Awaiting Dark
Tag: dobsonian
Restoring an Obsession
Obsession Telescopes are something of a standard in the astronomy community. David Kriege was one of the first to start building truss tube dobsonian telescopes commercially, bringing portable large aperture telescopes to the astronomy community. These telescopes were a bit of a revolution in the pursuit, with sizes unreachable only a decade before, when a 10″ or 12″ telescope was considered big. When I built my 18″ it is David Kriege’s book I used for much of the design, following in the footsteps of so many amateur astronomers.
A 20″ f/4 Obsession donated to the observatory has presented a challenge and an opportunity. The telescope was the prized possession of Bob Michael having been ordered new directly from Obsession. The telescope is serial number 004 with a manufacturing date of June 1st, 1990. As David started Obsession Telescope in 1989, this is a very early example of his work. For many years Bob and his wife used this telescope to observe, completing the Herschel 400 and other observing projects. Unfortunately he was forced to give up astronomy due to age and glaucoma, donating his equipment to the observatory.
Clamps
John Dobson (1915-2014)
Today an amateur astronomy icon passed away. John Dobson popularized the very simple design of telescope that came to bear his name, the Dobsonian. As a Vedantan monk John possessed few material means, pursuing a passion for telescope building in the monastery garden shed he designed a telescope that could be built from whatever scrap parts he could scavenge. He could often be found around San Fransisco showing the wonders of the night sky to anyone who would look through one of his telescopes. His infectious enthusiasm for astronomy led him to help co-found the San Francisco Sidewalk Astronomers.
The Dobsonian is a telescope that is characterized by an extraordinary simple and robust design. Made of plywood and other hardware store parts, there was nothing in the design that could not be built by hand.
The optical layout is a standard Newtonian design with the eyepiece at the front of the telescope. This allows the heavy primary mirror to be located quite close to the ground. The entire telescope rotates on a simple lazy-suzan azimuth bearing made of plywood, formica and teflon blocks. A simple set of trunnions allows the telescope to be raised and lowered in elevation.
The Dob brought large aperture astronomy into reach of thousands of backyard observers. Anyone with a modicum of skill could build a Dob in a garage with simple hand tools. Commercial designs soon appeared at very affordable prices.
Amateur telescope makers have built upon John’s ideas, creating elegant designs that far surpass those simple telescopes made from scrap. Aircraft grade plywood, machined aluminum frames, carbon fiber and computerized controls are common in modern Dobsonians. The design can be scaled up, Dobsonians are sometimes enormous, with telescopes of 30 or 40 inches aperture seen at many star parties. At OSP last year I setup next to a 40″ built by Chris Fuld, a monster telescope built by hand.
John spent much of his later life touring wherever dark skies, telescopes and people could be found. This often included national parks and regional star parties. I met John a few times across the years, at Grand Canyon Star Party and at an evening observing session at Starizona, an astronomy shop in Tucson. His signature graces the secondary cage of my 18″ f/4.5 Dobsonian, Deep Violet, beside the signature of David Levy.
John was also a proponent of a decidedly non-standard cosmology, believing that the Big-Bang model had fatal flaws. His alternate ideas make… Uh? Interesting reading. He describes a recycling steady state cosmos heavily influenced by the teachings of eastern religions and mystical thought.
John Dobson died today, 15 January 2014 at the age of 98 in Burbank, California. John leaves behind a son, many friends, and a community indebted by his contributions to amateur astronomy. My friend Dean Ketelsen knew John far better than I did, I suggest you read his notes on his passing.
I spent a few moments and put all of the photos of dobsonian telescopes that have appeared here on Darker View into a gallery. The photos are just a little sliver of what John Dobson meant to amateur astronomy…
Night of The Dobs
Star parties at Hale Pohaku are very informal affairs. You never really know who is and who is not going to show up. We just don’t plan that much.
It had just been too long, I had not had a good night out with the telescope for months. This dark moon Saturday was not going to be missed, I packed up Deep Violet and headed for The Mountain. I knew a few folks would be there, certainly Cliff and Tony. The rest of the folks were a surprise to me. As the evening progressed more and more ‘scopes showed up, a few more familiar voices in the darkness.
The only real issue was the wind, it was annoying. A couple of us moved our telescopes into the patio area of the MKVIS, where the building offered some shelter at the expense of blocking some of the sky.
Better Elevation Bearings for an Orion XT8
A telescope needs to move smoothly, without any binding or sticking. The elegantly simple bearing design used in dobsonian telescopes uses a smooth surface riding on a small pad of some low friction material. This requires a pad of some material that slides smoothly and does not stick, to provide the motion. The standard answer of telescope builders is polytetrafluoroethylene, simply referred to by the acronym PTFE or the trade name Teflon.
The price is excellent for an 8-inch telescope, but there is some sign of cost cutting in the more recent versions. One interesting difference I found in this ‘scope was the bearing material. The bearing pads that are supplied do not appear to be PTFE, but instead a much harder material, possibly HDPE instead, which would be cheaper than PTFE. replacing these with PTFE results in a nice improvement in the smoothness and ease of movement of the telescope.
This particular telescope was missing two bearing pads, so some replacement was required, whatever the material. Without access to the original parts it was necessary to obtain whatever material was available, and that would be PTFE. Since it came in a different thickness than the original bearing it was also necessary to replace all four pads.
In place of the original staples, a small flat head screw is used to secure the Teflon in place. The soft material is easy to work. As long as the tools are sharp drilling and countersinking the pieces is easy. Best use a new drill bit if you have one, or grab one of the odd sizes that never seem to be used.
Position of the bearing around the radius of the trunnion have some impact into how easily the telescope will move. Positioned closer together and closer to the bottom of the bearing and the telescope will move more easily. Positions further part and higher on the bearing sides will result in more friction as the trunnion presses outwards against the pads.
The repair worked quite nicely, with a ‘scope that move smoothly across the sky. A bit more cleaning and a few minutes collimating the optics had the small telescope again ready for the sky.
Deep Violet, an 18″ f/4.5 Dobsonian
In April 2001 I realized a dream that had been many years in the dreaming and a year in the making, a large aperture dobsonian.
- The scope had to have sufficient aperture to take advantage of the dark skies available near Tucson. I wanted to see spiral arms in galaxies.
- The design was to be visual only. No drives, but provisions for an equatorial platform at a later date.
- The mount would be a no compromise rigid structure, capable of allowing good optics to perform at their best.
- The scope had to fit through a standard doorway.
- The scope had to fit in the cargo compartment of a Ford Explorer Sport without dropping the seat for safety during transport.
- The eyepiece must not be an excessive distance above the ground, allowing use while standing on the ground much of the time. (But then, I’m 6’2″ tall)
Over a decade of engineering experience has taught me that a well defined set of specifications can make all the difference at the end of a project. With these design goals in mind the plan then progressed rapidly.
Rebuilding a 12.5″ f/5 Truss Tube Dobsonian
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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