NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Beginner
From: Bill B
Date: 2005 Sep 14, 16:30 -0500
From: Bill B
Date: 2005 Sep 14, 16:30 -0500
George wrote: "I should add that, just like Bill, I have no experience with the Davis model that Asbjorn was using, but Bill's comments tarred all plastic sextants with the same brush; and unfairly so, in my view." George It was not my intent to tar all plastic sextants, but merely pass along the feelings/observations of those more qualified then myself and with first-hand experience--like Celestaire and David Burch. Burch wrote in a PDF on his site http://www.starpath.com: "I would also propose ? as a broad generalization ? that using these procedures one should be able to obtain accuracy?s of some 5 or 6 miles as a general rule with plastic sextants. Maybe better in some cases, maybe a bit worse in others." Celestaire's seminar handout claims 3 to 10 miles. In your own words while speaking of your Ebbco and IE checks, "...and it never shows a discernable change (reading to the nearest minute, which is all the instrument is good to)." Given that is only a minute, not five or ten, it is still greater by an order of magnitude than one would expect from a quality metal sextant. Your results with the Ebbco are a testament both to the Ebbco and your abilities. But if you are in the mood to shoot the messenger...George also wrote: "Asbjorn described his technique, which as far as I could tell required adjustment of the index error to zero (using the grub-screws?) on each occasion, which appeared to me misguided. A better procedure would be to leave those grub-screws alone, after an initial setting, and just accept and allow for any subsequent index error: However large that index zero-error happens to be, if it's allowed for then it doesn't degrade the precision of the measurement AT ALL." I find this interesting. In my limited experience, adjusting for IE also affects side error. Again risking reprimand, what I take from the list is that a little side error is not a big problem, and may be useful. (Star-to-star or star-to-planet angular measurements?) I am not clear Asbjorn checked for side error after his index error adjustments. I suspect side error introduced by adjusting for IE would not account a 5' error. The problem being I have no idea how to quantify side or index-mirror perpendicularity errors. My texts explain how to adjust those errors out, and check for side error with a star or horizon, but little more. The texts I have read first have one adjust for index-mirror perpendicularity. Then there are back-and-forth adjustments for horizon-mirror side error and index error until the index error is close and the side error is eliminated. One check for side error involves rocking the sextant to see if the horizon remains aligned. Being landlocked I use a faint star. The questions arise: 1. How (other than the arc check for rear silvered and block check for front-silvered mirrors) to check for index-mirror perpendicularity error in the field? 2. What errors will be introduced, and to what magnitude, if the index mirror is not perpendicular? 3. What errors will be introduced, and to what magnitude, if the horizon mirror is not perpendicular? It strikes me that by using a star to adjust out side error, it may appear perfect, but still have problems in use if the index mirror is not exactly perpendicularity. Any thoughts or expertise you or the list could offer would be appreciated. Bill