Designing & Building the 18 inch f/4.55 Dobsonian Telescope
by Greg Babcock

Designing the Telescope

For some time, I have been planning to build a Dobsonian Telescope 14 inches or larger. I attended many Star Parties and observed many telescopes and how their owners interacted with them. I decided to draw up a series of parameters in which my design had to meet.

1.) I required Portability. It had to fit comfortably into the Ford Explorer and still leave plenty of room for camping gear,

2.) It had to be light and manageable by one person,

3.) I required quick set up and take down.

4.) I wanted enough light baffling to not require a shroud

5.) I wanted the lines of the Telescope to be smooth and simple.

The Mirror Box

In September of 1997, I purchased an 18 inch f/4.55 Mirror from Steve Swayze of Swayze Optical and a matching mirror cell from Astro Systems. The Mirror Cell is made of 1-¼ inch Birch and was designed to attach to the Mirror Box by means of a hinge. This is to permit easy removal of the mirror for transporting. I choose to rigidly attach the Cell to the Mirror Box for strength. The Mirror remains in the Mirror box during transporting. The Mirror can be accessed and removed through the top of the Mirror Box.

The Mirror Box is built out of ¾ inch Baltic birch. The sides of the Mirror Box are also the 18 inch radius Altitude Bearings. The Altitude Bearings are lined with an Aluminum strip that sit on Teflon pads, which are mounted on the Ground Board.

To reduce weight, large cutouts in the Mirror box were replaced by Kydex pieces. Handle holes were cut into the Box and additional holes were cut in the Mirror Cell to further reduce weight and to provide ventilation for the Mirror since no cooling fans are used. The Mirror box weighs only 64 pounds with the 2 inch thick 40 pound 18 inch mirror inside of it.

The handle holes are a good example of the design philosophy. Many telescopes have metal handles attached to them, adding weight. The handle holes reduced weight, but they also permitted the Mirror Box to retain a clean look and provided additional ventilation to the mirror. The handle holes achieved multiple objectives.

Mirror Cover

The cover over the top of the Mirror consists of 2 pieces of 1/8^th inch Mahogany. The first piece has a 19 ½ inch hole cut in it for light baffling. It attaches to corner braces on the Mirror Box and is removable to permit removal of the Mirror if needed. The second piece, “the door”, is attached to the Baffle via hinges. It acts as a dust cover when closed and a dew cap when open.

The Ground Board & Stand

The ground board is two ¾ inch pieces of Baltic Birch glued together to make a 1½ inch piece of wood accept for the hub and spokes, which are ¾ inch thick. The outside of the Ground Board is slotted with the ends of the slots angled accept the Altitude Bearings. The Altitude Bearings drop below the bottom of the Ground Board to clear the ground by a mere one inch. This gets the mirror and thus the center of gravity close to the ground.

The slots hold the Mirror Box in position laterally as the telescope is moved about the azimuth. The Ground Board rides on 3 Teflon pads attached to the Stand. The feet on the ends of the Stand are 1-½ inch thick while the hub and spokes are ¾ inch thick. The diameter of the stand is only 22-1/4 inches to tuck in between the altitude bearings. Though small, it seems to do the job fine. The low center of gravity holds the telescope firmly in place on the ground.

Cage

The Cage is an “Open 2 ring” design. The rings are made of ½ inch Baltic Birch. Because of the open design, a 9 volt heater was attached to the Secondary Mirror to reduce dewing.

Weight was a major consideration in the design of the cage because the balancing point around the Altitude bearing was so close to the mirror. Large holes were cut in the Focuser Board to reduce weight. Eventually, the Telrad was replaced by the 2 ounce Orion EZ finder to further reduce weight. A baffle was placed between the Focuser and Secondary (pre baffle) with a hole of 1 3/8 inches in diameter. The entire Secondary Mirror is visible through this baffle. This not only baffles light, but also permits the Kydex “background” to be smaller and lighter. A 4 inch hole is located in the “background” directly behind the Secondary Mirror. This hole is unnoticed through the Focuser because it is blocked by the Secondary Mirror. Holes were also drilled in the Spider Vanes to reduce weight, but also to reduce diffraction by reducing the metal surface, and, thus, reducing “cool down” time. The holes in the Cage components not only reduced weight, but they also reduced wind resistance. Not to be forgotten, the lighter weight makes the Cage easier to manage during the assembly of the Telescope.

The Truss Material

The Truss Tubes were purchased from a home improvement store. The tubes are a dull anodized aluminum. The dull finish reflects very little light. They are 1 inch in diameter with a wall thickness of .055 inches.

The trusses were flattened and forked on the ends to permit them to slide over the mounting bolts that hold them in place. The Trusses overlap at the four Cage connecting points.

Construction

The Mirror Box was glued and screwed together and the corners caulked. Up to 8 coats of paint were applied to the wood pieces. The paint used was a marine grade, “Brightside” polyurethane by Interlux in “Largo Blue”. This paint is designed for the top sides of boats and is meant to be walked on. This paint was difficult to work with because it runs as it cures. You have to allow it to dry, sand out the runs, and go again. For the Parts that are flat black, the polyurethane paint was sanded rough and spray painted.

I choose to glue only the wood parts. The only exception being the heater glued to the back of the secondary mirror. Parts of 2 different material types were screwed to each other using stainless steel screws. This permits the easy replacement of parts should it be necessary. I have this hang up that different material have different rates of thermal expansion and contraction . I was concerned parts would eventually break loose from each other over time or have to be chiseled off when they needed replacement.

Final Product

There are many design features in the Telescope that are the result of consultation with Mel Bartels. The prebaffle, the Kydex background design, the light baffle in front of the Primary Mirror, and holes in the Spider vanes all came from Mel. Other credits belong to Chuck Detloff who supplied the cage parts, Brendan Fitzpatrick who provided the woodwork on the Mirror Box, Ground Board and Stand and Dan Grey who helped machine the Trusses.

The telescope met all of the design criteria. It is portable, weighs only 85 pounds, takes up little space in the vehicle, and takes one person only 6 to 7 minutes to assemble. The Telescope is in balance around the Altitude bearing when using a 27mm Panoptic Eyepiece and Paracorr without requiring counterweights. To stand back and look at the Telescope, it appears to be nothing more than a tube assembly sitting on a rather insignificant stand.

First Light: Saturday, June 6^th, 1998, 8:38pm. Moon.

First Star Party: Saturday, June 20^th, 1998 at Larch Mountain.

A little History:

I have been interested in building an ultra light / minimalist designed telescope ever since a June 1979 Sky & Telescope article in which a Canadian Telescope builder, Michael Taylor, built a 15 inch truss tube, fork mounted telescope. It was so compact that when broken down into components that could fit into the trunk of his small car. At the time the design was a trend breaker. Many of us were still locked into the massive German Equatorially mounted telescopes with full length closed tubes that required a truck to transport if it were more than 10 inches in aperture. I felt very strongly at the time of this article, that this design was the future of telescopes.

When John Dobson built that familiar sauna tube / wooden box design, it too was quite extraordinary for the time. It was extraordinary in that a 24 inch telescope was built strictly for visual use and it could be transported, even if in a dilapidated school bus. It raised the aperture limits for the amateur astronomer.

Though the Dobsonian have evolved a lot since the first giants of the San Francisco Sidewalk Astronomers many are still bulky, heavy and cumbersome. So it was refreshing to see Mel Bartels 20 inch telescope for the first time in 1997. It immediately reminded me of Michael Taylors telescope and helped me develop my telescope design.

Essentially, the Dobsonian is a telescope of compromise. It is a visual telescope only, unless it is motorized, and most are not. We have given up the equatorial mount in favor of aperture, portability and saving money. Because the Dobsonian is a telescope of compromise, it has to provide those other advantages in a big way. For me portability was paramount. Building a minimalist design was essential thing to do, not the trendy thing to do. The design I went with had to meet my 5 listed criteria or it wouldn’t get used.

Frequently asked questions:

Are you going to put a shroud on this telescope?

The answer is no. Shrouds have a few qualities, but they act like sails in a breeze and they tend to collect dew, which may drip on to the mirror. If they collect dew, they can shift the balance of the telescope. I’ve seen many shroud covered telescopes collect heat during the day. The build up of heat during the day and the shroud blocking air flow at night, means it takes that much longer for the mirror to cool off come observing time. The final reason for leaving the shroud off is that it is aesthetically unpleasing in appearance. With all considered, it is an extra piece of equipment I prefer to leave off.

Is there a problem with Dampening?

A compromise in the design was making the stand small enough to tuck between the altitude bearings, which is also my mirror box. This was done so that the telescope could be placed low to the ground. Stands on rocker box equipped telescopes can be slightly larger because the rocker box is larger in order for it to accommodate the mirror box. My telescope does not have a rocker box. My mirror box sits directly on the ground board.

Though the stand is small, the only time I have noticed dampening problems was at Table Mountain when the telescope sat on huge clumps of grass. Those who have been at Table Mountain understand that you don't just pick up and move. Even so, dampening was not unacceptable under regular use. I have had many opportunities to observe at 600 power and have not encountered dampening problems any worse than most other scopes I have looked through.

Isn’t there a lot of vibration from wind being that the Telescope only weighs 85 pounds?

No. It isn’t lack of weight, it is surface area that creates this problem. Examples are tall mirror boxes, shrouds, and enclosed cages. One thing considered while designing the telescope was that a heavy cage with a lot of Kydex surface area, located way out at the end of the telescope, would translate into a lot of leverage and thus vibration and longer dampening times should a wind come up or the telescope get bumped.

Further, never mind the inconvenience of owning a heavy telescope, consider that a large profile, heavy telescope with a tall mirror box, must be designed to not only carry the optics of the telescope, but it must also support its own weight.

As the weight increases, gravity will attempt to pull, twist and deform tall mirror boxes and tall thin walled rocker boxes. Mass creates momentum when a vibration event such as a bump or breeze has occurred. The momentum from the telescope’s weight / design combination, may cause the dampening to actually take longer to dissipate.

Why Aluminum lined Altitude Bearings?

This is a moving part that carries a lot of weight. Moving parts wear out. Bearings rotating on skids (Teflon) are being polished and slowly worn down. I like the Aluminum bearings because it is durable, easy to replace, but they must be cleaned and waxed on occasion. They seem to be quite smooth, but not as smooth as ebony star. Because the aluminum strips are slightly wider than the ¾ plywood that they are attached to, they acts as spacers and guides as the telescope is rotated around the Altitude Axis instead of the sides of the painted wooden bearings.

Updates:

Retrofits during the "off season" of 1998.

Sorbothan Pads

Sorbothane Pads were placed on the feet of the stand to further reduce vibration and dampening time. Testing their effectiveness at the last Oregon Star Party. The 18 inch telescope, with pads in place, was placed on an outdoor carpet as is customary. The carpet rests on dirt of course. When the telescope was thumped, there was no detectable dampening time. The only movement made was a result of the thumping its self. This compares to approximately a 1.5 second dampening time with out the pads. There was concern that the outdoor carpet would negate, somewhat, the effectiveness of the pads. Apparently this was not the case.

Removed Side Panels

It was found in a comparative test with Mel Bartels' 20 inch Telescope, that more airflow improved image quality.

Cage "Background"

Additional holes were cut in the "Cage Focuser Background" to further lighten the cage and to further reduce wind resistance. The pieces cut away were not necessary to block stay light. A note on wind. I have yet to be affected by it while viewing.

Replaced the Telrad

Replace the Telrad with an Orion E-Z Finder. This was difficult because I like the Telrad, but this move reduced the weight of the cage by 1.5 pounds.

In Summary...

... the telescope is a little lighter and more stable. Cheap high technology to achieve stability, not brute weight.

More on Portability

In traveling to the last Oregon Star Party, I brought a passenger, an oversized awning, a table, 2 chairs, and other creature comforts and provisions to last us for 4 days plus. The telescope rode in it's customary position behind the driver seat of the Ford Explorer. There was no need to tow a trailer, or to pile luggage on the roof rack. There even remained a 360 degree view from with in the cabin of the vehicle during our travels. This is what a minimalist design permits.

On Tour

Since it's completion the telescope has performed very well at many Star Parties this summer. In addition, it has been on display at several events.

from most recent, back:

Was guest speaker at the Friend's of Galileo Astronomy Club of Southwest Washington in Longview, Washington's Astronomical Society, April 26th, 2000 meeting.

Was guest speaker at the Battle Point Astronomical Association in Bainbridge Island, Washington, May 12, 1999 meeting.

Was on display at the Rose City Astronomer's April 1999 general meeting. At this meeting, Richard Berry and Mel Bartels spoke on the Amateur Telescope, past present and future

It was on display at Telescope Optics Workshop in Bellingham, Washington, March 1999. Spoke on the design.

It was one of 5 telescopes featured at the August 1998 Oregon Star Party "Walk About". This summer, I have had an opportunity to address 300 people regarding the design.

It won the award for most innovative design at the July 1998 Table Mountain Star Party at Table Mountain Washington.