Re: ATM Folloup: I Hate Dobsonians

[Bratislav.Curcic@uab.ericsson.se (Bratislav Curcic)]

There is one more aspect of using metal in medium to
large scopes. The massive pieces of metal are not so good 
for local seeing conditions. Our club's 20" with excellent
optics by late Bill James seldom betters my 8" on planets,
mainly because massive (all metal!) mount, cast Al cell and 
mirror itself (almost 4" thick) store so much heat that
local turbulence practically never ends. If you want your
scope to be a planetary (as opposed to "stellar") performer,
try to avoid metal. Composite sandwitches (carbon fibre or
kevlar skin over plywood or foam) will be of almost the same 
strength as metal, much lighter, much more resistive to 
vibration and will store far less kinetic energy and heat. 
It may not look as "professional" as all metal, all machined 
scopes, but I couldn't care less. My scopes are made to look 
THROUGH, not AT.
And another thing - metal will expand and contract over a given
temperature range far more than almost anything else. My carbon
fibre Wright and friend's Maksutov and Schmidt scopes will keep
exactly the same focus during the whole night; in fact they will 
need no refocusing regardless of summer (HOT in Australia)
or winter conditions. Try that with you shiny "pro" looking 
scope.

Bratislav