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    Re: Irradiation and Manual Navigation. Haines and Allen. 1968/9
    From: Douglas Denny
    Date: 2010 Feb 28, 03:24 -0800

    Thank you Frank and Gary for the assistance/advice. Will try again...

    Hooray! It worked this time ! And so to business:-

    I was fully aware Goerge would Harumph and splutter into his teacup at reading about this business of irradiation as the implications are quite profound; and I hope the paper provides him with endless amusement in dissecting it critically. I admit to a slight personal amusement at my gentle provocation. I could well imagine him squeeking "I don't believe it!! in full Victor Meldrew style .. (for Americans -a british comedy TV programme),(Sorry George).

    I do think the conclusions are profound, and of great relevance. It is also amazing to me that it has never been fully investigated; but that might be because it relates to a physiological effect of vision, and therefore has an element of subjectivity in the investigation of phenomena like this which is anathma to physicists and astronomers who are used to dealing with very precise quantities indeed. Talk about something which "might" give a variation of of this or that and their eyes glaze over....

    Haines and Allen for example, show that the MAR (minimum angle of resolution) between a point source and extended circular source can be between 2 minutes of arc to over 30 moa with different observers (they used eleven "highly trained male observers, all who had 20:20 vision") and diffeent conditions; (a lot of the results around six or seven moa) which would have great implications for Lunars.

    But now you have the paper you can digest it and decide on any relevance yourself, and I shall let the paper speak for itself.

    The purpose of the investigation was to "identify the relation of this phenomenon (irradiation) to manual navigation tasks".

    There are caveats (there always are). Right at the beginning the authors make it clear no distinction is made between Earth bound and space observations (they were clearly interested in the use in space), and that no sextant was used only the unaided eye, no magnification, eye relief, (I assume they mean glasses), field of view, or atmospheric variables were investigated.

    The main conclusions were:-

    1) The apparent edge of high illuminance targets can be displaced as much as 15 moa from the targets actual edge under some viewing conditions.

    2) The MAR is a function of the contrast ratio between the target and background and the luminance of the point source.

    3) the detrimental effects of irradiation are not reduced by using optical filters across the entire field of view to reduce the illuminance of the extended source.

    It also says: "The findings are related to certain aspects of manual navigation presently in use".

    Conclusion 3) is very interesting because it indicates that sextant shades will NOT affect or reduce the effct of irradiation to reduce the illuminace of say a very bright Moon with lunars. This presumably is because the effect of irradiation is due to contrast ratio which will not be affected with shades.

    I still maintain that this important effect seems to have been largely ignored by the authorities of the Nautical Offices of both America and Britain probably because of a scarcity of real information. My Eplanatory Supplement for the Astronomical Ephemeris (1961) merely states:

    "The adopted semi-diameter (of the Sun) {16' 01".18} at unit distance is an enhanced value, which includes an allowance for irradiation, (1".15) although this should strictly be independent of the distance; the variation of the correction from its mean value can be about 0".02 i.e. 0.017 of the mean value".

    Well this paper by Haines and Allen makes a complete nonsense of that when the variations might possibly be some tens of minutes of arc.
    And if indeed the irradiation component in 'official' works from the Nautical Office have been subsequently dropped - it is not surprising in the light of this H&A paper of 1969.

    My Dad always said - "don't believe everything you read in books - as the song goes 'it ain't necessarily so'..."

    I hope you find the paper interesting.

    Douglas Denny.
    Chichester. England.

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