To: atm{at}shore.net
From: Jim Burrows 
Reply-To: Jim Burrows 


At 19:52 2003-04-24 -0600, Bill T. wrote:

>Star test your spherical mirror.  See how the diffraction rings look
>different inside and outside of focus.  That bright ring inside of focus
>is the calling card of spherical aberration.  Notice how the point of
>sharpest focus is a somehwat vague.
>
>Now start doing a few minutes at a time of parabolizing strokes.  Let the
>mirror cool down.  See how the inner and outer diffraction rings are now
>more similar in appearance.  See how the focus is now more defined.

I would definitely NOT recommend trying to figure using the star test. 
It's too hard for several reasons:  1) having to mount the mirror, 2)
waiting for good atmospheric conditions, 3) factoring out diagonal,
eyepiece, and collimation errors, 4) last but not least, it is very
difficult to tell from the star test what and where to polish.  The star
test's main function is a second opinion before sending the mirror out for
coating.

Get going with good ol' Foucault, it's worked for professionals and
amateurs for ~ 150 years (even Hubble - they did a Foucault test which said
the mirror was a chunk of junk - but the NASA managers didn't believe it).

         -- Jim Burrows
         -- mailto://burrjaw{at}earthlink.net
         -- http://home.earthlink.net/~burrjaw
         -- Seattle N47.4723 W122.3662 (WGS84)

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