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    Re: Removing Index Error with the Sun
    From: Greg Rudzinski
    Date: 2011 Oct 23, 17:51 -0700


    The plastic Davis MK 15 is not the preferred instrument for taking lunars. It is satisfactory for small craft celestial navigation which is what it is designed for. To improve results for your lunars you will need to check index error before and after the lunar by superimposing the images of a not too bright star. Plastic sextants can't be trusted to hold a constant index error from sight to sight. Well used plastic sextants are particularly bad about staying in adjustment. Don't squeeze the Davis either. This will cause a noticeable deflection. To see this effect just line up the images of the horizon and apply some pressure to the arc while still looking at the horizon through the scope. You should see the horizon start to gap apart. Remember to refocus the scope on the horizon before starting the index error check. Height of eye should be 6 feet or more to avoid sextant parallax. Beware of haze on the horizon which will cause inconsistencies.

    My advise is to start saving up for a metal sextant :)

    Greg Rudzinski

    [NavList] Removing Index Error with the Sun
    From: yhshuh---com
    Date: 23 Oct 2011 15:25

    Forgive me if this has been posted before, but I searched the forum and couldn't find the answer I was looking for.

    Recently I was reading about the method for removing index error in which one sights the diameter of the Sun via the upper and lower limbs, then reverses the images and splits the difference. Up to this point I was either using the horizon or superimposing the sun on itself with fair results.

    However, on the 21st of this month I went out to shoot a lunar and used the horizon to remove all I.E. Then, I checked my work by sighting the diameter. I found an I.E. of 2.0' off scale. I noted this and continued with the round of sights: Sun & Moon altitudes, the L.D. and a second set of altitudes.

    Back at home I used the method outlined on Frank Reed's "Easy Lunars" page to clear the distance. (By the way, I highly recommend this page. Mr. Reed provides a concise, easy method. A handheld calculator with trig functions and the Almanac are all that's required. All of my results have been verified by computer. Mr. Reed says he needs to update the page, but I say "If it ain't broke..." :) The page can be found here: http://www.historicalatlas.com/lunars/easylun.html )

    Anyway, I found that my result was -00:07:21 from actual GMT. Then I decided to ignore the I.E. and trust my horizon alignment. Re-calculated and found my result was now -00:03:21 from GMT. A lot closer to my first Lunar which (and yes, I am bragging) was only about a minute off.

    So, am I doing something wrong? FYI I am using a Davis Mk 15 plastic sextant and a natural sea horizon at about 5 mi. distance. All sources of error including watch error and temp./pressure were accounted for. Any help would be greatly appreciated.


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