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    Re: Reliable Index Correction to a Tenth Minute of Arc
    From: Hewitt Schlereth
    Date: 2010 Feb 27, 10:14 -0400

    Hi, Douglas -
    It's interesting to see this topic being discussed, but I had the idea
    the 1.2' correction for irradiation was dropped from the Nautical
    Almanac circa 1970?
    I just checked my my 2010 NA and it's not mentioned in the explanatory
    material at the back. Augmentation is, but not irradiation.
    On 2/27/10, Douglas Denny  wrote:
    > 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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