Here is my all-metal 47cm f/4.8 scope. It now sports a 11.3cm f/4.5 finder, which has about a 3 degree field of view. It's a big change from the old wood mount and wood/conduit truss OTA. The scope has lost 30 pounds in the process. Most of the weight savings has been in the OTA, which is now down to about 70 pounds, including 20 pounds of counterweights. The total weight is 100 pounds. Setup time hasn't changed much, it takes me 3 trips and 3 minutes to get from my garage to my backyard, and 6 minutes to set up. Most of the time I leave it assembled in my garage these days and have Kate help me move the OTA outside, fully assembled.
First light was May 20th, 2001, looking at Mars for a couple of hours. Tracking at 282x was great, vibration not a problem at all. I wanted to finish this scope for Mars' opposition that year, and damn it, I did it.
Thanks to Bill Prewitt for pointing me to a place for good cheap surplus aluminum, Ken Lowther for helping me cut up the metal with his tools at his place last year, and Bob Kerrey for doing the welding on the base, mirror box and secondary ring.
Clive Milne's scopes were certainly an inspiration to me!
Details:
Cutting quarter inch aluminum isn't too much trouble for a good sabre saw, just use WD40 or a light cutting oil, clamp your work down to something, and by all means, WEAR EAR AND EYE PROTECTION.
The truss tube end connectors consist of binder clips which have 1/4 inch holes drilled in, 1/4-20 round head hex bolts put through and bolted to the secondary cage and mirror box. 1 inch aluminum angle is mounted to the truss tube ends and a hole is drilled in the angle to snugly fit the bolt heads. I think I used a 9.25mm diameter drill for the holes.
The scope is driven by two Vexta 400 step per rev motors and roller drives. The roller drives are 2.5 inch diameter 0.25 inch thick aluminum disks reamed to fit on 5/8 inch stainless shafts. The steppers are microstepped at 32 microsteps per step using a control system from the MacDob project, which I’m writing software for.
The altitude bearings have a 15 inch radius. The mirror is plate glass, 21mm thick at the edge and 15mm thick at the center. I ground and polished it myself. It has an enhanced aluminum coating from Spectrum Coatings.
Full view |
looking into the mirror cage, showing motors and encoder |
other side of mirror cage |
close up of quick release truss clamp |
quick release clamp, disconnected |
I made my own Crayford! |
Back of scope, showing the ever-changing counterweight arrangement |
Altitude roller drive and stepper motor |
Azimuth roller drive and stepper motor |