RE: [ATM] over&under correction.

["Scott Milligan" <starzkey@charter.net>]

Not only is there no difference in summary quality measures, the in focus 
diffraction patterns are exactly the same. Also, the extrafocal diffraction 
pattern in one case exactly matches the infrafocal diffraction pattern in 
the other for the same amount of focus travel. So, based solely on optical 
theory there's no difference at all between the two cases.

Of course I defer to others on which condition is easier to fix.

Mike Peck


Extending this discussion to ask the question "which is worse, a tenth wave
edge rolled down or up" I would conclude that either of these defects,
occupying otherwise equal areas, position and magnitude on the mirror, will
have the same impact on contrast transfer.  I make this observation only
because over the years I have heard much commentary regarding the need to
avoid even the slightest amount of turned edge, but relatively little energy
spent warning against residual turned up edge.  In my (perhaps overly
simplistic understanding), a zonal error of slope Y and amplitude X has the
same impact on MTF regardless of sign, even if for other reasons, one is
more easily detected via the star test than the other.  Comments?

Scott Milligan


_________________

Michael Peck
email mpeck1@ix.netcom.com
Wildlife photoblog! http://wildlife-pix.com
Amateur telescope making
http://home.earthlink.net/~mlpeck54/astro/astro.html 

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