[ATM] Binocular Seeing and CCD : Re: Quantifying visual seeing error

["matt" <electro_optic@bellsouth.net>]

-----Original Message-----
From: Michael Peck <mpeck1@ix.netcom.com>
To: atm@atmlist.net <atm@atmlist.net>
Date: Tuesday, October 04, 2005 12:53 PM
Subject: Re: [ATM] Quantifying visual seeing error


>In the short exposure limit
><http://home.earthlink.net/~mlpeck54/astro/temppix/histrmsse.png> the
>histogram has the same basic shape but shifted to the left and
>compressed. Most (~88%) of the time the short exposure wavefront
>qualifies as diffraction limited by Marechal's criterion. If one were
>to create a movie of simulated PSFs produced by these simulated
>wavefronts what it would show is a well defined centrally
>concentrated PSF (almost always) that dances around randomly.
>
>
>
>>Still, the theory seems to make seeing harsher on optical quality
>>than what it appears to me. With r0~2.2 inch in a
>>typical 2 arcsec seeing, everything bigger than 60mm
>>in aperture would be - on average - less than diffraction
>>limited due to seeing alone (low 8 and worse on the scale).
>
>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. 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.
>
>Mike
>
>
>_________________
>
>Michael Peck
>email mpeck1@ix.netcom.com
>Wildlife photoblog! http://wildlife-pix.com
>Amateur telescope making
http://home.earthlink.net/~mlpeck54/astro/astro.html


Interesting topic. I found some similar simulations and conclusions from
2003 , in a Cambridge PhD thesis of a researcher who used images from the
Nordic Optical Telescope . He starts with considering Kolmogorov turbulence
as it affects various aperture telescopes and exposure times :

http://www.mrao.cam.ac.uk/telescopes/coast/theses/rnt/node4.html
http://www.mrao.cam.ac.uk/telescopes/coast/theses/rnt/thesis.html

There's also Fried's original old paper "Probability of getting a lucky
short-exposure image through turbulence ", from 1978 .


1 - One issue would be what is a quantitative relationship between exposure
time , turbulence degraded Strehl ratio, telescope aperture , imaging object
magnitude , and CCD parameters such as pixel size, readout noise , QE .

Intuitively, it might appear that as the telescope gets larger more photons
are collected and shorter exposure can be used . However, readout noise
limits CCD performance by degrading signal to noise for very short exposure
.

I'd like to see which is the telescope aperture that would allow enough
photons per pixel to increase signal to noise and reduce effects of CCD
readout noise , without increasing the telescope size so much that good
Strehl probability is reduced more than necessary . In other words, I'd like
to find the optimum telescope size for a given CCD technology (combination
of readout noise and QE ), let's say there are about 3 or 4 contenders in
the CCD arena . Kodak, Sony Exview, Marconi , Texas Instruments (these last
2 have solid state photomultiplier CCD's with readout of under 1e ).

2- a second issue is how visual Strehl is affected by turbulence and how the
brain deals with this for binocular viewing . I've heard the opinion that a
large binocular telescope with 2 primary mirrors has a clear 3D effect that
a similar mirror area single primary telescope equipped with a binoviewer
does not have. I wasn't able to verify this firsthand yet .

Since parallax is out of the question as the source for the 3d effect for
the 2 primary telescope, what causes the 3d effect ?

One hypothesis might be that the 2 primaries are separated by a significant
lateral distance many times R0 , and subjected to different instantaneous
seeing .

The brain might be able to correlate/integrate the less aberrated /less
fuzzy brief moments for each eye and rebuild a better image due to the 2
eyes seeing different aberrations and having unrelated moments of "lucky
seeing" . But how does that create a stronger 3d effect ?
I couldn't find any references to this issue, other than anecdotal .

best regards,
matt tudor




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