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A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Beginner
From: George Huxtable
Date: 2005 Sep 13, 10:30 +0100
From: George Huxtable
Date: 2005 Sep 13, 10:30 +0100
In reply to Asbjorn's comment- > > And when I look through my earlier measurements, all sights from the > > same "batch" (i.e those taken at about the same time and same set-up) > > gives about the same error in offset. E.g when I have an error of 5' > > in a sight, all sights taken at that time have about the same error. > > I have not yet understood why, and I have too little statistical > > material to give any conclusions about this. It might be coincidence. Bill wrote- >If the error is relatively consistent within a batch, perhaps index error is >the culprit. I have no first-hand experience with plastic sextants, but >from what I have read IE appears to vary to much more within a batch, or >from batch to batch, than does a metal sextant. and Asbjorn replied- >And it does. But I check for this after each sight. ============================== Comment from George. I don't think of index error as an "error" at all, in that sense. Index error degrades observations ONLY if it isn't allowed for, or if it happens to change between an index zero-check and an altitude observation. It's a simple check to make, and a simple matter to allow for. That should be part of the routine of every sextant user, of expensive metal sextants as well as simple plastic ones, whenever the instrument comes out of its box. Indeed, the check is so quick and easy to make that if the user of a plastic sextant is being ultra-cautious he can alternate index-checks within a batch of observations. That's what I used to do, at first, with my plastic Ebbco, until I became confident that it wasn't showing short-term changes. Now I check the index zero (against the horizon, in daytime, if it's sharp) when it comes out of the box, and again before it goes back, and it never shows a discernable change (reading to the nearest minute, which is all the instrument is good to). If a sextant shows real instability, it's likely to relate to looseness somewhere, probably in a mirror adjustment, which can be discovered by observing whether the reading responds to gentle pressure on each corner of a mirror. This can happen, even on a metal sextant, particularly if a "sextant-worrier" has over-slackened the grub-screws so that counter-pressure by the return springs has been lost. It's important to minimise backlash by always making the final adjustment of the index in the same direction, for index-zero observations as for the altitude observations. 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. If there's significant backlash, then Asbjorn's initial adjustment method, if I've understood it right, could be badly affected, though even then I would be surprised if it gave rise to a ten-minute error, which is what we're looking for. However, his recent communication makes it clear that he checks for index error after each sight, which is all that anyone could ask for. Bill admits that "I have no first-hand experience with plastic sextants", yet goes on to add "but from what I have read IE appears to vary to much more within a batch [of observations], or from batch to batch, than does a metal sextant.". In that, he is simply relating popular prejudice against the plastic sextant in general. In the hands of a careful user, there's no reason at all for index error to get in the way of a good observation, to the rather lower standards of accuracy that one accepts with a plastic instrument. 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. =============================================================== Contact George at george@huxtable.u-net.com ,or by phone +44 1865 820222, or from within UK 01865 820222. Or by post- George Huxtable, 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.