Re: [ATM] Re:How much of cell induced deformation is too much?

[Michael Peck <mpeck1@ix.netcom.com>]

At 17:45 26-07-04, john sherman wrote:



>So what is the % of light in the Airy disc for a perfect aperture? I'm 
>thinking it's 86%, but I don't have any books here with me (maybe I should 
>go read one?<g>). So 50% of that would mean there is 43% of the total left 
>in the disc, and therefore 57% elsewhere. Right? So if I look at a star

It never hurts to open up a book now and then, but yes those numbers are 
about right, at least according to my calculations.

>  in my scope the rings and background should appear bright compared to 
> the disc. Right? So why is

Not necessarily. In a modestly turbulent atmosphere and assuming that's the 
dominant defect in the optical train the diffraction rings will be broken 
and smeared over a large area, but a stellar image retains a bright core. 
In severely turbulent atmosphere the stellar image breaks up into a mass of 
speckles with no clear center at all. There are examples with real and 
simulated images here: <http://www.astrosurf.com/cavadore/optique/turbulence/>.

>it that when I look in my scopes the disc appears to be very much brighter 
>than the rings and background? Wouldn't this observation indicate that I 
>am getting much better than 50% final Sr out of my scopes? I know the disc 
>is concentrated into a smaller area, but still...
>

See the response I posted to Scott Milligan's post from the other night. 
Vlad's analysis looks to me like a realistic assessment of the median 
behavior of the entire optical train of a largish amateur telescope, 
atmosphere included. However in excellent seeing there should be a 
nontrivial percentage of time that the atmosphere is giving you a 
diffraction limited wavefront, and better optics should allow you to 
exploit those fleeting moments of clarity.

Mike Peck


_________________

Michael Peck
email mpeck1@ix.netcom.com
Wildlife photography page http://home.netcom.com/~mpeck1/index.html
Amateur telescope making http://home.netcom.com/~mpeck1/astro/astro.html 

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