From: "Richard Schwartz" 
To: "Bill T." 
Cc: 
Reply-To: "Richard Schwartz" 


So, does your foucault tester have a knife edge, ronchi grating, or wire to
cut into the focal point?    If the star test is so great, why don't you
just get rid of your knife edge and use an eyepiece?

I think that star testing can be useful... if you trash the eyepiece and
replace it with a razor blade, ronchi grating, or fine wire.  35mm film
cans fit perfectly into 1.25" focusers; cut a hole in the top and
bottom of a film can and...

----- Original Message -----
From: "Bill T." 
To: 
Sent: Friday, April 25, 2003 9:42 PM Subject: Re: ATM Fw: to parabolize or
not......II


>
>
> >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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