From: "CSC" 
To: "Atm" 
Reply-To: "CSC" 


        Using an artificial star and making sure the distance to source is
sufficient will allow you to get better star test data. The atmosphere is
less troublesome.

        I guess you could see 1/30 P-V wavefront, but it would be so subtle.
At
extreme magnification the star test patterns would be clearer, especially
at 1-3 waves defocus.  I star test spheres on the bench and use about 700x.
Maybe I'm seeing 1/30 P-V then, but how do I know?  Only a slight contrast
difference shows at this level, and it's way past foucault at this point
except to see roughness.

        You could send it out for Interferometric testing for a couple hundred
bucks and get some certification.
Your errors from collimation, seeing, tube currents, thermal currents off
the mirror and figure changes are more of a problem than the figuring on
the glass.
        I know it's nice to go for the ultimate surface, just to get that out
of
the way as a factor in scope performance.

Colin



-----Original Message-----
From: owner-atm{at}shore.net [mailto:owner-atm{at}shore.net]On Behalf Of Scott
Ewart
Sent: Thursday, February 27, 2003 1:51 AM To: ATM list
Subject: ATM Advanced star testing



     I wonder if anyone can help me with something.  It's hardly what you
could consider a problem.  I have an excellent 8" f/6 mirror which
performed flawlessly at WSP.  Encke gap, detail on Ganymede, split Sirius,
etc.  I was recently star testing my 12.4" that I'm currently figuring
and used the 8" as a control to compare seeing.  The numbers for the
12.4" showed close to 1/20th wave over-correct, and the star test
showed a bright ring appearing JUST outside focus with no such sharp ring
inside.  Seeing wasn't good enough to see detail further from focus with an
un-coated mirror.
    Of course I tested the 8" at the same time and found only one real
difference inside vs. outside.  Overall this one's way better than the
12.4", but I noticed that the innermost ring around the secondary
shadow is slightly brighter outside focus than inside.  Everything else
looks the same.  My question is, does anyone have experience interpreting
the test at this level?  Dick Suiter's book has nothing close to errors
this subtle. And just how good might this mirror be?  I'm thinking it may
be better than 1/30th wave P-V wavefront.  Bench test numbers are near
perfect, and probably vary more one run to the next than anything like the
real error. The star test is supposedly good down to 1/50th wave, but I've
never read anything about what it looks like at this level.  Anyone know of
reference materials on the subject?
    I know, I know, you probably all wish you had my problems.  I sure wish
I knew where this thing came from.  The back is signed "Meridian"
and if I remember right, the numbers "10/66" (a date?).  Any
ideas?

Scott

--- BBBS/NT v4.00 MP