Re: 4 inch reflector can't resolve Mars at all

A decent 8" Newt will outperform a 4" instrument for planetary viewing-
assuming of course that you've mastered precision collimation. Keep in
mind my earlier warning however: keeping a planet centered at high
magnification with a Dob can be a chore, and you do need to keep the
planet centered for the best view. Off axis, coma will quickly degrade
the image.

I disagree with much of what you posted. First off, precision collimation
is not required for good views in an 8" f/6 newt, if what you mean by
"precision" takes more than the 60 seconds that I usually spend. An
accurate laser/barlow is certainly sufficient for high-power planetary
viewing, and there are other less expensive options that work just as well
if less convenient.

Secondly, tracking a dob at high power is, for many folks, not a problem.
Our club is planning on offering a scope building class, where we will use
commercial optics, spider and focuser and teach/help folks to build a 6" f/8
dob. We had the prototype at a small star party last week, and I was
heartened to see that folks with little/no experience readily tracked Saturn
at 200x (6mm 60degree Burgess Planetary). Everyone was very satisfied with
the way the scope moved - smoothly and intuitively. I think that this the
key - the quality of the bearings. I agree that the bearings on the Chinese
dobs are not perfect, and this includes the ones sold by Orion. I wish they
were better, as there is no reason why they shouldn't be. However, they are
easily fixed by anybody who is even a bit comfortable with hand tools.

Finally, off axis coma is not a significant issue with an 8" f/6 newt, even
with wide-field eyepieces. In our 10" f/6 dob, we regularly experience
great high-power views while allowing the target to drift completely across
the field. I have never heard anyone else comment on coma when observing
with a newtonian of focal ratio in the range of 6 or higher.

Dennis


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