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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
Brad




-----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
consideration.

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?

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.




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