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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
post(s).

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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