Rebuilding a 12.5″ f/5 Truss Tube Dobsonian

12.5" Dobsonian
 
Some time ago my friend Bill Lofquist bought a dobsonian telescope from Roger Ceragioli. Roger had built the ‘scope to provide a home for a beautiful 12.5″ f/5 mirror he had made. The mirror is gorgeous, as is typical for Roger who is one of the best opticians I know. I have an APO triplet of Roger’s that is a prized possession.

Mechanically the scope had a few problems. The truss tubes were attached with separate hardware top and bottom, so that setup required over 20 minutes of sorting through screws and futzing with eight separate truss tubes waving around the whole time.

The elevation bearings had been set about 1/8th inch off from each other leading to a side to side twist when the scope was moved in elevation. This was not a major problem when using the scope visually but would make the use of digital setting circles impossible as DSC’s require orthogonal axis in the scope.

The ground board was a bit undersized, making the scope prone to tipping when used at low elevation.

The rebuilt scope is essentially finished with the usual tweaking and small adjustments remaining. Things are coming out very well and a few of the changes are worth passing along to the ATM community. In the sections below I will concentrate on practical details in hope of conveying some of the finer points in telescope making.

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Violet Haze, A 90mm f/13 Apochromat

Violet Haze
Violet Haze, a 90mm f/13 apochromatic telescope
I had wanted a high quality APO refractor for some time. Mostly for photographic use. Opportunity presented itself when Roger Ceragioli offered me a 90mm telescope he had finished the year before and was willing to sell. Working for the Steward Mirror Lab, Roger normally grinds very large optics, things like secondaries for six to eight meter telescopes. But as a hobby he makes somewhat smaller telescopes. This particular lens set had won him a merit award at RTMC in 2002. I had previously seen this telescope and after some negotiation we settled on a price.

The lens triplet is exquisite, providing absolutely perfect airy disks at high power. The photo below shows an example of the out of focus image of Antares taken with the telescope. Pulling out my copy of Suiter’s Star Testing Astronomical Telescopes shows nearly identical images for the ideal diffraction pattern. No wonder the ‘scope won a RTMC merit award.

Photographically it has proven to be almost perfectly free of color, corrected across the spectrum. There do not seem to be any detectable UV or IR halos around bright stars. This is partly a result of good design, and aided by the long focal length of f/13. No field flattener is required, with pinpoint stars across the focal plane.

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A Backyard Telescope Pier

Complete Pier
The completed pier with a telescope atop
Have you ever wanted to have a place to set up your scope easily in the backyard? with instant polar alignment? no tripod legs in the way? Even for someone with little handyman experience a pier is an easy weekend project that can be completed for around $60. Add the cost of a wedge for your scope, about $125-$400 new, less used, and you have a usable pier. A few bags of concrete, a little rebar and a sonotube will do the job. I know, we ATM’s usually use sonotube for telescope tubes, but this is what it is really meant to do, cast concrete.

A pier is also the first step in a real backyard observatory, build the pier first, then a building around it. The process shown here works for any pier and can be scaled as needed for larger scopes. The pier shown in the photos is intended to hold an eight inch SCT.

The plans and photos shown here have been used for several piers here in the Tucson area and have been refined with the experience. Feel free to improve on what is here, and if your idea works well send photos!

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A GPS Observing Clock

Observing Clock
A hand made GPS clock for the observing table
For years, when observing, I found myself wanting a clock on my observing table when recording observations. I have used either a wrist watch or a cell phone, but looking at these was uncomfortable as these modern devices use bright backlit LCD displays, not a nice night-vision friendly red. The cell phone also has the additional problem of using up its battery quite quickly when out of range of a digital cell tower at some remote observing site. I needed a simple desk clock for my observing setup.

Accuracy was also a question, accurate time is always important when observing. Asteroid occultations, lunar and solar eclipses, iridium flares, twilight, jovian moon transits, the list of things where accurate time is useful is long in astronomy.

The Specs

Of course being a electrical engineer makes designing and building a clock a fairly trivial exercise. But why stop there? Why not build in a few extra features…

  • Use red 7-segment LED’s and build in some type of selectable dimming mechanism.
  • Why bother setting the clock each time you set it up? Make the clock self setting and very accurate.
  • Since the clock is accurate add a serial port to allow the clock to supply accurate time to a laptop when taking astrophotos.

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Improved Rocker Box Pads

A small design detail in a dobsonian telescope is a method to restrain the mirror box in the center of the rocker box, to keep it from sliding side-to-side in the elevation bearings.

Rocker Box Pad
A standard carpet pad embedded into the wall of the rocker box
A common solution is to use a couple carpet pads to provide a lateral support that keeps the wood from rubbing. The pads do not add any friction that would keep the scope from tracking smoothly at high power. These pads are available in most hardware stores in both sheet form as well as pre-cut circles, usually one inch in diameter.

I have had trouble with these pads in Deep Violet. They do not stay put! Sometimes when inserting the heavy mirror box into the rocker I would catch a pad and simply shear it away from the wood. I ended up using a larger pad and using small wood screws to secure it instead of the adhesive.

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Primero

Every astronomer has a first telescope, mine is a 6″ f/5.1 Newtonian I first built as a teenager. As life progressed I was forced to dismantle the telescope and it dwelt for a time as a pile of parts in a box. Eventually I had an opportunity to rebuild the telescope, but as both my technical capability and my financial means had increased I was able to do a substantially better job.

Primero
Primero setup at TIMPA outside of Tucson
The result is the instrument you see to the right, Primero, or simply ‘first’ in Spanish. The original mirror was used but little else from that first telescope was reused. A completely new mount, a new tube and all new fittings. The only purchased parts are the focuser, the Telrad and the secondary mirror. Several parts of the mount were removed and reworked from a previous mount, this includes both the bearings, shafts, counterweight and saddle. The entire tripod, tube, mirror cell, spider and secondary holder were produced by hand for this scope.

The optical design of the telescope is standard Newtonian with a f/5.1 primary mirror of 6.0 inches giving a focal length of 777mm. The mirror was hand ground when I was a teenager and thanks to expert help during figuring is an excellent mirror.

The RFT design is deliberate and has proven to be a good choice, particularly with modern eyepieces that perform so well in short focal length scopes. With a 35mm Tele-Vue Panoptic eyepiece the scope provides a 22x image with just over a three degree field.

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A Box ‘O Broken Telrads

The Telrad finder is one of the most useful telescope accessories ever invented. A set of glowing red rings showing you, at a glance, exactly where your telescope is pointed in the sky. I have one on each of my telescopes. The Mauna Kea VIS also equips each telescope with one of these simple devices.

They do not work so well after hitting the ground a few times.

As I have mentioned in the past, the equipment at the Mauna Kea VIS gets used hard. It is setup every single night of the year. Thousands upon thousands of people use these telescopes to see the wonders of the night sky, the first time for many. The wear and accidental damage in the darkness takes a toll.

Broken Telrads
A box full of broken Telrad finders awaiting repair
When Deb and I were last at the VIS we spent the day cleaning eyepieces and making other repairs to the ‘scopes. One of the things I found in the storeroom was a small pile of broken Telrads. Some were missing windows, many had broken battery holders, mirrors were missing and reticle holders hanging loose. Many had been patched back together with tape or hot glue, attempts to keep them working for another night.

Quite a few had reticles that were missing or melted by exposure to sunlight. The lens that focuses the reticle’s ring pattern, projecting it into the sky, will also focus sunlight on the reticle, quickly melting the thin film if a Telrad is left in the sun.

Gathering up partial and scattered parts I collected a box of finders that I can work on later. It made quite a pathetic sight, a box of broken Telrads. A couple evenings later, five of the Telrads are now rebuilt and ready to return to duty. Four more are awaiting replacement reticles before I can call them completed. I will take them back up next time we are on the mountain, but I expect we will find something else that needs to be fixed.

Telescope Maintenance

At the Mauna Kea Visitor Information Station telescopes are available every clear night for the public to enjoy the wonders of the night sky. Every evening a set of telescopes ranging from 102mm to 16 inches is setup in the patio beside the VIS. The gear is used heavily, every night of the year, the wear on the telescopes does exact a toll. The abuse is constant, kids hanging on the eyepiece, volunteer operators who have never used a telescope, rain, fog, blowing cinder dust. Conditions that were never foreseen by the designers and far beyond what most telescopes encounter. Sometimes the condition of the equipment is embarrassing, dirty eyepieces, groaning mounts that refuse to track, much of the gear just looks worn and tired.

Dirty Primary
An 8" telescope mirror covered with a ridiculous amount of dust
It is hard for me to see this, but at least I can do something about it, I do, after all, fix telescopes for a living. It is not unusual for me to spend an evening repairing a telescope and I have made a point of getting some more extensive maintenance accomplished.

My first effort last year was to clean and repair the small dobsonians used by visitors every night. Two eight inch, a six and a 4.5″ Orion dob are put out for anyone to use, from adults to children. After years of use they were in horrible shape, bearings and focusers were coming apart, collimation gone, moisture dissolving the woodwork, a finder attached with duct tape, the mirrors so covered with dust it is surprising there was much of an image to see. One of the eight inch scopes and the 4.5 inch were in pieces in the warehouse after a fix attempt by another volunteer. It took a few days of work to put all to right. Stealing parts from an older scope, repairing what could be saved, cleaning and pounding out a couple dents. Clean, re-install and re-collimate the optics. Four dobs back in service and in better shape than they had been in quite some time.

Losmandy Teardown
Parts of a Losmandy G-11 telescope mount spread across a table top
The 16″ Meade LX200 should be the flagship of the equipment used at the VIS. But for all too long it refused to work properly, it would not track. A trip back to the manufacturer failed to correct the problem, despite nearly a thousand dollars in shipping fees for factory service the telescope still would not work most of the time. Most volunteers would not use it, having given up in frustration. Surprisingly the issue was obvious, just listening to the scope indicated gears not fully meshed and grinding on one another. An hour’s worth of dismounting the scope, opening the bottom panel and re-seating a motor mount had the telescope back on sky and slewing from target to target. The scope has failed since, but the problem was even simpler, a loose connection found after a half hour of poking around.

Currently, one of the three Losmandy G-11 mounts belonging to the VIS is in my garage, spread across the table in many parts. I spent a few hours yesterday dismantling the mount and cleaning the grease and cinder dust out of the bearings. It is in pretty good shape, a good cleaning, re-seat the worm gears and some new clutch pads and it will be ready for a few more years of service. I need to get some more grease before I can reassemble the mount, but otherwise everything is ready to put back together. Finish this one and there are two more like it in sore need of maintenance.

One thing at a time, of course by the time I get through it all it will be necessary to start over again…