for me was to upgrade to a
    metal one (Astra III B) which solved all of those issues nicely, plus
    increased my accuracy to where I can usually get within 1 NM compared to a
    GPS position. They show up on EBay fairly regularly, seems like I paid
    around ~$400 for mine back in 2006.
    
    --
    GregR
    
    
    ----- Original Message ----- From: Randall.F.Morrow@kp.org
    To: NavList@fer3.com
    Sent: Monday, February 20, 2012 7:51 AM
    Subject: [NavList] Index correction sun sights
    
      I spent some time yesterday doing repeated index correction sun sights
    with my Davis Mk 15,  using the techniques described in a Starpath article.
    In more than 20 observations I got 20 different values, both on and off the
    arc before I finally gave up.  Previously I had posted that sun and moon
    sights using the appearance of tangency always give me trouble due to the
    distortion I observe as the edges approach.  But in addition to this, when
    the images do not fall on a vertical line I adjust the horizon mirror as the
    article suggests, but when I repeat the sight for verification the images
    are again off line.    It has been suggested that I do not turn these screws
    on the mirror often but they do not stay in line from one sight to the next
    so re-adjusting seems necessary.    When turning the horizon mirror finger
    screws the image seems to be affected by movement of the plastic frame of
    the mirror by the pressure from my fingers as I turn the screw.  Is lack of
    rigidity in this component a factor?  Before making the sights I removed the
    telescope and looking at the reflection of the arc, brought the direct and
    reflected images in line for perpendicularity.  Am I missing something?
    Please advise.
    Randy
    
    
    Randall F Morrow PT
    Kern County - Bakersfield
    
    
    
    
    

       
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