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    Re: Reliable Index Correction to a Tenth Minute of Arc
    From: George Huxtable
    Date: 2010 Feb 27, 10:43 -0000

    Bill Morris reported a useful series of measurements on prismatic effects in 
    sextant shades.
    
    He also quoted a paper by Whipple from Kew in 1887, which indicated the 
    rather low standards for shade precision that prevailed then, which may well 
    have dominated the precision then achievable in Sun-Moon lunar distances-
    
    "In Whipple's time, if the shade caused no more than 20 seconds deviation, 
    it was marked K.O.1, if no more than 40 seconds, it was marked K.O.2; and if 
    more than 40 seconds, it was rejected. This gives us some insight as to why 
    nineteenth century writers dwelled on prismacity and steps to eliminate its 
    effects, but none on why "K.O." was chosen. In a time when ancient Greek was 
    widely studied, perhaps it was "ola kala" (everything correct) reversed."
    
    For the meaning of "KO", I think that was simply "Kew Observatory". (The 
    original observatory was built so that George III could observe the 1769 
    transit of Venus, and still exists).
    
    I understand that in some sextants, the shades were fitted in such a way 
    that they could be rotated, within their own plane, between stops, by 180º, 
    so that averaging the readings, with all shades reversed, would cancel out 
    any such error. If potential errors in shades were as great as the Kew 
    document indicates, that precaution could well have been an important one.
    
    As long as a shade can be unshipped from a sextant. and cobbled roughly into 
    position by some means instead, then a modern user could make an estimate of 
    error in a dark Sun shade from the change in index error, using the Sun, 
    when that shade was turned through 180º.
    
    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.
    ----- Original Message ----- 
    From: "Bill Morris" 
    To: 
    Sent: Friday, February 26, 2010 2:27 AM
    Subject: [NavList] Re: Reliable Index Correction to a Tenth Minute of Arc
    
    
    Frank,
    On 17 February (g11914) you wrote:
    
    "And yet, even so, I have spoken to people who have tried this who get 
    strangely different results under different conditions. Part of the problem 
    is probably slight prismatic effects in the shades. To do this test with the 
    Sun, we need full shades in front of the horizon glass, but this is really 
    the only observation when you would have full shades there. It wouldn't be 
    surprising to find a couple of tenths of a minute of arc error there."
    
    This led me to investigate prismacity of mirrors and shades. I can dismiss 
    mirrors quickly. In a collection of fourteen sextant mirrors that I stripped 
    prior to re-silvering, only one had prismacity exceeding 1 second and that 
    was a horizon mirror, where prismacity is of no account, as the angle of the 
    rays striking and leaving it is always the same.
    
    In 1883, G M Whipple communicated to the Royal Society of London "A 
    Description of an Apparatus...for the Examination of the Dark Glasses..of 
    Sextants" (Proc. Roy. Soc. Lond., 35 (1885), pp.42 - 44). In a paper written 
    in 18 years previously, in 1867 on sextant calibration, Balfour Stewart had 
    written as a concluding paragraph:
    
    " In conclusion, it ought to be mentioned that perhaps no artificial light 
    easily obtainable is sufficiently powerful to allow of the darkest glasses 
    of a sextant being examined, and for that purpose, we may ultimately have to 
    resort to other means." (Proc. Roy. Soc. Lond., 16 (1868), p.6)
    
    Whipple did resort to other means, using a heliostat to direct the sun's 
    rays into his apparatus. I used a 40 watt halogen bulb in a desk lamp to 
    illuminate the cross wires of a collimator and focussed on the wires using a 
    Watts Microptic autocollimator as a micrometer telescope. It allowed me to 
    examine all but the darkest of the "dark glasses" of a selection of 
    sextants, by interposing them between the collimator and the telescope and 
    noting the maximum displacement of the cross wire image.
    
    In Whipple's time, if the shade caused no more than 20 seconds deviation, it 
    was marked K.O.1, if no more than 40 seconds, it was marked K.O.2; and if 
    more than 40 seconds, it was rejected. This gives us some insight as to why 
    nineteenth century writers dwelled on prismacity and steps to eliminate its 
    effects, but none on why "K.O." was chosen. In a time when ancient Greek was 
    widely studied, perhaps it was "ola kala" (everything correct) reversed.
    
    I was able to get results for six out of seven shades for each of the 
    sextants I looked at.
    
    Bronze octant in the style of Spencer, Browning and Co., ca 1850:
    Average deviation  7.0 seconds. Worst deviation 14 seconds.
    
    Hughes and Son sextant, 1920
    Average deviation 2.2 seconds. Worst deviation 4 seconds.
    
    Tamaya  sextant 1957
    Average deviation 2.0 seconds. Worst deviation 6 seconds.
    
    Freiberger sextant ca 1964
    Average deviation 2.8 seconds. Worst deviation 5 seconds.
    
    
    It seems clear that things improved between 1850 and 1920, but little 
    progress was made after that. Six seconds is of course a tenth of a minute 
    and may indeed account for the "strangely different results" obtained.
    
    To obtain index error using the sun, an eyepiece shade can be used (it is 
    one of the reasons why they were provided) and this will eliminate errors 
    from prismacity of the shades, though only "lunartics" will need to do so. 
    However, they will still have to use shades when obtaining the distance 
    between the sun and moon and the height of the sun.
    
    Bill Morris
    Pukenui
    New Zealand
    
    
    
    
    
    
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