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    Re: The Moon is a "nuisance"
    From: Michael Bradley
    Date: 2006 Oct 12, 09:05 -0700

    Hi List
    Beg leave to report a little personal investigation following up the
    summer's debate on index errors etc., particularly
    with respect to moon sights. No claim to anything massively original
    here, just a bit of reading from a 'Visual Optics' textbook since
    picking up a little but not all of the discussion while away on the
    summer cruise. I may have missed/deleted a post discussing diffraction,
    and I haven't followed up Frank's telescope magazine article reference
    so please disregard this post if it unknowingly repeats any previous
    Diffraction causes sharp edges of images to become 'comb like', with an
    edge pattern of brightness and darkness.
    The diffraction limit for the human eye (8mm pupil) is calculated to be
    approximately 0.5' in the textbook.
    The pupil size of humans varies congenitally ( ratio 2 to 1) , with age
    ( ratio 2 to 1), and with light intensity.
    The overlaying of two separate diffraction patterns ( eye and
    telescope) will produce a complex pattern of dark and light at
    the edge of an image: sometimes the diffraction effects mutually
    adding, sometimes mutually cancelling.
    What follows is my guesswork, and may easily be wildly wrong:
    The diffraction limit for, e.g. a 40mm x3.5 star telescope might be,
    say, 0.1' but the diffracted image  will
    presumably  be magnified x3.5 to give a 0.35' comb at the eye, which
    will combine with the eye's own diffraction pattern.
    The diffraction effect of a 40mm sight tube might produce a 0.1' comb
    at the eye, which will combine with the eye's own diffraction pattern.
    I suggest that the combined eye and telescope diffraction light/dark
    pattern errors would be different for
     lunar distance type images ( bright against bright )
    as against traditional sights brought down to the horizon ( bright
    against dark or darker ).
    The index error from 4SD as against horizon would also differ because
    of the differing effect of bright/
    bright joint diffraction as against bright against dark joint
    diffraction, and the difference between sight tube and telescope
    results could well arise for the same reason.
    The estimated size of these errors is of the same order of magnitude as
    Frank's previously reported lunar inconsistencies.
    Just a set of thoughts...
    Mike Bradley      55 01.2' N 01 27' W
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