Re: ATM Collimation, the Truth?

[andydtg@phoenix.net]

Bob Pfaff wrote:

>That statement is correct if you have an OFFSET  diagonal.
>If you adjust things so everything is concentric with an
>offset diagonal what you have done is skewed the optical axis
>a little so you do NOT have a offset diagonal
>ANYMORE. For OTA's with offset diagonals you must use the spot
>method of collimating. 
>
>The offset  diagonal will not appear  centered in the focuser and the diagonal
>reflection will not be centered in the primary when collimated.
>See All About Telescopes by Sam Brown

For collimating after the secondary has been offset and 
aligned to the focuser/center of primary, I put a circular
mask over the rod that holds the secondary and defines the
mechanical center of the OTA. This mask is circular with a 
diameter about 1/4 to 1/2" less than the primary & looks, thru
the focuser sight tube, like an enormous secondary. It's
*easy* to center this in the primary as the ring of light
surrounding the mask looks very asymmetrical id the tilt of
the primary is off even a little bit. The scope is pointed at
a uniform light background like the sky for this to work. 

This gets it close enough for star collimation very quickly.


Andy Saulietis / DTG Alt-Az-Fp Drive Systems
HDPE Worm Gears, Custom designs & Machine work
12617 Harriet Ln
Santa Fe, TX 77510 USA
409-925-8854 Voice/Fax
e-mail: andydtg@phoenix.net