NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Index correction sun sights
From: Lu Abel
Date: 2012 Feb 20, 15:01 -0800
From: Lu Abel
Date: 2012 Feb 20, 15:01 -0800
One thing that I've discovered with my Davis Mark 20 (discontinued, half-way between Mk 15 and Mk 25 in features) is that I get a different IE coming down than going up. So I follow the precautions and procedures Alan mentions, but in addition I make sure I measure IE in the same direction as I will be bringing the body (up to the horizon or down to the horizon). And, yes, absolutely measure IE both before and after a string of sights.
I've also heard (but not confirmed) that the Mk 15 is made of a lower quality, less stable plastic than the Mk 25. Anybody know for sure?
Lu
From: Alan S <alan202@verizon.net>
To: NavList@fer3.com
Sent: Monday, February 20, 2012 1:21 PM
Subject: [NavList] Re: Index correction sun sights
Randall:For whatever it might be worth, based on what I've been told by people who know better, as well as what I've read of sextant manuals, the following might be of interest.Those mirror adjustment screws should be turned as seldom as possible. Respecting index error, unless it gets to be something outrageous, check index error at the start and again at the end of a series of shots, and average the results, using that number as the index error correction. Fiddling with adjustment introduces wear to plastic sextants one is better off without.Like you, I'm a landlocked navigator, I find celestial navigation an interesting pass time, something to do when I get to the beach, or in Pittsburgh "down to the Mon", unless I break out an AH, and shoot in the parking area of our apartment complex.With a Davis Mk. 15, I use the above described approach. With a metal frame sextant, an Astra 111B, I check index error whenever I shoot, and usually take the average of 2 readings. Otherwise I might adjust mirrors once a year. With the Astra sextant, index errors are small, perhaps a couple of minutes or less, most always “on” the arc, at least so it is with mine.Alan----------------------------------------------------------------
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