To: gfbranden{at}earthlink.net
From: Mark Suchting 
Cc: atm{at}shore.net
Reply-To: Mark Suchting 


At 11:21 PM 4/25/2003 -0400, 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?

My point was that for detecting the most important abberation for beginning
ATM's, namely Spherical Abberation, the Ronchi test does admirably with F6
and slower mirrors. The Ronchi test is also extremely sensitive to
astigmatism. I find that even with 1/10 wave astig that barely effects the
star test, the Ronchi bands orientation will rotate slightly on either side
of focus. By the time you are seeing any `S' shape in the bands you have
gross astigmatism. Small amounts of turned edge are not so easy to see but
with experience can be seen. I'm talking here about having no more than one
line visible at focus with say a 100 lpi grating.

Just because you can see a `sign' of TDE in your star test doesn't mean it
is necessarily going to have any significant effect on your image. The star
test can be sensitive to errors of sharp slope of 1/50  wave , and a slight
but narrow TDE may only reduce the Strehl ratio of an otherwise perfect
mirror by  a few percentage points, but be perfectly obvious in the
defocussed star.   I believe that with a little experience ( particularly
in constantly comparing the view slightly inside and outside focus) that
the Ronchi test is sensitive enough for any level of TDE that is
significant to the final result.

~Mark

--- BBBS/NT v4.00 MP