From: "Bill T." 
To: atm{at}shore.net
Reply-To: "Bill T." 



>Guy Brandenburg wrote:

>It's not all that sensitive, though. My 8" f/6 homemade scope looks
>absolutely perfect (to me at least) using a ronchi grating at prime focus,
>but a star test reveals that it has a little bit of a turned edge - now
>that I know what to look for. The ronchi test also doesn't do a very good
>job on detecting mis-alignment.
>Anybody care to differ?

The ordinary star test in unbeatable for picking up the slightest edge
problem, and all but the most subtle astigmatism.  But for evaluating every
other aspect of the mirror, IMO nothing else available to an amateur is as
unambiguous as the Ronchi null test in either the double-pass or star
versions.  It is, after all, a true null test.  Artifacts in the viewed
image directly relate to problems on the mirror - while viewing the image,
you have an assistant put down pieces of masking tape to mark the exact
locations and boundaries of zonal problems.

In some ways it's almost too sensitive...I've marveled as the Ronchi null
image of same mirror goes from curved-out, to straight, to curved-in and
back and forth between all of these as the evening temperature fluctuates.
For best results, both the Ronchi null and the ordinary star test needs to
be done repeatedly over a long period of time, or at 4:00 a.m. after your
mirror has spent the night outside cooling down and stabilizing. 
Assistants are hard to find at those times.

Bill T.


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