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    Re: Learn the stars, by phone
    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?
    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. 
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