Re: ATM Unobstructed Optics Viewing

[epabcc@epa.ericsson.se (Bratislav Curcic)]

> Are you convinced that this "off axis" unobstructed technique really does
> increase contrast for planetary viewing?

NO. Unfortunately, the side effect of "Dobsonian revolution" is many 
light buckets with unacceptable quality of optics. In (too) many cases,
off axis mask will help reducing wavefront error (by decreasing aperture
size of Airy disc increases, and tollerances increase proportionally; so
your one wave bucket with an off axis mask may become fine quarter wave
scope). But all this is valid for BAD OPTICS ONLY. People will come
with all sorts of excuses and talk about images being "less bright",
"less sensitive to atmospheric effects" etc. etc. to which I say : B.S. !
I have NEVER seen any positive effects from reducing the aperture in a
good scope (but I have to say that scopes I've used are smallish, definetely
NOT 40+ inches you're discusssing about; on the other hand I haven't
heard of anyone stopping down Catalina or Pic du Midi to "improve" images 
either). 

> You said " Playing with a good ray trace program can be very revealing". I
> wonder why 
> "  field aberrations as well coma and astigmatism " occurs near the eyepiece
> rather than at the mirror........'Cuz the light is a concentrated beam near
> the eyepiece, thus very critical to manipulation??

No, it is _a bit_ more complicated than that ... It comes down to having
different parts of mirror forming images in different areas of final image.
Any more complicated explanation than this, and I'd have to drag out some
unpopular formulas and terminologies ...   ;-)  

Bratislav