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    Re: Star sparkle in sextant image
    From: Ken Gebhart
    Date: 2004 Sep 27, 09:54 -0500

    on 9/27/04 7:49 AM, Fred Hebard at Fred{at}ACF.ORG wrote:
    
    > ------------------------------------------------------------------------
    > 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
    >
    Sorry guys, but I have to speak up in defense of the Astra scope after Fred
    having said it was awful.  We have always felt that the scope was the better
    part of the sextant.  A few years ago, some engineers at Eastman Kodak
    tested several sextant scopes, and found the Astra to be the best.  A fellow
    named Bill Cook, who was then the head of the American Optical Society,
    wrote a magazine article saying the same thing.  Lastly, the last two
    hundred C. Plath Navistar Classic sextants sold before they became extinct,
    actually came with Astra scopes.
    
    It's possible that the scope Fred looked through was damaged in some way.
    It's also possible that a manufacturing run had a flaw in it.  I would
    certainly welcome any other expressions of quality from other List members
    (Joel?).
    Ken Gebhart
    
    
    

       
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