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    Re: Learn the stars, by phone
    From: George Huxtable
    Date: 2009 May 27, 00:00 +0100

    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{at}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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