Re: [ATM] Folded Stevick-Paul

["Jerry Reddell" <jerry.p.reddell63@sbcglobal.net>]

Great Philip. The Stevick-Paul design is my favorite. My eventual telescope
building goal is to build an 8 inch f/8 Stevick-Paul and then eventually
move to something larger.

Sounds like you have a real challenge with the f/15. Good luck..

Jerry P. Reddell

-----Original Message-----
Of Philip Lardner
Sent: Friday, February 10, 2006 8:38 AM

Seeing the recent posts in "RE: [ATM] Off axis reflector" prompts me to
de-cloak and stop lurking ;-)

I'm currently working on the mirrors for a 14" f/15 Folded Stevick-Paul and
have completed (for now) the f/15 primary mirror. I thought that making and
testing such a long focus spherical surface would be relatively straight
forward, but have learned differently!

At f/15, getting a clear and unambiguous null at the RoC is not easy. The
'focus' seems very 'soft' and I have to spend some time deciding where it
actually is. This is not helped by the very long light path, which I can
only partially enclose in a tunnel of hoops and blankets to reduce air
turbulence in my house (the weather's too cold and nasty outside!) I am also
hampered by the roughness in the lens of my video camera on the Foucault
stage. I have to rock the camera slightly to see which errors are on the
mirror and which errors are being superimposed by the camera. This problem
does not seem to arise with the f/4 and f/6 mirrors I have also tested with
this set-up!

The other thing that has been bugging me is the apparent residual primary
ripple. I have worked hard to eliminate it as best I can using 0.4 micron
'Black Rouge' This stuff is the pits to work with by the way! It has a
glacially slow polishing action, it gets *everywhere* and is hell to remove
from work surfaces and clothing! It does give a nice finish, however. But
back to the point in question...

At f/15, am I being over optimistic in expecting to be able to remove *all*
traces of ripple or are even tiny errors just made glaringly obvious at
f/15? Should I go back and continue working them until they are sensibly
perfect or be happy and get on with life? This is intended to be a
high-contrast planetary scope after all.

I'm currently polishing out three 10" flats that I will test by interference
before regrinding the two leftover surfaces into a matching convex/concave
pair.

Fun times ahead!

Philip.


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