Re: Telescope resolution.


"canopus56" <canopus56@xxxxxxxxx> wrote in message
news:1138907512.571683.269040@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
bratislav3...@xxxxxxxxxxx wrote:
Your calculations do not apply to LONG thin objects.
It is well known that naked eye can greatly surpass its resolution when
seeing for example power wires against the bright sky. See Sidgwick.
Cassini division itself was discovered by a 2.4" scope (a
NONACHROMATIC one at that !).

Even assuming the lower Cassini division discovery criteria ( 89 /
D_mm), that still isn't enough to detect an _0.06"_ wide band - the 350
km wide gap. But I'll defer to your superior experience. Pickering
did do additional tests with a wire on the ground claiming to go to
R'/15. Personally, I don't know if I buy it because it conflicts with
the inherent size of the Airy disk.
<snipped>
The Airy disk, is _not_ the limit of resolution!... It is the limit to
_fully_ resolve something. If (for instance), you have a pair of
wires/dots on a background, and look at these with a scope, with them
seperated at the detector/eye, at the Airy disk diameter, you can get the
full 'peak', 'trough', 'peak' of intensity. However space the wires/dots
closer than the Airy disk diameter, and you can still tell there is a
'dip' in intensity between the peaks. The 'limit'of resolution, depends on
what percentage change, you accept as the minimum. The Dawes criterion, is
normally taken, as the approximate point where the dip disappears, for two
lines like this, and gives a significantly lower figure than the Airy disk
size. However if instead of trying to separate two lines/dots, you switch
to just detecting the presence of one, line/dot in an otherwise uniform
surface, the detection limit (not 'resolution'), can go much lower. So
(for instance), you can happily 'see' that a white ***, with a white dot
on it, is 'brighter' than an identical *** next door, even though you
cannot see the dot a all. If you see a street light ten miles away, there
is no way that your eyes can approach 'resolving' this by several orders
of magnitude, yet, you can still 'see' the light. In the daytime, the same
light cannot be detected.
So there are a whole 'crop' of limits. Full resolution, resolution, and
detection (for specific types of object).

Best Wishes


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