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    Re: Learn the stars, by phone
    From: Brad Morris
    Date: 2009 May 14, 13:19 -0400

    Hold on there!  I never claimed any pointing accuracy.
    It works to my satisfaction.  It beats the snot out of the 2102-D method of 
    finding the navigational stars.  Lots easier and basically more fun.  I 
    pretty much won't use it to 1 degree in any case, the navigational stars just 
    aren't that close.
    The Celestron Skyscout has two basic functions.
    1) If you ask it to find an object, it directs you to point the device in 
    various directions until it tells you that you are pointed right at it.
    2) If you point at an object, it can tell you what object that is.
    How it determines the magnetic deviation is not divulged.  I have had the 
    device tell me that there was a deviation when standing next to my vehicle 
    and when I was too close to power lines.  One internet reviewer who was using 
    it for astronomy indicated that it detected an anomaly due to a ferrous screw 
    in his kit.  That's right, a screw. I suspect bringing a bar magnet to the 
    device would set off the immediate complaint.  I will give this a twirl 
    tonight.  It doesn't care what mode you are in, it just starts complaining 
    the moment it senses the deviation.
    To Frank's question as to how the batteries don't affect the magnetic field: 
    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.
    Best Regards
    -----Original Message-----
    From: NavList@fer3.com [mailto:NavList@fer3.com] On Behalf Of George Huxtable
    Sent: Thursday, May 14, 2009 1:03 PM
    To: NavList@fer3.com
    Subject: [NavList 8267] Re: Learn the stars, by phone
    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.
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