NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Learn the stars, by phone
From: George Huxtable
Date: 2009 May 15, 09:14 +0100
From: George Huxtable
Date: 2009 May 15, 09:14 +0100
In response to my earlier posting, in which I wrote, about the Skyscout- "Brad hasn't addressed that claim of 1� of accuracy, for direction, ..." Brad wrote- "Hold on there! I never claimed any pointing accuracy." Yes, that's exactly what I was trying to say! Brad has, so-far, made a partial test that it can detect the presence of external magnetic fields, but hasn't yet got to the crucial stage of measuring the change in measured angles at the point where that detection threshold is crossed. I suggest that next time, he doesn't even need to take it out in the rain; it doesn't even need to see the sky. Just choose a reasonably non-magnetic location indoors, prop the instrument up to point towards a reasonably populous part of the sky (even though no view appears), note what it chooses as the nearest star to where it happens to be pointing. Then apply some external magnet as we've discussed, just short of crossing the detection threshold, to see what is then identified as the nearest star. By comparing the azimuths and altitudes of those two stars, Brad should get a good notion of the threshold for magnetic deviation, in its effect on measured angle. He adds- "there is a mu metal shield provided with the device that is placed around the batteries. Mu metal is a well known material which actually reduces magnetic fields across its boundary." Now, that really does surprise me! Mumetal, with its remarkably high magnetic permeability, makes a remarkably good barrier, for isolating magnetic fields, just as Brad says. It would do the trick well, of keeping any magnetic field from the batteries from affecting the rest of the instrument. But it has another effect, precisely because of that high permeability. It tends to draw into itself any magnetic field lines, from the Earth, that would otherwise pass through unaffected, uniform and parallel (in much the same way as a ferrite rod concentrates radio signals, though that analogy is imperfect). So the presence of the mumetal shield, in itself, would distort the ambient field from the Earth, in a way which would vary with the orientation. Is Brad certain that it's mumetal? George. contact George Huxtable, at george@hux.me.uk or at +44 1865 820222 (from UK, 01865 820222) or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---