From: "J&S" 
To: 
Cc: 
Reply-To: "J&S" 


Hi Mike,

>I am concerned about how accurately centered the mirror must
>be...What are people's opinions regarding accuracy of centering a primary?

     Here's my 2 cents:
     As long as three conditions are met, centering of the primary is not a
big issue (with reservations).
Condition 1:   The primary can be accurately adjusted to aim squarely at the
secondary.
Condition 2:   The secondary can be accurately adjusted to aim squarely at
the primary.
Condition 3:   The baffle tube protruding through the primary can be
accurately aimed at the secondary, independent of the primary mirror (so
that optical adjustments of the primary will not affect mechanical
constraints of the optical path, ie, the baffle tube).
     As long as those conditions are met, centering of the optics in the
main tube are of no consequence - with the understanding that uncentered
optics will usually result in the optical axis not being coincident with
the mechanical axis of the main tube, which is no big deal if you are not
going to use setting circles on your mount. (That's the reservation.)
     But from an "ease-of-use" point of view, nicely centered stuff is
always to easier to deal with when collimation time comes around.
     For more on mechanical concerns and consequences regarding centering
and mounting of the optics/baffle go here:

http://home.att.net/~jsstars/cassbaff/cassbaff.html

     Bear in mind that with a Cassegrain (especially with an F/3 primary!)
good collimation is CRITICAL for best performance. Don't forget how bad the
images in an F/short Newtonian are when the collimation is out of whack. A
baffle tube that is mounted directly to the primary is almost guaranteed to
make proper collimation a huge hassle, if not impossible. Been there done
that many times...

- Jim S.

--- BBBS/NT v4.01 Flag-5