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    Re: Sextant precision
    From: Henry Halboth
    Date: 2004 Oct 5, 00:10 -0400

    For an excellent review regarding the determination of centering error, I
    would refer you gentlemen to a work previously referenced here, i.e.,
    Hydrographical Surveying by Wharton & Field, 1920 edition. For Fred's
    information, I do briefly quote as follows ...
    
    "The centering error is determined at Kew Observatory for certain angles
    by fixed collimators ...
    .......", etc.
    
    Also Fred, you might refer to Lecky who, in his work, refers to the use
    of the collimators at Kew Observatory, as the most expiditious way of
    determining this error..
    
    The stellar method for determination of centering error recommended by
    Messrs. Wharton & Field, as the most accurate is, and I quote .......
    
    "The most accurate method, because it employs a large number of
    observations for the same, or nearly the same, angle, is by observation
    of pairs of circum-meridian stars in the artificial horizon, at various
    altitudes. Double the difference between the resulting latitude by each
    star, and the mean latitude, will be the centering error for an angle
    equal to the double altitude of that star, that is the angle actually
    measured by the sextant, INDEX ERROR BEING CAREFULLY DETERMINED AND
    APPLIED BEFORE WORKING OUT (emphasis by me)".
    
    It is further stated ...
    
    "Centering error may also be obtained by careful measurement of the
    angles between stars. The correct apparent distances must be found in the
    same manner as in clearing a lunar distance; the true distance being
    first calculated .......", etc.
    
    Regardless, at the apparent precision that you gentlemen seem to be
    seeking, I seriously recommend to you the referenced text which seeks to
    define the utmost accuracy in the determination of position by celestial
    observation.
    
    On Mon, 4 Oct 2004 08:39:41 -0400 Fred Hebard  writes:
    > On Oct 4, 2004, at 1:05 AM, Alexandre Eremenko wrote:
    > > After many repeated measurements I found the star-to-itself
    > > method more precise than the Sun method. This is the case
    > > with my sextant, my scopes and my specific observation
    > > conditions, I do not generalize this to all situations.
    > > (Unfortunately I have no possibility to use real
    > > horizon at this time).
    > >
    > > I can post the numerical data if this is interesting.
    > >
    > > However I don't know of any other method (other than star-to-star
    > > distances) of measuring what you call "centering error"
    > > without special equipment.
    > > This should answer your sentences:
    > > You say that the better method was:
    > >
    > >> At one time, an instrument
    > >> could be sent off for testing by a system of
    > >> collimators,
    > >
    > > But I don't have a collimator. And many other people don't.
    > > Bougainville :-) who used the stars for a similar test,
    > > as described in the recent messages:
    >
    > Alex,
    >
    > I would like to see some of your star data for index error.  I post
    > below some sun data, with a few moon determinations.
    >
    > Regarding machines for determining centering error (the centering of
    > the pivot of the index arm), I don't believe those are called
    > collimators.  I believe a collimator is for determining centering of
    > the telescope.  The machines and operators, together encompassing a
    > service, for determining centering error are not available in the
    > U.S.,
    > to the best of my knowledge, except possibly at the Warren-Knight
    > Instrument Co., .
    >
    > Fred
    >
    > date&zone time  object,sextant  on'     off' IndexError"        SD'
    >
    > 09/05/2004 15:15        sun, C+P        32.4    31.1    39
    > 15.88
    > 09/05/2004 15:15                32.4    31.1    39      15.88
    > 09/05/2004 15:15                32.4    31.2    36      15.90
    > 09/05/2004 15:55        sun, C+P        32.3    31.5    24
    > 15.95
    > 09/05/2004 15:55                32.2    31.3    27      15.88
    > 09/05/2004 15:55                32.2    31.5    21      15.93
    > 09/06/2004 10:30        sun, C+P        31.7    31.75   -1.5
    > 15.86
    > 09/06/2004 10:30                31.8    31.9    -3      15.93
    > 09/06/2004 10:30                31.8    31.9    -3      15.93
    > 09/10/2004 19:00        sun, C+P        31.6    32.1    -15
    > 15.93
    > 09/10/2004 19:00                32      32      0       16.00
    > 09/10/2004 19:00                31.8    32      -6      15.95
    > 09/11/2004 16:40        sun, C+P        32.1    31.8    9
    > 15.98
    > 09/11/2004 16:40                31.9    31.8    3       15.93
    > 09/11/2004 16:40                32      31.9    3       15.97
    > 09/19/2004 17:45        sun, C+P        31.8    32      -6
    > 15.95
    > 09/19/2004 17:45                31.7    32.1    -12     15.95
    > 09/19/2004 17:45                31.7    32      -9      15.93
    > 09/20/2004 18:15        sun, C+P,       31.2    32.6    -42
    > 15.95
    > 09/20/2004 18:15                31.4    32.4    -30     15.95
    > 09/20/2004 18:15                31.3    32.3    -30     15.90
    > 09/21/2004 17:00        sun, C+P        31.5    32.4    -27
    > 15.97
    > 09/21/2004 17:00                31.7    32.4    -21     16.02
    > 09/21/2004 17:00                31.8    32.2    -12     16.00
    > 09/22/2004 14:00        sun, C+P        32.1    31.9    6
    > 16.00
    > 09/22/2004 14:00                32      31.9    3       15.97
    > 09/22/2004 14:00                32.3    31.9    12      16.05
    > 09/23/2004 17:30        sun, C+P,       31.8    32.6    -24
    > 16.10
    > 09/23/2004 17:30                32      32.4    -12     16.10
    > 09/23/2004 17:30                32.1    32.4    -9      16.12
    > 09/24/2004 22:30        moon, C+P       32.5    32.7    -6
    > 16.30
    > 09/24/2004 22:30                32.3    32.6    -9      16.23
    > 09/24/2004 22:30                32.4    32.7    -9      16.27
    > 09/26/2004 09:45        sun, C+P,       31.7    32.5    -24
    > 16.05
    > 09/26/2004 09:45                31.8    32.4    -18     16.05
    > 09/26/2004 09:45                31.9    32.4    -15     16.07
    > 09/29/2004 05:50        sun, C+P,       31.5    32.65   -34.5
    > 16.04
    > 09/29/2004 05:50                31.5    32.8    -39     16.07
    > 09/29/2004 05:50                31.4    32.7    -39     16.02
    > 09/29/2004 22:00        moon, C+P       30.3    31.3    -30
    > 15.40
    > 09/29/2004 22:00                30.2    31.6    -42     15.45
    > 09/29/2004 22:00                30.3    31.4    -33     15.43
    > 09/30/2004 17:20        sun, C+P        31.7    32.45   -22.5
    > 16.04
    > 09/30/2004 17:20                31.5    32.5    -30     16.00
    > 09/30/2004 17:20                31.5    32.4    -27     15.97
    > 10/01/2004 16:40        sun, C+P        31.5    32.6    -33
    > 16.02
    > 10/01/2004 16:40                31.6    32.4    -24     16.00
    > 10/01/2004 16:40                31.6    32.4    -24     16.00
    > 10/02/2004 10:00        sun, C+P        31.7    32.3    -18
    > 16.00
    > 10/02/2004 10:00                31.7    32.4    -21     16.02
    > 10/03/2004 13:45        sun, C+P,       32      32      0
    > 16.00
    > 10/03/2004 13:45                32.1    31.7    12      15.95
    > 10/03/2004 13:45                32      32      0       16.00
    >
    
    
    

       
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