Ok, have to add my 2 cents:

The telescope forms an image that is 'diffraction limited' with respect
to the focal plane of the telescope.

The eye forms an image that is 'diffraction limited' with respect to its
focal plane.

Eg: a pair of 10x50 binoculars. Airy disc is about 2 Arcseconds of the
true field. given a 50 degree apparent field, the Airy disc is about 20
arcseconds of the apparent field. For the eyeball, in the daytime, with
a 2mm pupil and 25mm focal length, the eye's ariy disc is about 56
arcseconds of the apparent field, so the eye is the limiting factor. At
night, when the eye's pupil is 6mm accross, the eye's Airy disc is 18
arcseconds of the apparent field, so the telescope is now the limiting
factor.

for an 4" f/10 refractor, at 100x, the airy disc is about 113 arcseconds
of the apparent field. the eye's airy disc is 18 and 56 arcseconds
respectively at daytime and nighttime, so the telescope is always the
limiting factor.

did I get it right ?

Eric.


Alan French wrote:
"Frank Bov" wrote in message
...

[SNIP]
Now, at low power, the eyepiece does not magnifying the image enough for

the

resolution at the focal plane to matter; the eye's the limiting factor. So
at very low power, once the exit pupil exceeds the dilated eye, the
resolution in the perceived image stays the same as if the exit pupil just
filled it. [SNIP]


Frank,

Yes, that's one reason the debate I'm in elsewhere is so strange. Folks
want to believe they are utilizing the full resolution of a pair of 8x42
binoculars on a bright sunny day, yet 8 power is not enough magnification to
use the resolution of even a much smaller lens.

Clear skies, Alan