Re: ATM Collimation, the Truth?

[Bratislav.Curcic@uab.ericsson.se (Bratislav Curcic)]

Steve,

in a well collimated Newtonian there will NEVER be "all concentric
rings". If your diagonal is not offset, the image of primary won't be
concentric with outside rim of the secondary; if you offset the
diagonal, the reflection of the secondary as seen in the primary won't
be concentric with the rest (as it is physically offset).  So yes, the
truth is : in a fast Newtonian you CANNOT have "concentric circles
within circles" no matter what you do (providing you want collimated
scope).

>  In my opinion, it is impossible
> for a scope that has a diagonal offset, to form a completely symmetric,
> beautiful non-flared Airy disk with completely concentric diffraction
> rings at best focus.

Hmm. I think you got it wrong here. There is no physical reason for
this. Try this : once your scope is perfectly collimated (i.e. your
in focus image doesn't flare, and has nice set of even diffraction 
rings), place a circular mask slightly larger than your existing 
secondary, offset it a few millimeters and look again. There should 
be no coma present and rings should stay concentric.

But don't forget every now and then to look at the images,
not just aberrations. It does wonders for me (and indeed any
case of acute TN-itis).

Bratislav