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    Re: Index Mirror Correction
    From: Wolfgang Bosswick
    Date: 2003 May 14, 13:17 +0200

    Dear Gary,

    My Sextant is an elder Navacom Yacht Sextant. Thanks a lot for your advice, it is very comprehensible and looks like a perfect procedure for adjusting the index mirror precisely. I found the standard method (dismount scope, index at 30°, adjust to unbroken arc when looking from above through the index mirror) very difficult - no way to get a precision of a few minutes in perpendicularity. With the cylinders, I can imagine an excellent precision. I will try it (of course followed also by correcting the horizion mirror both for perpendicularity and a minimal index error).

    BTW, there came no correction table for the arc error with the used sextant, and I would also like to check the overall precision of the sextant. Do you or someone else know a source for sights between stars (not to the horizon) in the range from 10° to 100° on the Northern hemisphere (LAT ~ N48°)?

    Thanks a lot,


    (HTYCGARY@aol.com) schrieb:

    In a message dated 5/13/2003 5:54:42 AM Eastern Standard Time, wolfgang.bosswick@sowi.uni-bamberg.de writes:

    Any advice would be very much appreciated.

    You don't say what kind of sextant you have so...I'll try to describe a method that should work equally well for any normal marine sextant.  Find 2 cylinders about 1/2-3/4" in diameter and about 3/4-1" long.  Rest the sextant horizontally on a flat surface and place one cylinder on the arc at 0 degrees, place the other on the arc at 100 degrees.  look "down" on the arc from above the index mirror such that you can see 1/2 of the cylinder that is on 0 degrees.  Play with the coarse adjustment arm until you can see the other cylinder reflected in the index mirror.  Make sure that your eyes are at the same level as the tops of the cylinders.  Adjust the index mirror until the tops of both cylinders are at the same level.  Don't forget that you still have 2 adjustments on the horizon mirror that can throw your sights off.  Both adjustments are important, but the most critical adjustment is for index error.  You either need to know your index error so that you can factor it out during the sight reduction or you need to adjust it to zero.

    Hope this helps,

    Gary Harkins C400 #140 "Cygnus"
    Website   http://www.hometown.aol.com/htycgary/myhomepage/index.html
    Harbour Towne Marina, Muskegon, Michigan Slip C-28
    Harbour Towne Yacht Club, Life Member
    C400 National Association
    United States Power Squadrons-JN

    NRA Life Member
    MCRGO (Michigan Coalition for Responsible Gun Owners)
    JPFO (Jews for the Preservation of Firearms Ownership)

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