Re: [ATM] difference 1/8 1/16 1/20 wave?

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

That test was relevant only due to the fact that 6" is a size that is close
to being not affected by atmospheric turbulence on an average night , having
a theoretical resolution of 1 arcsecond . With atmospheric turbulence out of
the way , the dominant factor becomes the telescope quality , and the
observer's ability and skill. In other words, a small scope's Strehl is NOT
ruined by atmosphere as much as a large telescope Strehl .
If we're talking telescopes in the 20"+ aperture, with a theoretical
resolution of 0.3 arcseconds or better , they will never have diffraction
limited performance due to atmospheric turbulence still creating average
seeing in the 1 arcsecond or worse . The difference between a 1/8 and 1/800
wave for a large scope will NEVER be measurable by any means, human ,
optical, electronic etc. due to seeing .

best regards,
Matt Tudor

p.s. if that large scope is an obstructed reflector , then its PSF is also
ruined due to central obstruction diffraction effects . With a 20%
obstruction, PSF is ruined much more than the difference between 1/4 and
1/800 wave , so it makes no sense to worry about it . Rather than figure for
2 extra years for that elusive 0.9999 Strehl (or is it becoming more
fashionable nowadays to quote 1.0001Strehl...) , just move the secondary 2mm
farther away , reduce its size by 2mm , or make your spider vanes a hair
thinner than they are and improve the PSF by reducing diffraction effects
much more than what you lost by having a 1/4 mirror rather than 1/800 wave
(all these apply to large scopes , not to the 4-6" range) .

-----Original Message-----
From: Bob May <bobmay@nethere.com>
To: atm@atmlist.net <atm@atmlist.net>
Date: Thursday, July 07, 2005 1:25 PM
Subject: Re: [ATM] difference 1/8 1/16 1/20 wave?


>You might want to go back in time and see what Peter Ceravolo did with a
set
>of 4 6" telescopes.  He did the 4 telescopes to differing amounts of errors
>in the mirrors and found that it took an excellent observer to see the
>difference between the 1/8 and the 1/16 wave error telescopes and then only
>on an excellent night of seeing.  There may be somebody out there that
whose
>eye's will demand a better quality mirror but that person will be
relatively
>rare.  This set of testing tends to put the perfect level of telescope to
be
>on the order of 1/15 - 1/20 wave mirror.  Anything better, of course, is
>nice but not needed.  Anything less can lead to a loss of detail when
>conditions permit.
>Bob May
>bobmay@nethere.com
>http://nav.to/bobmay
>http://bobmay.astronomy.net
>
>_______________________________________________
>ATM mailing list http://www.atmlist.net/
>

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