The advantages of having a fixed scope installation are many and with the 20" scope I use for my deep sky CCD imaging work it is a necessity due to the shear size and weight of the system. I wanted to have a smaller imaging scope setup though that could deliver high performance tracking and guiding for a wide range of scope types and sizes ranging from a 12.5" f/4 Newt f/15 classical Cassegrain to a combo of an 8" TEC Maksutov and 5" AP wide field refractor. The main design criteria for the mount was stiffness and transportability, but not necessarily what one thinks when the word "portable" is used.
The mount was designed to handle a payload of 150lbs. while maintaining a minimum resonant frequency of greater than 10Hz to minimize motion and settling times due to disturbances from wind, motion corrections and bumping. The heaviest component could weigh no more than 70lbs for the limit of lifting and the whole mount had to be able to be assembled and polar aligned to within a few arc minute in less than 10 minutes. This was achieved using quick disconnect flanges between the RA and DEC axes, screw in shafting for the counter balancing and handscrews for mounting the pier legs. The mount has 4" diameter hollow stainless steel polar RA and DEC shafts, a polar borescope, fine adjust altitude and azimuth adjustments for polar alignment, 25 degree range of latitude adjustment, 12" Mathis worm gear drive for RA, 10" Byers for DEC, clutches in both axes, DC servomotor control and GOTO electonic control by Astrometric Instruments.
The worm drives are zero backlash design with adjustable spring preloads while the bearings are precision machine tool angular contact ball bearings. The total weight of the mount is 350lbs including counterweights and cost approx. $600 (I am a professional scrounger!) to build less the GOTO controller. At the 2003 Winter Star Party observers were encouraged to literally punch the pier to see how much disturbance was created while watching Jupiter at 600X. The image always stayed in the field of view and settled out in less than 1/2 second to the disbelief of viewers. The cost to build this mount including the GOTO controller was less than 1/3 that of an AP1200 or Paramount 1100ME.
The mount was entirely machined by myself using our company's toolroom lathe and vertical mill. Working in a machine shop to pay my way through university was very helpful in this project. I did need help with the aluminum welding, a skill I would love to master sometime.
The mount was entered at Stellafane in August 2001 and won 1st place for the "Special" category. It has worked flawlessly in hot (+86F) and subzero(-6F) weather so far and I am counting on it to give me a lifetime of high performance in my portable imaging journeys. I still need to design covers for the worm gearing to complete the design and decide on the surface finish for the mount.
In late 2004 the mount was upgraded with MAXON high performance DCServomotor gearheads to provide higher slew speeds and more torque.
Following are some photos showing how it was built.
Composite stainless steel DEC shaft assembly being machined
Finished machined RA and DEC shafts with brass bearing preload handnuts
Stainless steel conterweights, counterweight shaft and DEC shaft housing
Finished machine parts for the equatorial head
Mid assembly stage of the equatorial head
Equatorial head showing the fine adjust azimuth and altitude adjustments
Equatorial head showing the RA DC Servo drive and preload assembly
The equatorial head nearing completion
The finished mount including the 12" diameter strain layer damped pier
The finished mount components ready to travel
The mount (with 8"TEC MAK and TAK 4" scopes attached) being judged at Stellafane 2001!
The mount (with 8"TEC MAK and AP 5 f/8 EDT" scope attached) at WSP 2003
The mount (with new 12.5" f/4 homebuilt Newtonian and AP 5 f/8 EDT" scope attached) at WSP 2005