Building A Super Heavyduty 20” Astrophotographic Telescope
by Allan Guthmiller
As an active participant at the Riverside Telescope Makers Conference for a number of years, I have seen many large telescope designs, especially those for astrophotography. Unfortunately, after their debut at the Conference, some of these marvelous instruments are tucked away in the garage and seldom used. A common reason appears to be portability. In some cases extra people are required for the assembly process. Recruiting help can become an issue, thus leaving oneself to the mercy of others. This can take the fun out of operating the “Big Scope.”
Another observation that might influence the effectiveness of the large portable scope is the ratio of the optical tube verses the mount. Some tube assemblies appear to be considerably larger than the mount they are posted on, which makes me wonder if they act more like wind sock than an observing platform? I prefer the opposite. After all, many professional observatories have proportionally large mounts for their telescope, such as the 200-inch Hale Telescope at Mt. Palomar.
With these things in mind I set a goal to construct a large super heavy-duty portable telescope with a tube-to-mount ratio fashioned similar to that of professional observatories. The instrument would be designed to withstand winds up to 15 mph, single man assembly, and could be ready for astrophotography in about 30 minutes.
I admit having a bias towards to 200-inch Hale Telescope at Mt. Palomar, CA. It’s my all time favorite. So I fashioned my design with a split-ring (horseshoe) yoke in mind. I had acquired a 20-inch f/5 Galaxy Optics mirror and set out to draw some plans to fit it. A 48-inch diameter split ring made of aluminum, machined surfaces, and trailer mounted for portability, became the final draft.
To turn a 48-inch split ring required a large machine shop. Rettig Machine in Redlands, CA was the place. I knew Dave Rettig (now deceased), owner, for he was an amateur telescope maker himself. Dave was quite a character. Those who knew him know what I mean! I showed Dave the plans and he said, “That horseshoe will cost you way too much money to build. Besides, I've got one out back I'll let you have for free.” Free? Wow! The price seemed right and it certainly fit into my budget.
Out back in the shop yard, buried under a pile of scrap iron laid in ruins this rusty split-ring yoke mount. Made completely of cast iron, with 1/2” wall thickness, it weighed over 600 lbs! The split ring was only 36 inches in diameter, somewhat less than my original plans, but for the price, I was willing to make a few modifications. Dave warned me that this mount could never be portable and that I would need to build an observatory. My thoughts differed, after all, I wanted heavy-duty!
I called upon my brother, David Guthmiller, machinist and owner of Advanced Machine Dynamics in Highland, CA, to help haul the mount over to his shop where the project took place. I am eternally grateful to my brother for all his expertise, craftsmanship, and the use of his facility over the four years it took to restore and build this instrument. I also want to give thanks to his partner Wally Kaiser, and Jim Robertson of Burns Engine for their inputs as well.
Upon examination of the mount, it was clear the maker had some form of engineering and knowledge of sand casting. The mold for the mount appeared to be created from 1/2” plywood and pine. Embedded grain and knots were visible throughout the casting. Almost every component was made of either cast iron or aluminum including a 22-inch fine tooth drive gear and clutch assembly.
During the restoration project Anthony Cook at Griffith Observatory in Los Angeles, CA contacted me. He was doing some extensive research on the genesis of the 200 inch Hale Telescope for his book “NO SMALL DREAMS: Evolution and Impact of the Design of the Hale 200-inch Telescope.” He explained to me that the man behind the mount was a dentist and amateur telescope maker, Dr. H. Page Bailey of Riverside, CA. He made several mounts between 1931 and 1933. The first was a wood prototype, while another made of “I” beam steel. The location of these two mounts is unknown today. He later made two mounts of cast iron that still exists. One is located at the San Bernardino Junior College in California; the other was Bailey’s personal mount, which is now in my possession. Both mounts held 16” Cassegrainian telescopes, although they were design for optics up to 24 inches.
Anthony found me by following the mount’s somewhat ominous trail. After Bailey’s death in 1962, Warren Estes acquired the mount and telescope. When Warren Estes died in 1976, the instrument was passed on to Clifford W. Holmes, founder of RTMC, where the mount and scope parted company. Cliff Holmes passed away in 1993 and the mount drifted over to Dave Rettig's Machine Shop. Dave told Anthony that he gave me the mount. A year later Dave also died. Hummm?? I began to wonder if the mount had a curse? After all, each succeeding owner after Bailey failed to put it to beneficial use. I hoped the spell would finally be broken through this project.
The mount was severely weathered. Cracks in the casting allowed water to seep through, filling the upper yoke cavities with water. Holes had to be bored on the underside to drain the water and air out the spaces. The cast iron declination shafts were rusted tight in the bushings, which took two days of soaking with penetrating oil before they were loose enough to be pounded out by a sledgehammer! Another causality to oxidation was the 22-inch cast iron gear drive. Many teeth were severely eroded rendering the original drive mechanism unsalvageable.
After all parts were removed, the casting was sandblasted. You wouldn’t believe the chunks of rust that were removed! Afterwards it was sealed with Bondo, sanded, textured, and painted with primer to prevent further rusting during the construction. As for the machined surfaces, such as the horseshoe, there was no need to regrind. Even though it was pitted from many years of oxidation, there was ample surface area for smooth contact with the rollers. Therefore, a hand full of steel wool and a little “elbow grease” made the split ring look like new!
The next phase of construction was the trailer. It had to be designed carry the 1,200 lbs. of telescope and mount, yet be easy for a single person to maneuver during set up. The trailer was constructed in two units. The base frame holds the split-ring yoke. The “A” structure to the rear mounts the south polar bearing, while a set of rollers cradle the split ring at the front. The base frame rides on top of the main frame of the trailer and is free floating, no hinges. The main frame includes the axle, suspension system, wheels, jacks, and all the necessary items for safe towing. Both units are constructed of 3”x 6”x 1/4” rectangular tubing for strength and rigidity. Fortunately, friend of my brother happened to be a welder working at a trailer manufacturing facility. He did an excellent job welding the frame, and included many road safety features as well.
After painting and making a few adjustments it was time to mate the split ring yoke to the base frame. An industrial forklift came in handy lifting the 600 lbs. yoke casting. Note the rollers that support the horseshoe.
Once the mount had been successfully mounted onto the trailer, the optical carriage could be lowered into position between the declination axis. Again, the forklift came in handy for this maneuver. The optical carriage is constructed of ¼” and ½” thick steel plates that are welded together. A Novack 18 point floatation mirror cell bolts onto the back plate. Here you can see me installing the mirror cell into the optical carriage.
The truss tubes are 3” inch diameter and 1/64” inch wall steel. Each tube is filled with urethane foam to dampen any vibrations. They are mounted to the optical carriage and upper truss assembly via special made ringed sleeves, which lock the tubes into place. The upper tube assembly is constructed of 3/4” inch marine grade plywood and attaches to the truss tubes to support the spider and focuser. The spider is made by Novac and the focuser is a 2” inch Crayford design. A shroud design made of solar screen replaced the original design that is seen here. This allows wind to pass through it, yet maintaining adequate cover around the diagonal mirror for improved contrast. Now, you're probably asking yourself, “Where's the Serrurier truss?” Don't need it. 3” inch tubes are more than sturdy to keep the tube assembly aligned. Remember, I want heavy duty.
As mentioned earlier, many of the parts were unsalvageable. This included the 22-inch drive gear. Instead of spending a few kilo bucks for a new 22-inch gear, I opted to build a friction drive using the mount’s horseshoe. A Byers 7 1/2” inch 359 fine tooth gear, clutch assembly, and a Hurst 4 RPM 120 volt synchronous motor was installed to one of the split ring rollers, in effect producing a 36 inch drive system that yields very little periodic error. Equally impressive was the smooth transition of the split ring and rollers. With the clutch disengaged, one could easily move the 1,200 lbs. telescope with a push of a finger!
The declination drive is a tangent arm. Unlike most designs, this system uses a ball screw. This eliminates any backlash or hesitation. A ball screw is a specially machined threaded rod in which ball bearings are seated within the threads. A special housing seals the balls. As the rod rotates, the balls roll between the threaded rod and housing, thus, the housing travels the length of the rod. The tangent arm is attached to this housing with a special slip nut, and renders an ultra smooth transition. No preload or clamping is necessary. With the telescope completed, it was time to take it out for a test. See On The Road With the Super Heavy Duty 20” Telescope for further information.