Re: ATM Tube sag?

[andydtg@phoenix.net]

Bob Pfaff writes:

>IMHO it is not tube sag that is a problem but the optical
>components that sag. The diagonal sags a little, the primary
>mirrors cell sags a little. Back in the days before the off-axis
>guider for astrophotography we used guide scopes. To get
>photos without trailing stars the scope needed to built like a tank.

I agree. Our 32" has a 7.5" minor axis 1.5" thick secondary, it's
mounted on an aluminum spider with 1/8" thick vanes tapering 
from 3" in the middle to 3/4" at the ends. We have to collimate it
at 45 deg elev so the differential sag and defocusing is minimized 
over the range of elevation motion. We're going to replace the 
aluminum vanes with SST so it'll be more stable for imaging.

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