One approach to the dobsonian telescope.

Designing a telescope can be a game of give and take. What performance features do you want? What cost, materiel or technology limitations are you under? What compromises are you willing to make? Decisions, decisions....

Here are some photos that describe my sixteen inch, f/6, lightweight dobsonian. Some photos date from 1998 shortly after it was first built. Others are more recent and show evolution and improvement of design. Some of the ideas here may be useful to those planning to build their next telescope.

First, a great resource for building your own large dob is The Dobsonian Telescope. If you want to start out with simpler, smaller scopes then check out this book by Richard Berry. I have both and I have referred to them repeatedly since purchasing them.

Overview photo of my scope. I chose to build an upper ring to reduce the wind profile because at the time I was living in east central New Mexico - a treeless, windy plain. Lots of clear nights, but wind would often spoil observing opportunities if you did not take steps to mitigate it. (Behind the scope is an observing ladder with extra steps added between the original ones. I find this makes it easy to stand on a ladder without the need to hunch over or go on tiptoes to reach the eyepiece. On top of the ladder is one of the earliest GPS models the military used...circa 1993, if I remember correctly.)

Another point about observing ladders. Don't be afraid to use them, even when the eyepiece is high enough to require a ladder. Why? You can use the ladder to brace your upper body when you are standing at the eyepiece. If your scope is motorized, the seeing is good, and you are using high magnification...if you are standing at the eyepiece (with no ladder to support you) you may find your body (and eyeball) swaying, spoiling the view. Put the ladder next to you, grab it in some fashion, and brace your body with it. Don't just grab the ladder at arm's length. Bring the ladder as close as possible to you. Brace your body up as high as possible with the ladder. There have been times I have placed the ladder besides my chest and cheek (off to one side of me, not in front of me), grabbed the ladder with my far hand (so that my arm crosses my upper chest/neck area)...and used this arm as a 'chin rest' to hold my eye very steadily at the eyepiece. (In cases when I've observed objects at lower elevation, with shorter telescopes...I've sat in a chair, propped my elbows on my legs, and propped my chin with my arms. Same effect...your eye is held in proper location, with minimal effort, allowing better, longer, more productive, more relaxed observing. If you don't have a ladder, any broom stick or other brace will do.)

I have taken other steps to reduce my scope's vulnerability to wind.

- I cut away the central portion of my spider vanes, so that only the outer edges remain. It is almost as if I have a wire spider that uses broad flat wires. Less surface area is present to flutter in a breeze. (A wire spider would be even better to combat the wind.)

- I placed damping material around the ends of the four spider vanes (and outside the light path of the main mirror). Take your spider and pluck one of the vanes...listen to it 'twang'...it's not a well damped mechanical setup, is it? Once it starts vibrating, from wind, or drive motors, it will keep vibrating for some time. One source of damping material is adhesive putty - the kind used to mount posters to walls without tearing away the wallpaper. You can find it in stationery departments of stores like Wal-Mart. It is easy to apply, and easy to remove if you change your mind.

- I placed a loose, limp section of clothesline inside each truss tube. (This is a technique amateur radio operators use to keep their antenna rigs from fluttering in the breeze so much and failing early from metal fatigue that repeated flexing produces). On breezy nights, if I put my hand gently on the telescope's truss tubes...I'd feel them vibrate and flutter in a stiff breeze. Because each of my truss tubes has a wood plug in each end it was easy to add a screw eye to the inner end of each plug and connect a limp rope.

- I placed a baggy cloth jacket around each truss tube. I used felt, and my wife sewed up some oversize sleeves the length of the truss tubes. The key here is to use a limp material, such as felt, and more importantly, to loosely couple the cloth to the tube. In other words, as the tube vibrates, it needs to bump and 'mush' against some of the folds of the cloth. If the cloth is too tightly coupled to the tube, then the entire unit will move as one solid object and the cloth will not damp the vibrations of the tube. How do you determine the proper amount of 'loose coupling' for your baggy cloth jacket? Hang one of your truss tubes from a rafter or door frame with fishing line or dental floss. (Give the middle of the tube a bump and see how long it vibrates...a very minimally damped condition.) Put the cloth jacket over the tube, and tape the top end to the tube. Twist the jacket so that it begins to tighten up around the tube. Bump the tube and see how fast vibrations die out this time. Experiment with the jacket twisted different amounts to find the optimum 'coupling' parameter.

- Good grief, is there anything else I can do to combat vibration?! Yes. My secondary mirror (and possibly yours also) vibrates in another way I have not yet mentioned. It can oscillate about the cylindrical axis of the secondary holder. The spider vanes act as springs to keep the oscillation going, and there is little damping action taking place. (If you use fans or drive motors on your scope you may have seen this problem...your star images become elongated in one direction...caused by this oscillating rotation of the secondary holder.) What can be done? Rebuilding the spider to an 'offset' vane design will help reduce this problem. (An offset vane design does not have the vanes in a '+' configuration, but instead a '>O<' configuration.) Another method (which I have not yet tried) is to attach a small bottle of oil and BB's to the spider's hub (and out of the light path of the main mirror). This oil/BB filled bottle would act as a viscous damper. This approach could also work on truss tubes...strap oil/BB bottles to the middle of your truss tubes instead of using baggy cloth jackets.

Overview of the upper ring. (Old photo, before spider vanes were modified.) It is a laminate of two 1/2 inch plywood rings for a total thickness of 1 inch. If I were to do it again I'd use 1/2 inch plywood to further reduce weight.1/2 inch is stiff enough for this size ring, with perhaps some additional bracing near the focuser. The focuser is a low profile model, but it is not in a low profile position. Why? Two reasons. First, a truly low profile focuser is more difficult to baffle well against stray light. You would need a very large baffle disk opposite the secondary mirror. Secondly, such a large baffle disk in a windy climate was unacceptable for me.

More discussion about baffling the upper half. Here is a view looking along the 'baffle train.' (Image overexposed to show black surfaces better.) If you look through the focuser you can see the secondary mirror, but do you also have a direct view of things next to the secondary, or behind it? Thanks to an aperture baffle below the focuser, stray light from a very oblique angle can not reach the top of the focuser. (However, things just to the side of the secondary are not blocked by this baffle.) Thanks to the baffle disk behind the secondary, no stray light from behind the secondary can reach the top of the focuser. I used flocking paper, and in daylight I have good enough contrast to observe all five bright planets when the sun is above the horizon.

Here is a view of the 'baffle train' from a different angle, clearly showing the aperture baffle several inches below the focuser. (Image overexposed to show black surfaces better.) The focuser aperture baffle is approx. 1 1/4 inches diameter. This produces mild, unnoticeable (to me at least) vignetting when I use a 20mm Nagler eyepiece. With higher magnification eyepieces (smaller field stop) vignetting is not an issue. Because of the small aperture baffle, my baffle disk behind the secondary mirror is only 8 inches in diameter. This presents a small wind profile. (Also note flocking paper placed on secondary mirror holder, which is darker than some of the black painted surfaces. Black velvet would be an even darker material.)

Here is a look at the bottom half of the telescope. Note that I am using virtual spring counterweights. This was detailed in the Nov '99 issue of Sky and Telescope magazine, pg. 131. Also, you will see a protractor on the elevation trunnion, and a tape measure wrapped around the circular ground board. These were used as alt-az setting circles, with a 386 laptop converting RA/Dec to alt/az in real time. (If anyone wants a copy of this free, home made QBASIC software, please contact me.)

Here is a close-up of the elevation setting circle. It is divided into 1/2 degree intervals, and it is easy to interpolate to 1/4 degree. It's merely a protractor from a drafting supply house.

Here is a close up of the azimuth setting circle. It is merely a tape measure refill wrapped around the circular ground board. On this size ground board each 1/16 inch division represents 1/3 degree, and it is easy to interpolate to 1/6 degree (10 arcminutes). If I were to do this again I'd find a metric scale, because I can think in tenths better than I think in sixteenths when using the setting circles at 3AM.

Why use setting circles these days? It was far less expensive than adding encoders, encoder reader, and other electronics, plus I had an old (and otherwise obsolete) laptop laying around. Pointing is accurate enough that I did not need a finder scope after the scope was aligned on a known object...I could put an object in a low power field of view with the setting circles alone. In daylight setting circles also come in handy. I've been able to find and observe Mercury, Venus, Mars, Jupiter and Saturn when the sun is above the horizon, or in very bright twilight. I did not do this as a circus stunt. Frequently the seeing is best just before or shortly after sunset, and gets worse as temperature drops through the night.

One more helpful hint. Make a close fitting mirror cover that fits over your main mirror like a jar lid. Why? The smaller the volume of air that you enclose over the face of your mirror...the slower the rate of dust accumulation. I made my cover from a disk of plywood and a lip/short cylinder of scrap formica. This cover is about 3/16 inch larger diameter than the mirror for a reason...I then attached a length of 'V' shaped plastic weather stripping to the inside of the mirror cover's lip. This 'V' shaped weather stripping gently contacts the edge/side of the mirror and does not disturb the front surface, and makes the cover nearly airtight. (Don't put this kind of cover on your mirror when it is covered in dew.) Dust accumulation has dropped significantly.

All feedback and comments are welcome, and will make this a better information resource for amateur telescope makers.

email: t-k-r-a-j-c-i-@-s-a-n-.-o-s-d-.-m-i-l (remove the dashes)

Last update: 26 Oct 2002