Re: [ATM] Quantifying visual seeing error

["vladimir" <svla@isp.com>]

Michael Peck wrote:

> That may be too pessimistic. I think at visual wavelengths a Fried
> parameter r0~10-15 cm. would be typical for poor to average seeing,
> with r0~20-25 cm for good to excellent seeing. If the eye-brain's
> response time is closer to the short than long exposure limit you'd
> actually perceive diffraction limited conditions most of the time
> with an r0 sized telescope.

It was a bit too pesimistic. Schroeder (Astronomical optics 1st.ed. p315).
gives seeing limited angular resolution (a) in radians as

a=1.27(Lambda/r0)

This also approximates seeing blur diameter in large
telescopes. This gives r0 in terms of seeing disc in arc
seconds (s) as

r0~26Lambda/s

for Lambda in um, and "s" in cm. For the visual pick,
Lambda~0.55um,  an average 2 arcsec seeing would have
r0~7.1cm. This would imply that a 4" aperture under
these conditions would be - on average - more of the
time bellow than above diffraction limited seeing.


>Of course with, say, a 3r0 sized
> telescope you'd almost never experience truly diffraction limited
> conditions. You would, however be diffraction limited by Fried's
> standard of RMS < 1/(2 pi) waves about 75% of the time.

Mi impression is that this standard is for "diffraction limited resolution"
(~Lambda/D), not "diffraction limited" in the 0.8 Strehl sense. Even
if the FWHM at r0~D is about 1/3 wider (~1.35Lambda/D), vs. perfect 
aperture,
it is considered that diffraction limited resolution is still achievable.
However, more than half the energy of the Airy disc is lost to the rings.
No visual observer would be happy with that kind of "diffraction limited".
I know some that aren't quite happy with the 0.8 Strehl already.

Vlad

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