"Florian" wrote in message .. .

Trapezium - Could only resolve the 4 main stars with either scope. With
the 10" things got really murky with higher powers. With the 3" stars
stayed a lot sharper but still no E or F components. Maybe i'm expecting
a bit too much from the little TV76. I'm also starting to think the
mirror in my 10" isn't that great...

Three closing comments... 1. I'm very happy with the Tele Vue 76. Very
sharp. Sharper than the 10" dob at higher powers...

Yes, I'm sure the TV-76 is sharp; a small, high-quality refractor is
a wonderful thing.

But don't be too quick to condemn the mirror on the Dob. Most mirrors
are pretty good these days; the problems you are describing sound much
worse than could be ascribed to a mediocre mirror. Are you talking less
sharp at identical powers, or less sharp at identical exit pupils, i.e.
3X higher for the Dob than the refractor?

My best guess is that the problems are thermal. Deserts are famous
for cooling off quickly at night; a Dob would be unlikely to keep up
unless carefully designed for good thermal characteristics and/or
equipped with a fan. See Bryan Greer's article in the May S&T
for how to diagnose this.

Other things to think about are that at identical magnifications,
the exit pupil of the Dob (and the amount of light collected)
are likely to show up aberrations in your own eye that are
invisible in the smaller scope. Also, whereas the seeing is
often good enough to allow a 3-inch scope to perform at or
near its theoretical limit, that happens much more rarely with
a 10-inch scope. That is one of the biggest mental adjustments
required when moving from a small scope to a big one.

Finally, it sounds as though you are still somewhat stuck in a
small-scope mentality; most of the targets that you mention are
small-scope targets rather than big-scope targets, and when you
talk about big-scope targets like galaxies, you talk about them
in small-scope terms -- detectability -- rather than in big-scope
terms. In a 10-inch, you should be looking for the spiral arms
in M51 or the dust lane in NGC 3628, not just thinking about
detectability and maybe gross morphology.

- Tony Flanders