Science. Education. Community. Fun.

Refurbishing the CUAS 16″ Cassegrain Dome Telescope

by James Wehmer

This document was derived from images in my Power Point presentation that was shown to CUAS on April 12, 2018.

The motivation for this project was the need to keep an astronomical object centered in the eyepiece during our monthly Family Skywatch open houses. Drive motors needed to be added. We were spending more time recentering the object as it drifted outside the field of view than the public was getting time to view the object. Sometimes an inexperienced guest would grab the telescope and move it off of the target necessitating reacquisition. It was also hoped that we could use this telescope to take time exposure photographs of dim objects. When I joined the club three years ago, I was very excited to start using our equipment. I envisioned attaching my camera and taking long exposures of dim distant galaxies. Imagine my disappointment the first time I visited the dome to find one of our members being very unhappy because the polar axis had frozen up and the telescope wouldn’t move. Clearly improvements needed to be made. Thus began our year and a half long project. As a research engineer at the U of I, I was accustomed to figuring out how to do things that no one else had ever done before. But, I have never done a project like this before and had great trepidations over how it would all turn out.

History

First, a little history of this 115 year old telescope mount. It has been repurposed many times from commercial to scientific purposes, and photoelectric photometry to photographic research surveys, to its current public amateur use. It is the long history of this telescope that makes me proud to modify it once again and add my name to its history. In March 1903 it was purchased from the John Brashear Company for $7500 for the purpose of focusing the sun’s rays to make large synthetic rubies from small natural ones. This is why I shall henceforward refer to this telescope as “Ruby”.

Over the past year and a half, although we have had many arguments, I have developed a close relationship with her. In 1912 the University of Illinois purchased Ruby for $1500 to use her for photoelectric photometry measurements of star brightness using photoelectric cells recently developed at the U of I. Previous to this, star brightness’s were measured using visual polarizing photometers. It is important to note that the electronic technique pioneered with this telescope of electrically measuring a stars’ brightness is the exact same technique used on current satellites to find extra solar planets orbiting around other suns. Although, more sensitive modern light detectors are now used. Ruby was modified and a new observatory building was constructed for her in 1914 just north of the Marrow Plots and south of the Astronomy department building which houses a 12 inch refractor. The 1914 dome is still the one housing Ruby at our CUAS Prairie Winds Observatory. In 1925 Ruby got a newer 30 inch mirror with a longer focal length and was moved to Florida Avenue just south of Mt. Hope cemetery. The new location likely perpetuated the telescope’s association with ghosts as younger astronomers walked through the cemetery on their way to do a night’s work would retell the tale of the instrument “being used to observe the flight of souls”. On the evening of 27 May 1933 a new photometer triggered the opening of the Chicago A Century of Progress World’s Fair using the light from Arcturus.

The Fourth Reincarnation

In the fall of 1939 the reflector was replaced with a 0.1 m Ross-Fecker photographic refractor. Each 8×10 inch glass plate covered a large 20 by 16 degrees of sky. Photographs were used for counting of stars in the Milky Way down to the fifteenth magnitude in order to achieve an understanding of the distribution of stars in the Milky Way. The telescope soon became the primary research instrument at Illinois and resulted in multiple publications. The South Observatory’s dome and the Ross Camera were removed in 1965 when Florida Avenue was widen. The telescope was reassembled in 1968 at the university’s new Prairie Observatory site located near Oakland, Illinois. The Ross camera was stolen and the mount damaged in 1983. I personally used this camera in 1976 fitted with an objective prism to record the spectrum of many stars at the same time for the purpose of identifying stellar types.

Above photo is of the 4-inch Ross-Fecker camera with the 4-inch Fauth refractor as the finderscope.
The instrument was a refractor of 4-inch aperture and 28-inch focal length (f/7) with a scale of 290”/mm. It held 8×10 inch glass plates covering 20 by 16 degrees.

First plate taken October 10, 1939, last recorded plate October 8, 1980. Many of the plates are now at Yerkes Observatory where they will be digitized and preserved.

The Champaign-Urbana Astronomical Society (CUAS) Acquires It

The Ross camera was stolen and the mount damaged in 1983.

The mount that vandals had tossed down the stairway at Prairie Observatory ended up in Bob Lozars’ barn! The fork and drive were broken, but the 4-inch shaft and bearings were in good condition.

CUAS acquired both the dome and the polar axis in 1991. The fork mount was built by Scout Hough. This is Ruby as she was first installed at CUASs’ Prairie Winds Observatory. On July 11, 1992 a ribbon cutting officially opening the C.U.A.S. Observatory. “First Light” was July 13 at 9:15pm the light of the star Arcturus entered the telescope, just like it had done in 1933. M13 was the first Messier object found.

Ruby was refurbished in April 2005.

This is how I first saw her in 2014 and when we were evaluating the condition of the RA bearings in 2016:

OK, Now It’s MY Turn

For decades, I have been collecting parts and planning to build a large reflecting telescope on a permanent mount. I am now 67 years old and realize that I am never going to move to Arizona or purchase property here in the country. What I now need is someone with a rural sight who will allow me to build my telescope. Eureka! That is exactly what CUAS has.

Design Considerations

  1. Make it as cheaply as possible by doing all of the work ourselves and using as many free parts and materials as we can find. At this time, CUAS was trying to raise $40,000 to build a roll off shed observatory to house our many additional telescopes and I did not want to compete for funding with that project. All total, we got $350 funding. $120 for steel and $230 for the two self aligning declination bearings from CUAS. Many members donated parts, time, and paid for services. Scott Glick got the fork and many of the steel parts powder coat painted. Mike Conron purchased a new polar axis bearing for $444. Mike Rosenburger donated the fork steel from old farm equipment parts. Trigby Emilsson donated 100 pounds of lead for counterweights.

    The key parts were the two worm gear wheels. Years ago I had salvaged these and without them this project would not have happened. It would have cost us $2500 to purchase these gears. The brass Declination gear came out of a NORELCO PHILLIPS X-RAY Chrystal Diffraction Goniometer. I have no idea what that thing was that I took the big Right Ascension gear from.
  2. Keep the electronics simple. I could have easily just copied the electronics that I used for my own eight inch Schmidt–Cassegrain telescope. It uses two microprocessors to do spherical coordinate system transformations using data from optical encoders to calculate position and Pulse Width Modulation speed control of the DC gearmotors. But who is going to fix it when it breaks if I am no longer around? You can’t just go out and buy another one. Plus, the programming I did for it is terrible and it is very hard to use. Alternatively, use stepper motors, but they also have a limited speed range. I finally settled on a simple DC system where a variable voltage controls the motor speeds. It easy to understand and repair or duplicate. The problem with this design is that with a fixed gearing ratio, it is impossible to get both fast slewing speeds and precise slow motion tracking without stalling out the motors at slow speeds. I would need some kind of a gear transmission and clutch which would require a lot of precision machining work. My solution was to forget the transmission and just build a clutch system to engage and disengage the worm gears. The telescope would be manually pointed to the part of the sky to be observed, just like it was always done. Then, the worm gears would be engaged and the observer could center the object in the eyepiece using the motors and tracking would commence.
  3. Ease of maintenance. I have cursed many a designer when something breaks and it is difficult to access the mechanisms. By removing a few bolts and one electric connector, either RA. or Dec. drives can be removed and taken home to work on. The electronics are also easily removable.

Work Begins

We waited until most of our 2016 public skywatch events were over before we disassembled the telescope. Considering how filthy they were, I was very pleased and surprised at the excellent condition of the optics after we cleaned them. The polar shaft must weigh over 300 pounds.

Note in the picture the $444 bearing we replaced. Water from condensation poured out of the bearing cover when we removed it. Very bad! I installed zerk fittings on the covers and pumped the housings full of grease to remove this problem.

We did not want the dome to remain empty during the refurbishing project. One of our members had donated a 14 inch Meade LX200 GoTo scope so I built an adaptor to temporarily install it on top of the existing polar mount base. We used this scope in the dome for all of the 2017 observing season.

Below, Mike Rosenburger welds together the parts I cut for the tube cradle.

The Electronics

I already had all of the motors, cables, and electronic components leftover from previous projects. Building the electronics was a good indoor project for the winter. I laid out the artwork for the printed circuit board and got some old coworkers at the U of I to make the boards for free. I then soldered the components into the boards and tested them. The entire circuit runs on 12 VDC at 1/2 amps. There are 4 adjustable DC regulators; Clutch motors, Dec Slewing, Dec tracking, and the RA tracking has a voltage speed adjustment knob on the front panel. Motor direction reversal is digitally controlled using two MPM3304 “H” bridges. The cables have redundant wiring for reliability and are easily replaceable. I tried to make the pendant controls simple and intuitive. I have already planned ahead and installed space and wiring for a Sky Commander digital setting circles which I hope to eventually install when funds become available. The one in the picture is just a photo attached to the front of the controller pendant. For a complete description of the circuits, refer to Ruby’s operating manual at -link to manuals-.

Someday I want to install a Sky Commander digital setting circles like this one:

Machining the Steel

The fun part of this project was figuring out how to do things that I had never done before. This project would not have been possible without the machine tools which I inherited from my father-in-law.

A funny thing happened on the way to the machine shop. In order to replace the $444 RA bearing, the flange on the south end of the polar shaft needed to come off. Many of us tried and failed. Mike and Guy pounded on it. Conron stripped paint off looking for set screws. We gave up. I backed my truck up to the door to load up the 300 pound shaft and take it to a machinist I know. We scooted the shaft out the door into the sunlight. Rosenburger walked up, saw it, and asked “what are these two little set screw holes for”? He loosened them a few turns and the flange screwed right off. That saved us a lot of work and me the embarrassment if I had taken it to my machinist friend.

The last piece of the puzzle was how do I connect the base of the fork to the north end of the polar axis. I needed a cylinder. I searched locally at Kurland Steel, Marco Steel Supply & Scrap. I emailed all our members. The only thing I could find was a $350 mail order piece. Then, Mike Lockwood got this piece (shown below) from John Pratt. I did not even need to cut or turn it. I just drilled and tapped six holes radially. Usually the world for me is more of a struggle than that.

Reassembly

I had worked all summer on the steel and winter was coming. In November 2017 we finally began reassembly. I still was not certain if all the pieces would fit together. We moved the LX200 out of the dome and into our new roll off shed observatory. We used Dave Thompsons’ engine hoist to remount the polar shaft. We pumped grease into the bearing housings to keep out condensation. I then spent days assembling the new fork and cradle. It all went together perfectly. I only had to drill out one hole slightly larger.

I found that in the vertical position the two springs holding the Declination worm against the worm wheel were not strong enough due to the weight of the assembly. I redesigned it so that instead of the motor disengaging the worm, it would now push to engage the worm. This is a much stronger more positive engagement.

I was unable to make the final piece to attach the RA drive assembly until the polar axis was reinstalled and I could make measurements. I was very pleased that it only took one single piece of 90 degree angle steel to make all three dimensions adjustable. This piece is the copper colored one in the photo with the seven adjustment slots. I also use the original motor mount. (the dark blue piece) I found it in the corner of our shed.

It took 140 pounds of lead counterweights hidden in the fork base to balance the Declination drive mechanism. This is because the weights are not as far out from the polar axis as is the drive. The mirror mount already had cooling holes so I added three 12 VDC fans and small size screens to keep the bugs out of the main tube. When we first disassembled Ruby, dead ladybugs poured out.

One of the simple pleasures of life is having things work out OK even without my planning it. I thought for days about how to make a better tube cover. The old one let bugs, mice and birds into the tube. I did not want to drill any attachment holes into the tube. I decided to just go ahead and make the cover and worry about attachment later. I laid a piece of plywood under the end of the tube and traced around it. After I cut out the disk, I screwed a strip of metal to the perimeter. I was surprised to find that this assembly would not fit onto the end of the tube. The tube is not round! I rotated it until it popped on. Eureka! Rotate it to tighten!

It was now December and cold and I was done with this project until spring testing and optics installation.

Spring 2018

In March of 2018 it finally warmed up enough for me to get back to work on Ruby. Now in theory everything should work OK. Right? No not quite. Some dummy wired both drive motors backwards and one clutch engaged while the other one disengaged. I had a 50/50 chance with each motor so why were 3/4 of them wrong? Fortunately, the designer had built the cable wiring for easy access. All I had to do for each motor was loosen two wire buss clamp screws, reverse two wires, and retighten the screws. Brilliant design! That engineer is fantastic! Actually, knowing it would be this easy is why I never bothered to worry about the motor directions in the first place.

Mike Conron and I installed the primary 16 inch mirror and secondary. It took a few tries to get it collimated. The limit micro switches on the RA clutch were glitchy and need replacing. I had damaged them during the RA drive installation. RA tracking was too fast even at the slowest speed setting on the adjustment knob. Too fast is better than too slow. I easily reduced the motor speed voltage by changing a resistor in the voltage regulator circuit.

I did not want to drill holes into the primary tube in order to mount the six inch refractor finder and associated counterweights. We disassembled the tube cradle and I took the cradle straps home. The strap material is only 3/16 thick. I wish we had welded mounting bolts onto them before we painted them. By using flat head screws and countersinking the holes from underneath, these screws should be strong enough to hold the finder and not protrude below so as to scratch the primary tube. The finder really is not very heavy. I cast five lead counterweights to offset the finder. acks recycling has lead for $0.80 cents per lb. I used my pottery kiln and a propane weed burner to melt the lead and cast it into tin cans. Turning on the lathe removed the can and drilled the central hole.

One fantastic thing is that vibration damping on the mount is great. Never having done a project like this before, there was some concern about the strength and thickness of the material for the fork. Hit the scope and oscillations stop within a few seconds.

March 21, 2018 was “first light”. I tried to follow tradition by first viewing Arcturus just like in 1933 and 1992.

May 6, 2018; first time exposure while tracking was M13 at prime focus.

In The Future

I still need to work on the RA tracking to make it perfect and someday a polar alignment using the drift method will need to be done on the mount. The Dec clutch pivoting mechanism needs to be tighter. Better Collimation. Build a cover for the RA drive. Reinstall the Dec drive cover. Paint the base. As stated earlier, I hope someday to install a Sky Commander digital setting circles. Declination slewing speed is good but RA slewing is too slow. This is due to the gearing ratios and the fact that I was afraid of damaging the drive mechanism by rebuilding it with a different ratio. Right now the best way to acquire an object is to position the scope slightly to the west of it and engage the clutch. Then drive the scope to the east to center the object. This way, earths’ rotation adds to the eastward motor slewing. I may remount the RA drive motor in such a way that we drive the worm axle directly through a pair of gears similar to the way I built the Dec drive. Ultimately, it would be nice to never need to disengage the worms. Each time the gears are not perfectly aligned when they mesh, repeated engaging will wear the gears. It would be ideal to just keep them engaged and install a better motor drive system capable of both fast slewing and slow precision tracking. Both drives can easily be removed from the mount and taken to a machine shop for new motor installation. This commercial system will also allow computer control of the mount.