21st Century Dobsonian Telescope Making Myths

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Last touched 2002 March 28

Originally published in Amateur Astronomy Magazine

Robert Bunge

I've been building telescopes for the past twenty years and have completed scopes that range from a 20-inch dobsonian, a brass 6-inch refractor and a series of compact travel scopes. I have built many different dobsonian telescopes, most of them to suit a specific type or style of observing I want to accomplish. I built many dobs in the late 1980s, at one time co-teaching or teaching dobsonian making classes where groups of up to a dozen students, working from plans, banged out great little 6-inch f/8 or 8-inch f/8 general purpose telescopes in three to six weeks.

Over the past few years, I've seen two or three "facts" spring up within the Amateur Telescope Making community that I believe are actually myths. Most commonly, newer ATMs, especially those who researched commercial Dobsonian telescopes followed by reading recent ATM books and elect to build a smaller scope on their own have appeared to pick up two of these myths. A third has been around a long time and has most often infected those with large truss dobsonian scopes.

Below, I'll touch on them and attempt to trace the history or source of the myth and opine why I don't think it is true.

1) You have to have use Baltic Birch plywood to build a telescope. This appears to have gotten started when a cabinetmaker introduced Dave Kriege to this stuff as he was researching the building of his original Obsession telescope. Now, if you've ever had the chance to meet Dave, you realize that he's obsessed with only the best. Baltic Birch is certainly a good, perhaps the best (well, perhaps Appleply), material for use for a wooden telescope.

But this begs an interesting question? So what exactly did they use to build dobsonians before the discovery of BB? Stone? Bricks? Solid oak? How about construction grade (in those days "AC") plywood? You just went down to the local hardware store and sorted through the stack and picked out the best piece. I made several scopes in the mid-late 1980's out of construction grade ply and they continue to work just fine many years later. Here's a good example: a 4.25-inch f/10 super-planetary reflector made from 1/4-inch AC ply (now known as SVSC-3) (http://www.ladyandtramp.com/4-inch/) - a scope that I built in 1987 and has spent large parts of it's life either outside under a porch (1988-1991) or wrapped up under a Desert Storm shield (1999 and 2001) sitting out on the deck. It was originally stained and coated with Polyurethane. After 14 years, I've recently noticed it seems like the polyurethane has disappeared so I've been considering disassembling the scope and recoating it. In the early 1990's I built a very successful 12-inch f/5.4 two-truss scope (http://www.ladyandtramp.com/ellie/) out of interior grade BC quality plywood. This wood was really cheap, and I was on a starving college student budget. I just made sure to stain and seal it real good, after all, most telescopes don't get all that wet.

My 20-inch f/6.4 semi-Obsession clone (http://www.ladyandtramp.com/tj/) is a mix: the focus cage rings are 1/2-inch BC wood. The mirror box is 3/4-inch AC. The rocker box is mostly soft-core/birch veneer from Lowes Hardware, with some AC 1/2-inch scraps thrown in for good measure. The mirror box was made 15 years ago and spent two years stored under a tarp on a porch and another year under a tarp in a backyard. Structurally the box is in fine shape. During the most recent rebuild of the scope, I sanded off what was left of the original stain and polyurethane and applied new.

In Kriege and Berry's book, they have an excellent section on the selection of plywoods for telescopes. I suspect most people have consumed the part on Baltic Birch - the first part of the section - and stopped reading. Here is a quote from Kriege/Berry from after the BB section:

"Don't sell softwood plywoods short. These plywoods are strong, inexpensive, and can be used to build nearly everything imaginable. Softwoods like Douglas Fir, for example, have a high modulus of elasticity and can provide all the strength or stiffness you need at about two thirds the weight of a typical hardwood plywood." (pg 374)

Mmm... did I read that right? If you want to build a telescope that has good strength/stiffness and is lighter than hardwood plywoods, you could use the softwood core plywoods available from the store down the street.

In some discussions with ATM's, I've heard the following: "Baltic Birch is better because it has more plys (layers) then regular hardware store plywood. Here's another quote from Kriege/Berry:

"The number of layers in a panel of a given thickness does not appreciably affect the stiffness of the panel" (pg 369).

I recently priced a 5'x5' piece of Baltic Birch from Colonial hardwood in Springfield, VA at $50 ($2 per square foot) and a 4'x8' piece of AC/exterior piece at Home Depot for about $15 ($0.50 per square foot). So common softwood plywood is about as strong, and only one half to one quarter the price. One difference? Quality; some of the hardware store stuff is warped or damaged, but often a look through a stack or a visit to another store solves that problem. Another difference? The softwood types have more "voids" - empty spaces between plys. Again, careful selection of the piece and careful planning of cuts can minimize this problem.

Using Baltic Birch to build a scope is great. It looks great, is strong, and expensive. Do you have to use Baltic Birch? No. If you are building a smaller telescope and want to save weight so it is easier to carry around, use softwood plywoods. If you are on a budget, or don't care that your telescope don't look like a piece of furniture, consider construction grade plywoods and be proud of it!

2) It ain't a good scope unless you use Ebony Star for the bearing surface material. An article started this in the now-defunct _Telescope Making_ magazine where Dave Kriege explored different materials as bearing surfaces in order to reduce friction while using large dobsonians. In the pre-Obsession/Tectron/Starmaster days, large dobs were really large, and very heavy. Richard Berry's original 20-inch f/5 had a solid plywood tube (no weight saving truss tubes) and weighed in at 400 or more pounds. The original solid sono-tube version of my f/6.4 20-inch was about 800 lbs. These scopes were almost impossible to turn on standard Teflon/Formica bearings, especially in azimuth (on my 20-inch, we used a large roller bearing that was originally from a H-53 helicopter rotor shaft). Later truss tube dobs were better, but still had azimuth stiffness problems.

Kriege set out to solve this specific problem: reduce friction for large, heavy dobsonians. Ebony Star was a great discovery (as was glass board) in this regard. My current Ebony Star equipped 20-inch borders on being too loose and blows too easily in the wind. Small dobsonians (anything less than say, 12-inches) never had this problem and have never needed to use Ebony Star as the bearing material. Any plain, smooth Formica will work just fine. Really. I've seen dozens of dobs built with it and they work great. As Kriege and Berry say in their book, pay more attention to putting the proper pounds per square inch on the Teflon in order to get that classic buttery smooth Teflon motion.

3) Truss tube scopes can't be used without a shroud. To some extent, I think the need to have a shroud is a hangover from the days "it ain't a telescope unless it's got a tube" days. My first tubeless/truss telescope, a 6-inch two-truss travel scope was built in 1983, based on a design I had seen in _Telescope Making_ magazine by St. Louis amateur Thane Bopp. Thane argued that you didn't need baffling/shroud if you observed from a perfectly dark location. I didn't observe from a perfectly dark location, so I used cardboard baffles to keep stray light down.

Later came 10-inch, 12-inch, 4-inch (f/4) and 16-inch two-truss telescopes. I finally put a shroud on the 12-inch, mostly because my standard method of baffling added obstruction to the light path and I wanted to clean up the diffraction pattern. When I finished my 20-inch, I said, "Ok, until I find a sewing machine, I'll just use these pieces of black (16"x20") cardboard and duct tape to baffle the scope." That was two years ago and as of February 2001, I was still using them. I've recently changed the focus cage design of this scope so that I'm now using a piece of Kydex as a light baffle. This design is a more permanent version of my cardboard system and also reduces the affect wind has on the telescope.

Actually, some cases, I think shrouds might hurt more than they help, for two reasons;

1) They trap heat coming out of the mirror box. To quote Astrophysics owner Roland Christian "That huge hunk of glass is the best heat radiator on the telescope field." Cloth is not a good transmitter of heat and many shrouds are made from material also designed to keep the moisture out, thus preventing the space inside the shroud from breathing. So now, all that heat has to make its way up the inside of the "tube" all the time distorting your ever so precious image. Experienced owners of shrouded dobs often roll the bottom part of the shroud up, so the heat can escape. So why have the bottom half of the shroud in the first place?

2) They turn telescopes into great big weather vanes. At the 2001 Winter Star Party, I watched several large dob owners remove their shrouds after the wind proved too strong for their azimuth friction, making the scopes unusable. Then they had to deal with loss of contrast because their scope wasn't baffled correctly. Either that, or they moved their scope to somewhere that was sheltered from the wind, often with a loss of horizon as a result. So why not make a set of Kydex or cardboard baffles that do the actual baffling without all the sail making qualities. I think this is smart TM design as opposed to the classic throwing the entire kitchen sink at the problem.

All that said, there are times a shroud is useful, perhaps the most apparent if you are using the scope where there are a lot of unshielded streetlights around. About the only time I wished I had made a shroud for my 20 was during an Astronomy Day observing session where we were observing from a school parking lot while the lights were turned on (don't ask!) and glare was everywhere.

These are three issues I see all the time in the current world of telescope making. I believe they are myths - just like the days when it was thought all mirrors had to have a 6-1-thickness ratio and all telescopes had to be equatorially mounted or they weren't telescopes. The first two myths are carry over from ATMs misapplying technology meant for large telescopes to small or buying into a popular craze. Shrouds? It is commonly justified for baffling and contrast, but deep down, I suspect many people just like the look of the shrouded dobsonian, or that's just what everyone else is doing.

It is almost always better to use smart design and the proper materials for a job and price range. When I hear ATMs discuss with ATM wannabes that "you have to use..." and the best looking and most expensive materials are quoted, it worries me. These newbies might be on a budget and scared off because of the high cost of Baltic Birch or the trouble/cost of hunting down Ebony Star, when other, cheaper materials are just as good and have a track record for doing the job just fine.

Bob Bunge
Northern Virginia Astronomy Club
Bowie, MD

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