A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Mark Coady
Date: 2015 Dec 22, 07:07 -0800
My conclusion exactly. Once I have it zeroed on the 20 x scope. and I put on the 7x or 4x agian. I just check it with three or four quick tests...if it all seems to still average around zero....happy day...
The adage goes..."don't fix waht ain't broke"
On my comments regarding published "accuracy" around the 9"-12" seconds for the high end brass racks, and 18"-20" seconds published for aluminum racks. Obviously that range is .15' TO .33'...for lunars, if that were average measurment error, that would be huge...but....
As i have come to understand it, based on the accuracy achieved in Lunars, its an overall number covering the arc extremes based on manufacturing precison. It doesn't seem to equate to anything near "slop" error for index setup, simply because we seem do much better than that.
If there are any sextant experts out there, i would love to have someone explain how that number is actually measured, so that I might understand why its impact seems elusive.
My Astra has an excellent characteristic of any good Sextant, timepiece, or measurement system, the 20x scope proves it: REPEATABILITY. I can deal with error, as long as the error is consistant and repeatable.
With the 20 x scope..I seem to get to pretty much the same place every time. Well within the published "errors" ...thus an excellent result. The inaccuracy I experience on the same day same conditon as I go back to 7x or 4x has a lot to do with the idiot holding it....and the inadequacy of his vision and technique.
THis may also be why some sextants, set up by the book, always seem to average consistantly a little "high" or "low" checked agianst a GPS positon. The error is there, but fixed.
Like when I kept Loran C charts with my own marks....sure...it's chart accuracy was far less than perfect...but if i had my own marks on my lobster trap string...or favorite fishing rock...it was pretty much gonna match my actual recorded coordinate...tommorrow, next week...next month......... ( I want to slap the guy who took away my Loran C, dammit..I liked it....... easier to do in your head without a plotter)
If I want to drive myself insane, I suppose I could log every practice sight agianst a charted precise position, then average them out and build an arc chart that consistantly proves what part of my arc might be averaging high or low....over a consistant history... that might tell me a lot about my sextant's secret sins..or i might be taken away in a white coat that buttons in back.........
I am already insane enough..so I am content to check my own longhand sight reductions agianst a computer...and live happily ever after if the errors are within accepted tolerances.