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Frank Reed introduced this thread by pointing out that digital compasses
were now available that could determine horizontal pointing within "one
degree or better".

Questioned about the implausiblility of using the Earth's magnetic field to
such precision, because of the effects of local perturbation, he quoted as
evidence the Celestron Skyscout, and claimed for that device even greater
precision, with an "accuracy of about 0.5 degrees", which "was not bothered
by minor magnetic interference" (whatever that meant). No other evidence has
been produced; just that example.

As a hadn't found find such a claim in the Skyscout information, and asked
for further detail, he wrote- "That half-degree claim comes from the
official specs. It's a believable claim, based on performance..."

But it turns out that Celestron's current website now states- "The SkyScout
has a pointing accuracy of 2-3 degrees (a thumbtip at arms length)". Just
the sort of error limit that would be expected from a compass-type device,
indeed.

Frank remained unperturbed- "If we were talking about a sextant or other
measuring instrument, those statements would be contradictory, but we're
not." He followed that in his latest posting with- "No, there isn't a
contradiction because we're using fairly vague terminology, and both
descriptions can fit just fine."

Well, Frank used the "vague terminology" he thought best at the time. If he
regards his statement of an "accuracy of  about 0.5 degrees" as "fitting
just fine" with the present specification of 2-3 degrees, then that puts
Frank's attitude to errors in a new light. It's a somewhat breathtaking
admission to be putting forward at this stage. If he had said, from the
start, "well, when I say 0.5 degrees, that would be perfectly compatible
with an error of 2 or 3 degrees", nothing would have remained to argue
about.

Readers may recognise a pattern here, in line with earlier claims for
implausible precision made by Frank for observations, proposals,
instruments. Details about the applicability of those claims become elusive,
when challenged. Perhaps this ratio, between 2 to 3 degrees, and 0.5
degrees, constitutes a "Frank factor", that we should be prepared to apply
to other such claims.

Dave Walden led us to the Scope City website, at
http://www.scopecity.com/detail.cfm?ProductID=6058
which read this, about the Skyscout-  "Accuracy - Pointing accuracy within
1/2-degree."

And indeed it does. Which fits in with Frank's statement that he had seen
that figure in the specs. I contacted Scope City to ask, and have had the
following reply, from their General Manager, Don Pensack-

"The claim was from the text provided us by Celestron way way back. It is in
error and I will change the text (our web manager is not an astronomer).
The accuracy is no better than the standard Sky Scout which is about 3
degrees."

So Celestron, and Scope City, have accepted their earlier specification was
untenable, and have loosened its precision by a factor of 6. That's an
increase in the sky area defined by its identification, by a factor of 36.
It accords with the precision that most compass users would expect.

In his latest mailing, Frank shouts, in capitals- "What I HAVE SAID (or at
least I tried to get this across) is that NONE OF THIS MATTERS. This is not
a measuring instrument. It's an educational device. It does what it does. It
works "as advertised"."

It matters, because Frank was quoting this instrument as an example, his
only example, of the availability of precise pointing devices. What's more,
even as a toy, if the pointing accuracy is no better than 3 degrees (which
could put the identified pointing direction right at the edge of the outer
viewing ring) then that would seriously degrade its utility at doing its
job. Celestron now parade their increased internal database, from 6,000 to
50,000 sky objects, only a tiny fraction of which will be naked-eye visible.
Within a circle-of-uncertainty of 3 degrees radius, I make it that there
will be, on average, 34 such stars. How likely is it that the Skyscout will
identify the "right one", of those 34? If the user could restrict himself to
the menu of 20-or-so bright stars, there would be no such problem. But if
one wanted to identify the stars of the Pleiades, that would become an
impossible task for the Skyscout (and difficult enough, even with 0.5
degrees precision).

All this reinforces the point I made at the start of this thread; that we
should treat all such implausible-sounding claims with an appropriate degree
of scepticism.

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.




storicalAtlas.com>
To: 
Sent: Saturday, May 23, 2009 1:26 AM
Subject: [NavList 8393] Re: Learn the stars, by phone



George, you wrote:
"There's the contradiction."

No, there isn't a contradiction because we're using fairly vague
terminology, and both descriptions can fit just fine.

Let me give you an analogous example: suppose one navigator says "celestial
navigation with a good metal sextant is as accurate as 0.5-1.0 nautical
miles" and another navigator says "celestial navigation with a good metal
sextant is only as accurate as 2-3 nautical miles". Those could both be
reasonable statements since neither one of them makes any real statistical
claim. Neither you nor I have said anything about standard deviations or
expected error or probable error or any of the other things that would
QUANTIFY the expression "pointing accuracy". What I HAVE SAID (or at least I
tried to get this across) is that NONE OF THIS MATTERS. This is not a
measuring instrument. It's an educational device. It does what it does. It
works "as advertised".

And you asked:
"Frank informed us his quoted half-degree "came from the official specs".
Well, did it? Did Celestron make such a claim at some time, then diluted
it?"

I can tell you that I definitely saw that in printed materials from
Celestron back when the SkyScout was first released. Beyond that, you're on
your own. If you're really so absorbed, indeed so obsessed, by all of this,
then I suggest you contact Celestron.

And you wrote:
"That would be interesting, in itself."

Yes, it might be interesting. Publishing the real accuracy in practice would
only get the company in trouble. Publishing much broader error ranges later
would probably save some customer complaints. And again, you might be able
to ask someone at Celestron about it, but I doubt they would reply since
litigious folks... well... you could imagine.

As for your comments suggesting that I'm just trying to find "wiggle-room"
or that I am trying to sweep something "under the carpet", I learned within
weeks after joining this group five years ago that you make comments like
this when you're frustrated. I can understand your frustration, George, but
I can't help you with it.

-FER







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