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Originally Posted by alan2  Unless one has previously taken shots with this sextant, how does one arrive at the referenced index correction, or might this be a printing or editing error that got by? |
You should periodically check for index error, as has been described. If the error is gross you can fix it with the adjustment screw on the mirror. Once the error is less than (iirc) 3 min, then it can just be applied as a correction.
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Re the Celestaire Practice Bubble Horizon, page 13 of Celestaire’s 2009 catalog, see page 12 for Davis unit, one finds the following instruction for determining the BC (Bubble Correction). Using the Known Position Method, “take several observations from a known geographic position (GPS does serve a purpose), and compute the lines of position normally. You may attribute the average error to the bubble, and subtract as a correction (BC) for future use”. My questions follow.
Re the several observations mentioned, I assume the following. One takes several sets of sun shots. Reduce each set of shots, and plot them. You have already plotted your Known Position. Measure the displacement between calculated position plots and the plot of Known Position. Say you took three sets of shots, the average error, displacement between their plotted positions and that of Known Position being say 12 nautical miles, the BC would be 12 minutes of arc, 1 minute of arc equaling 1 nautical mile. Next question is, re “subtract this as a correction (BC)", subtract the obtained BC from what number. I use the USPS SR 96a form for reducing sights.
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I assume you're just using the bubble horizon for land-locked practice - if that's the case then I'd ignore it as Dave has suggested. If you still want to correct for it, then I would suggest charting the errors between your practice LOPs and your known position. The bubble error will be like index error - it will always be on or always be off. You can apply it in your calcs with the IE or you add or subtract from your run when plotting your LOP. Obviously the error between GPS and LOP is going to vary widely, so you will have to take a lot of shots to determine an error from the averaging method. Assuming all other errors will cancel each other out the only remaining error will be a close approximation of the bubble error.
Bowditch has a pretty good section on the sextant, but I don't recall if he talks about artificial horizons.
Kevin