"Bob May" wrote in message ...
Getting rings around a bright star when at very high power is just looking
at the Airy Disc - go see what they look like before going any further.
If it isn't an Airy Disc that you are seeing, you have some very interesting
problems with the lens as optics generally don't work that way.
I suggest that you do a Ronchi or Knife Edge test to see what the optical
surfaces are like on a bright star. The Ronchi test should show straight
lines if the optical path is good and with the KE, the light from the star
should dim evenly across the aperture if the optics are good.

Thanks for the replay!

At high magnification IN FOCUS I can see the Airy disk +5 or 6
bright diffraction rings.
Of course the atmosphere make those rings dancing like ballerina on
ice....
I think this is a " extraneous junk (borrowing R.C.) " which is
destroying my contrast.
At medium and low mags. inside focus I can see the Outer 2 rings much
brighter than the outside of focus outer rings. Top of that there is
the 3rd. ring from
outside, doing the opposite what is the surrounding rings do, I guess
this is a zone error
@70% of aperture. I hope the description is not sound like garbage....
My hope is, if the under correction is coursing the major problem,
maybe a "slight"
polishing of R1 edge fixing the spherical error and live the zone
alone. Of course
the mgf2 coating is partially destroyed.

I appreciated this is a cheap lens, but cutting a long story short:
I started with baying the Vixen VC200L cassegrain. That was a big
mistake!
Open tube, 40% obstruction, thick vane, field corrector lens, made
sure that I never
see a star only a big snow bowl! Then I asked the dealer to change it
to a 5" refractor.
I can see the Airy disk now at least, but the resolution, despite the
colour aberration
should be better. Regard to " throwing good money after bad" : 2
teenager kids, mortgage
and a less than sympathetic wife, is not an option. ATM is. And if I
can fix just a bit
of the correction I can share the experiment with other less then
happy Synta achromat owners....
Thanks for reading, and please forgive me my less than perfect
English!
Miki.