ATM MakNewt Notes/Cervit Frustration

[Bill Marriott <btk@pacbell.net>]

Hi folks;

Once again, the recent comments on Maksutov Newtonians has pushed me to
get off my butt and try to finish the 6" f8 NewtMak I have been
'working' on for almost two years now.

The corrector has been polished to the proper radii for the past year,
while I have been beating my head on the Cervit primary. )As previously
noted, I may only have 1-2 days a month where I have time to work on
this)

In the past, I have had no problems polishing out wonderful spheres on
maybe a dozen mirrors, before parabolizing. However, with this Cervit
6.25", I have had nothing but problems. Full size, undersize, oversize
laps, with more or less the same results, a significant turned down edge
that I can't seem to eliminate. The best result was with a 7" lap, MOT, 
fairly small turned down edge, but after several hours of further work,
couldn't seem to budge it.

To try to put things into perspective, last week I ground out a 6.25",
f8.3 Pyrex mirror, threw it on a polishing machine at work, TOT, and
after the pitting was gone threw it on the test stand, and almost a
perfect sphere, slightly oblate, needing just a little hand work to make
it as nice as could be.

So it is the Cervit!!

So I figure there is two possibilities:

1)This material is tough. I have noted that it took a lot of work to
change the ROC, even using pads, where with Pyrex, one can do it easily
with pitch, flipping MOT, TOT (we're talking .0001"s in Sag here)
So maybe I need to just be more patient, and just realize it will just
take a lot of work to lower that polished plane to meet the edge of the
mirror

2) I have been told there is good Cervit, and bad Cervit, and while I
doubt that it has any relation to figuring the glass, who knows

If anyone out there has any comments as to figuring Cervit, i.e. what
others have been through, I'd appreciate some feedback

In any event, I now have a Pyrex primary done, (while the low expansion
of Cervit is of interest, I suspect with a 6" mirror, Pyrex will
stabilize fairly quickly anyway) its time to hand figure the corrector
(yes, one can with time can descern the ronchi fringes off the back side
of the convex surface through the front concave surface)
I have a stout aluminium tube, alum. plates for the mirror mount, and a
8" dia. by 1.5" aluminum disc for machining the cell.....

It time to FINISH this thing

Sorry about the rambling

Later

Bill
-- 
Bill & Tracy Marriott
Woodacre, Ca. USA
btk@pacbell.net

http://www.starlightoptical.com
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