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    Re: Star sparkle in sextant image
    From: Fred Hebard
    Date: 2004 Sep 27, 08:49 -0400

    ------------------------------------------------------------------------
    Frederick V. Hebard, PhD                      Email: mailto:Fred{at}acf.org
    Staff Pathologist, Meadowview Research Farms  Web: http://www.acf.org
    American Chestnut Foundation                  Phone: (276) 944-4631
    14005 Glenbrook Ave.                          Fax: (276) 944-0934
    Meadowview, VA 24361
    
    On Sep 27, 2004, at 7:48 AM, Jim Thompson wrote:
    
    > I am still working to get my intercepts down to acceptable distance
    > from a
    > known position on land or at anchor.  All too often I am about 2-4
    > miles
    > away. Perhaps my biologics are against me?  Even when I think that I
    > have
    > perfectly lined up the sun's limb with a clear horizon, my sights tend
    > to be
    > out by that much at times.  Perhaps I cannot see the limb and horizon
    > as
    > clearly as my brain thinks it can.
    >
    > For now, though, I would like to explore star image quality.  We have
    > discussed star sparkle before on this list, but I finally got around to
    > attempting to draw what I actually seem to see.  Sparkle makes it
    > difficult
    > for me to precisely line up the star's images on top of each other.
    > This
    > means that I have to take a series of sights to obtain an Index Error
    > before
    > and after each round of sights, and then average them. Typically my
    > variation in a series of IE attempts ranges to about +/- 0.5' of arc,
    > which
    > seems like a lot to me.  I would have thought that a navigator could
    > expect
    > less variation, say +/ 0.2' or even 0.1'?
    >
    > How do the images on the following page compare with what you see
    > through
    > your sextant?  Is there more sparkle, or less?
    > http://jimthompson.net/boating/CelestialNav/CelestNotes/
    > UsingSextant.htm
    
    Jim,
    
    It's taken me about a year and a half to get my technique down with an
    artificial horizon.  I think the secret is practice, practice,
    practice!  A key component is timing the sight.  With a fast rising
    body, an error of 1 second in time can be 0.2' of arc.  Often, I wait
    on the objects to settle to the position rather than trying to adjust
    the index arm to a perfect setting.  Then it is a matter of knowing
    when they are touching.  For an artificial horizon, good practice for
    this can be obtained by measuring the index error in conjunction with
    determinations of the sun's semidiameter.  I don't know an acceptable
    substitute for judging a sea horizon.
    
    I had an opportunity to go sailing this summer and took two sun sights,
    with errors of 9 and 12 minutes of arc, so I definitely need a lot of
    practice on water!  My attempts to determine index error by measuring
    the sun's semidiameter also were not very satisfactory.  On the beach I
    got three horizon sights all within 1 minute; part of the difference
    may have been having firm ground under foot.
    
    Like you, I have difficulty using stars to determine the index error.
    However, the sparkle I see is not as bad as what you depict in your
    link.  I remember once looking through an Astra telescope and being
    struck at how awful the optics were, in comparison to a Plath I also
    looked through (in a shop).  You might try going to a shop and asking
    to look through a few of their telescopes.  The Astra will take other
    telescopes, such as a Tamaya.
    
    I also read somewhere that it is easier to determine index error using
    dim stars rather than bright ones; reduces sparkle.
    
    Fred
    
    
    

       
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