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    Re: Reliable Index Correction to a Tenth Minute of Arc
    From: George Huxtable
    Date: 2010 Feb 27, 14:15 -0000

    Douglas Denny, speaking of irradiation, tells us-
    
    "There is little published information about this phenomenon, and little
    practical means of dealing with it, but the effect can be in the order of
    fifteen minutes of arc for the bright sun's disc, which appears larger than
    it really is; and in the order of three to ten minutes of arc between a
    point source and horizon."
    
    Well, with or without a working scanner, he could have told us a bit more
    about those alarmist figures and the conditions under which they were
    obtained. There's no doubt of irradiation being a real effect, though it
    might strike different observers to differing extents, but I regard those
    numbers, taken at face-vale, as quite absurd. They would negate the whole
    practice of astro-navigation.
    
    Is  "the bright sun's disc" is to be taken, perhaps, as the unshaded Sun? If
    so, it has no relevance to any instrument observations, except perhaps, to
    users of early versions of the cross-staff.
    
    Douglas will have to tell us more before we begin to believe what he is
    saying.
    
    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: "Douglas Denny" 
    To: 
    Sent: Saturday, February 27, 2010 12:44 PM
    Subject: [NavList] Re: Reliable Index Correction to a Tenth Minute of Arc
    
    
    Colleagues,
    
    I note there is considerable discussion on this forum about the necessity of
    obtaining as 'good' an estimation of index error as possible in a sextant -
    and quite rightly so of course - in the interests of obtaining practical
    accuracy for what is, or rather was, a working instrument in daily use for
    thousands of people throughout the  world in their everyday business of
    navigating ships around the world.  Now it is more of an academic issue with
    interested parties such as ourselves.
    
    I have made the point before, however, there is a danger of becoming
    besotted with the fine technicalities and detailed physics of instumentation
    and astronomy at the expense of _practical_ navigation, as it would have
    been actually used by sailors.  They were only interested in results and
    simple practical methods.  We might be looking too minutely at the wood with
    a microscope and loosing sight of the forest.
    
    I am not suggesting there is anything wrong in our modern dissection and
    analysis of techniques and instruments to the nth degree, as it is a
    fascinating business and interests us a lot (I know because it has been a
    passion of mine for a lifetime), but I warn of loosing sight of the
    _practical_ side of our passion, and a more overly view of the subject at
    the same time.
    
    To discuss measuring down to a tenth of a minute with a sextant seems to me
    to be attempting to 'Guild the Lilly' as there are so many factors which
    preclude making this a practical, though much desired, observation goal to
    attain.  One such factor I mention below:-
    
    Bill Morris has the ability with sensitive autocollimators for observing
    angular deviation of optical elements down to a tenth of a second of arc;
    and I note others have discussed endlessly the ability to take sights to a
    tenth of a minute of arc; ....but there is an aspect of practical navigation
    which has not been discussed here before as far as I know which negates a
    lot of this, and that is a natural phenomenon of the eye itself which is the
    spreading of a bright image laterally across the retina called
    'irradiation'.
    
    It has the effect of displacing the apparent edge between a bright area and
    a darker area towards the darker.
    
    There is little published information about this phenomenon, and little
    practical means of dealing with it,  but the effect can be in the order of
    fifteen minutes of arc for the bright sun's disc, which appears larger than
    it really is;  and in the order of three to ten minutes of arc between a
    point source and horizon.
    
    Standard practice in terrestrial navigation incorporates only one correction
    for irradiation of 1.2 minutes of arc in the table for the Sun's upper limb
    in The Nautical almanac.
    
    The only paper published on the subject that I know of is a scientific
    appraisal made by Richard F. Haines and William H. Allen of NASA at the Ames
    laboratory,  Moffett Field,  California in 1968/9.
    
    See Journal of the Institute of Navigation Volume 15.  No 4.  Winter 1968/9.
    
    I presume this investigation was towards better accuracy for space
    navigation as the astronauts used sextant measurements to determine accurate
    positions when in orbit, or transit to and from the Moon. Although radio
    telemetry and radar methods could do this too, I believe the use of manual
    sextant observations was considered important and was certainly used by
    astronauts as far as I know.
    
    Haines and Allen used a criterion of minimal angle of resolution (MAR) for
    various sources including:  two point sources;  a point and extended
    Circular source; Point source and simulated horizon.
    
    There is a comprehensive reference listing too at the end.
    ----------
    
    I would have scanned the copy I have to make it available here as it is a
    very important research document for navigation, but unfortunately my
    dinosaur computer has gone Awol with the scanner, and won't scan anything.
    So I cannot. Sorry. If I get it working again I shall do so unless someone
    else can look up the article and post it here.
    -------
    
    Around the time I graduated forty years ago in Cardiff, S.Wales, I knew
    Captain Cotter very well and he encouraged me to look into this phenomenon,
    as he was fully aware of the scarcity of information on irradiation in
    practical navigation. Accordingly, with the blessing of the staff, I made
    some experiments in the optical laboratory of the university.
    
    I set-up a metal plate at the end the longest lab I could find with
    accurately cut holes of different diameters, and used a bright lamp source
    behind with a diffuser. Measuring light intensity at the source plate with
    an accurate photometer, I then measured fifty settings each of the apparent
    discs just touching (on both sides) with a standard marine sextant at
    gradually increasing light levels.
    The idea was to plot a graph of irradiation effect of increased MAR between
    the two extended light sources Vs. light intensity of source.
    To my eternal shame and intense irritation I lost the results many years ago
    when moving from Cardiff.  I could still kick myself heartily when I think
    of it.
    
    Douglas Denny.
    Chichester. England.
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