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    Re: Learn the stars, by phone
    From: George Huxtable
    Date: 2009 May 14, 18:03 +0100

    Thanks to Brad and Frank for their responses to my questions about magnetic 
    direction sensing, and for links to additional information.
    I was questioning one aspect, and only that one, of the claims that were 
    being made about such a device, the ability, in Frank's words, to "determine 
    true direction in three dimensions in most parts of the world to an accuracy 
    of one degree or better."
    If that were possible, I could see an obvious application arising on my own 
    boat, and no doubt so would many others. The marine world would beat a path 
    to its door. But I was somewhat sceptical (as usual) and asked a number of 
    questions, recommending that such claims should be taken with a pinch of 
    salt, and asked who was making them.
     Brad hasn't addressed that claim of 1� of accuracy, for direction, but 
    Frank has taken it further, by writing- "it can determine where you're 
    pointing in the sky from anywhere on Earth at any date and time ... with an 
    accuracy of about 0.5 degrees." , but he still doesn't state where this 
    figure comes from, what instrument it refers to, and under what conditions 
    it applies. Nor have I found any claim for such directional precision in the 
    websites I was directed to, though I may have missed it, somewhere. It 
    doesn't seem to be included in the many pages of promotional guff about the 
    Celestron Skyscout, which as far as I can work out doesn't even display the 
    compass direction it's pointed at, just names a likely star that's in that 
    direction. Have I got that right?
    The review article that was pointed to is of little help, as its author 
    clearly failed to understand even what the accelerometers were for (they are 
    to establish which way is up).
    Brad tells us of his satisfaction with its behaviour when used for 
    identifying stars (which is, of course, its purpose) but little about its 
    pointing accuracy. He tells us "It is sensitive to local ferrous materials 
    and in fact informs you when the deviation is greater than some internal 
    value.". I'm sceptical about that magic ability. How could that be done? It 
    could, perhaps, by detecting any significant difference in the total field 
    strength, or the dip angle, from the value predicted for that location on 
    the Earth. But could it establish, by such means, a deviation that changes 
    the magnetic direction by 1�? I don't believe it!
    Frank, as is his style, belittles that problem by addressing it in 
    exaggerated terms- "Clearly, as you suggested, if there's significant 
    magnetic or acceleration interference (you wouldn't want to use it inside an 
    iron carousel), then it would have problems.. ". Well, of course, I wasn't 
    considering an "iron carousel", I was talking about real-life difficulties 
    that interfere with real-life compasses, difficulties that need serious 
    Brad could make an interesting little project of it (except that I know he's 
    involved in several interesting projects already).  He could point his 
    Skyscout to where two stars appear, reasonably close together side by side, 
    near the horizon, and establish the threshold point between  identifying one 
    and the other, by swinging the instrument a bit from side to side to see at 
    what direction it switches. Then try to fool it a bit by bringing a little 
    bar magnet towards it, from East or West,  pointing horizontally, one pole 
    towards the Skyscout,  at the same level as the instrument. It would be 
    interesting to see how far the dividing-line can then be shifted, before it 
    warns that magnetic material is present. Frank states that "It's not 
    bothered by minor magnetic interference", but to what extent has he tested 
    that, and how? What results did he find?
    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
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