On 13 Apr 2005 18:01:38 -0700, Clarky wrote:

I was just looking at the dobsonians again, and had another question.
Both the Celestron 10 inch and the Orion 10 inch both say that they
have a "maximum useful magnification" of 600. The Hardin 10 inch
Dob's "maximum useful magnification" is only 250. I know that
magnification isn't all that important, but that seems like a pretty
big jump, although I'm not sure 'cause I'm new to all this. Does
anyone have any thoughts on why the Hardin would be so much lower than
the other two?


As a general rule, with really good optics, 50x per inch is max power
for extended objects (moon, planets) then image deteriorates rapidly.
60x per inch for stellar objects like really close double stars.

However, since dobsonians are hand-driven, it is hard to keep an object in
view for more than a few seconds at a time with any power over 300x. This
is a much more practical limit, especially when the atmosphere bounces the
image about. Much like trying to read the
date on a penny at the bottom of a swimming pool.

Cheers,
Larry G.