17.5" f/4.5 Lightweight Portable Dobsonian

Albert Highe

I’ve been using my 12.5" f/5 ultralight dobsonian (the smaller scope in the background in figure 1) for two years. It is a minimalist design, using three parallel, cylindrical struts. Its total weight is approximately 50 lbs. The heaviest component, the mirror box, including the primary mirror and cell, weighs just 18 lbs. It is a joy to transport and use. It sets up quickly and delivers wonderful, high-contrast views. Because it is so easy to transport, I’ve spent far more hours under dark, remote skies than ever before. I was very pleased when it received a Merit Award at the Riverside Telescope Makers Conference 2000.

Figure 1 - The author seen behind his 17.5" scope. The 12.5" scope is off to the right.

Like many observers, I developed aperture fever and wanted a larger scope. Initially, I didn’t think the three parallel strut design would scale up to a larger scope. However, at the urging of a friend, I investigated the issues of scale-up more seriously. Also, after I built the 12.5", I developed some new design ideas that I was anxious to incorporate into a new scope. My final motivation was the acquisition of a good 17.5" f/4.5 Coulter mirror. So I forged ahead with scale up of the design to house the 17.5" mirror and to incorporate a few new features.

Over the past few years, I’ve built quite a few scopes using parallel struts. I am intrigued by the following advantages:

1. Altitude bearings attached to the parallel struts allow easy and adjustable positioning of the balance point.
2. Finders can be attached anywhere along the strut and maintain collimation when reattached or repositioned.
3. In particular, the finder can be used as an adjustable counterweight, its position changed to accommodate eyepieces of varying weight.
4. Different length struts can accommodate mirrors with different focal lengths.

For larger scopes, I prefer the stability and symmetry of three struts. Some of my earlier scopes used C-channels. Their flat surfaces make them particularly easy to cut and drill. However, I finally arrived at making a scope using three parallel cylindrical struts. I find their shape integrates well with the other cylindrical elements in a telescope, and I felt it would be straightforward to make clamps to hold cylindrical tubes so that no tools would be required for assembly.

From my experience building the 12.5", I judged that the larger scope would require struts with a diameter of approximately 2". I quickly realized that, during transport, three 2" diameter struts would be quite bulky. Instead, I decided to use struts with different diameters. There are a number of advantages:

1. Only one 2" strut (with the other two placed inside) takes up less space, and I don’t have three struts rolling around in the back of my van. I put some plastic caps on the ends and everything is secured for transport. 
2. The same struts go in the same place each time. If they have slightly different lengths, there is no effect on collimation.
3. The three tubes vibrate at different frequencies. Since they don’t resonant with one another, vibration damping time could be shorter.

However, there are some potential disadvantages:

1. A structure using three struts with different diameters is not as stiff as one that using three struts with the same (largest) diameter.
2. If a tube is dinged, it is possible the tubes may no longer fit within each other. If the outer tube is dinged when the others are nested within it, I may not be able to get them apart.
3. If dirt gets in between the tubes, it may be difficult to get them apart.
4. Unless the tubes have widely spaced diameters, the wall thickness must be rather thin.

Given the variety of tradeoffs, I selected struts with OD’s of 1-3/4", 1-7/8", and 2" and with a wall thickness of 0.049". I could have used a sequence of larger struts, but larger struts would require the scope to be larger and weigh more. So, I decided that I would rather sacrifice some stiffness in order to keep the scope smaller and lighter.

Wooden clamps attach the struts to the mirror box, the altitude bearings, and upper cage assembly. These are the most time-consuming parts to make. I start by laminating strips of ½" Finnish Birch plywood together to make 1" thick stock from which I cut forms that are used to make each size of blocks (figure 2). For the side clamps, I cut along the grain to make a form with a width of 3-1/4". For the top clamps, I cut across the grain to make a form with a width of 4-1/2".

Figure 2 - 1" thick forms used to make the clamp blocks. The top two forms are used to make 1-7/8" and 2" side clamps, respectively. The lower form is used to make the 1-3/4" top clamps.

I use a template to draw outlines of the clamps and mark the location of the holes. It is important to draw sets of clamps that are mirror images. It is also a good idea to make a couple of extra clamps. The diameter of the holes for the 1-3/4", 1-7/8" and 2" struts are drilled 0.025" larger than the struts.  I sand and finish each form before I cut individual clamps.

I cut individual blocks from the side clamp form with a table saw. I cut top clamps differently. First, one corner is removed with the table saw. Then I cut an arc matching the mirror enclosure tube surface using a router to separate that block from the form. Afterwards, the holes for the ¼"-28 bolts are drilled perpendicular to the strut holes. I also counterbore these holes from the bottom almost halfway through the block so the bolt heads are recessed.

Once the clamps have been cut into blocks, I cut some of the excess off the corners that become the rounded curved spring section of each clamp. I then use a belt sander, turned on its side, to sand smooth curves 3/8" thick.

I locate the side clamps on the rings and trace out the curve where each meets the telescope "tube". I trim the corner with my table saw, and then finish shaping the inside curves with a drum sander mounted in my drill press. The final shape is illustrated in Figure 3.  The articulating section of each clamp must move freely when they are attached to the rings. Consequently, I shave approximately 1/16" off of the inside face of the articulating section (figure 4) of the side clamps that are installed on the mirror enclosure and all three clamps on the upper ring assembly. This allows each clamp to move freely after it is attached. Cutting open each clamp with the table saw is the last step.

Once I’ve made all the clamps, I glue and screw each clamp to the already-sanded rings, ensuring that each is properly aligned with the clamps on the other rings.

The upper ring assembly is cut from 5/8" Finnish Birch plywood. It has the same shape as the end rings on the primary mirror enclosure (figure 5), but its diameter is 23". The ID of the opening is the same size as the OD of the Ebony Star tube, 19-1/2".

Figure 5 - The wooden rings for the upper ring assembly (center) and the mirror enclosure (left and right), and the Ebony Star tube.

Figure 6 - The upper ring assembly showing the focuser shelf, spider brackets, and Kydex baffle.

The finished upper ring assembly is shown in Figure 6. The focuser is a FeatherTouch available from Starlight Instruments.  I attached the focuser to the upper ring using a aluminum angle bracket that I cut and shaped from a piece of large aluminum channel (figure 7). 

Its finished dimensions are approximately 1-1/2" x 4-7/8"L x 3-5/8"W and ¼" thick. I drilled and tapped all holes in the aluminum so that I could attach the focuser to the bracket and attach the bracket to the ring without using any nuts.

In order to keep the focuser bracket as short as possible, I attached the spider above the plane of the ring and mounted the secondary mirror as close to the plane of the ring as possible. I made the brackets that hold the spider from 1-1/4"X1-1/4"X1/8" thick aluminum angle (figure 8). The brackets are ½" W X 1"HX1-3/16"L. They are held by 8-32 screws that pass through the upper ring and screw into T-nuts, inserted into recesses drilled into the underside of the ring.

Figure 8 - The spider brackets are cut from aluminum channel before drilling and shaping.

The focuser is positioned 7.5 degrees from vertical. The eyepiece is 76" off the ground at Zenith.

I cut the baffle opposite the focuser out of Kydex plastic. A baffle made from a flat piece of Kydex will tend to deflect into the light path. So, prior to cutting the baffle, I molded a strip of Kydex to have the same curvature as the internal circle of the upper ring. I sandwiched the Kydex between two strips of laminate and rolled the strips into a cylinder with the diameter of the inside of the plywood ring. After 15-20 minutes in a convection oven at 200 ° F, the Kydex takes on a permanent set. This resulting baffle behaves as if it were cut from a cylinder. It shows little tendency to sag or deflect into the light path. Two pairs of mating strips of Velcro on ring and baffle, 1-1/2" long, permit easy installation and removal. The complete upper ring assembly, with baffle, weighs under 7 lbs.

I also mounted the secondary mirror as close to the plane of the ring as possible. I used a 2.6" Protostar diagonal holder to support the 3.1" diagonal. The undersized diagonal holder is smaller and lighter than a 3.1" diagonal holder. In addition, I can introduce the proper amount of offset by attaching the mirror off-center using silicone sealant. In order to reduce reflections, I painted the side of the diagonal with flat black paint. 

The enclosure housing the primary mirror is essentially a fiberglass tube with wooden end rings (figure 9). The spacing of the three struts is nearly 120º. The centers of the two outer struts are 6.475" below the center of the mirror enclosure. 

Figure 9 - Bottom view of the mirror enclosure, showing the three pairs of collimation screws and the support feet.

Figure 10 - Image showing the altitude bearings flush against the side supports of the mirror enclosure. The black knob is a 1/4"-20 bolt passing through the bearing and screwing into a T-nut inserted behind the side support.

The sides of the mirror enclosure top and bottom rings are straight and parallel from the halfway point down to where the struts are clamped. In the 12.5" scope, the edge of the mirror enclosure top ring provides a stop for the altitude bearings, preventing them from rotating inward. The 17.5" scope has a lower balance point relative to the size of the mirror enclosure and altitude bearing diameter. Consequently, the bearings no longer align with the edge of the mirror enclosure top ring. I added side supports to each side of the mirror enclosure (figure 10). To make the scope stiffer, each bearing is attached to a side support using ¼"-20 screws that pass through the bearings and screwed into T-nuts, inserted into recesses drilled into the inside of the side supports.

I cut the top and bottom tube rings 22" wide from 1/2" Finnish Birch plywood. On the inside surface of each ring, I used a router to cut a circular groove, 3/16" wide and 1/8" deep, with a diameter the same as the fiberglass tube (figure 11). The top ring has an internal diameter of 18-5/8",  large enough to insert the mirror through it. Because the lower ring supports the mirror cell, I cut a smaller opening in the bottom ring to permit airflow through the mirror box. After all the holes are drilled and the clamps are attached, I epoxy the end rings to the fiberglass tube. It takes less time to apply finish to the rings before they are joined together with with Ebony Star tube.

Figure 11 - Top and bottom rings showing routed grooves that match the diameter of the Ebony Star tube.

Ebony Star laminate is a tough and resilient surface. After all, that’s why they use it for countertops. I make a form (figure 12) with the same OD as the groove routed in the wooden end rings. I cut the laminate 8-1/4" wide and 66" long. This length allows at least 4" of overlap when I roll it into a tube. After roughening the surface of the laminate in the overlap region with sandpaper, I glued the tube together with epoxy. After allowing the overlap region to cure overnight, I epoxied a fiberglass mat onto the interior surface. The result is a fiberglass tube with an Ebony Star laminate surface.

Figure 12 - The Ebony Star tube was formed and fiberglassed while held by the plywood form.

The mirror enclosure without optics or mirror cell weighs 9 lbs.

Note the bottom of the mirror enclosure in Figure 9. In addition to the mirror cell adjustment screws, there are three feet that prevent the scope from resting on the screws should I ever set the scope on the ground. Rubber feet slide over 1" diameter dowel rods that are glued into holes drilled approximately ¾ of the way through the bottom ring. The three feet are longer than the adjustment screws, but short enough in their positions so they don't contact the bottom of the rocker box when the telescope is in use.

The primary mirror is the heaviest component in a reflector. The 17.5" f/4.5 Pyrex primary is 1-5/8" thick. It weighs 28.8 lbs. In order to keep the center of gravity of the OTA as low as possible, I designed the mirror cell to attach directly to the bottom of the mirror box. The distance from the bottom of the mirror to the bottom of the mirror box is approximately 1".

I used the program "PLOP" made available by David Lewis (http://www.eecg.toronto.edu/~lewis/plop/) to design an 18-point floatation cell. Many mirror cells for large mirrors are made of heavy steel. Even the commercial aluminum cells are quite bulky and support the mirror a considerable distance from the bottom of the scope. So, I designed my own mirror cell (figure 13).

Figure 13 - Front and rear views of the light weight, low profile 18 point floatation mirror cell.

It has a very low profile and weighs less than five pounds. The large triangular mirror cell frame is aluminum. I don’t have the equipment to machine such an aluminum piece. Paying someone to machine such a complex shape would have been prohibitively expensive. However, the cost of sand casting the mirror cell frame was surprisingly affordable. I talked to a local aluminum casting house and learned the important factors for building the wooden form used for sand casting. Fabricating the shape out of plywood was straightforward. I had to build the form approximately 2% larger than my intended dimensions to allow for shrinkage. In addition, in order for the form to release cleanly after pressing it into the sand, the form contains no sharp edges and the sides are tapered a few degrees.

After I picked up the casting, I cut the sides square with an aluminum-cutting carbide blade mounted in my table saw. Finally, I drilled and tapped the holes for the mounting bolts and rocker bar pivot bolts. The unfinished aluminum pieces for the mirror cell are shown in Figure 14.

Figure 14 - The unfinished aluminum pieces of the mirror cell.

Each of three rocker bars, cut from 3/8" X 1/2" aluminum, is mounted to the sides of the triangular frame and supports two aluminum triangles. The small aluminum triangles are cut from aluminum sheet 3/16" thick and 4" wide. The rocker bars pivot on stainless steel shoulder bolts with a diameter of 1/4". Each of the small triangles hold three ¾" diameter stainless steel T (or weld) nuts. Once the cell was completed, I attached the mirror to the support pads using silicone sealant. Even though the total adhesive area is approximately 3% of the mirror area, it is very firmly attached. I decided not to support the heavy mirror on springs. Instead, the cell is supported and adjusted by a set screws at each corner, arranged in a push-pull configuration. Clamp knobs pull down on the 5/16-18" bolts screwed into the aluminum cell. 1/4"-20 thumbscrews, screwed into T-nuts inserted into the bottom of the mirror enclosure, push up on the mirror cell.

The altitude bearings

I made the two altitude bearings from one Finnish Birch plywood disk that I created by first gluing a ¾" disk with a diameter of 21-1/2" to a ¼" disk with a diameter of 22". This creates a bearing form with a lip ¼" high and ¼" wide. I then drill four holes that allow me to have a convenient place to start and stop my router when cutting the the inside arcs (figure 15).

Figure 15 - The form that is used to create the two altitude bearings. Shown are the internal arcs of the two individual altitude bearings.	Figure 16 - The bearing form has been cut in half and the remaining internal material has been removed with a router.

After cutting the disk in two, I rout out the remaining material from the inside of the bearings (figure 16). Then I mark the locations of the strut clamps and drill 1/4" diameter holes to accommodate the 1/4" carriage bolts that pass through each clamp. I countersink each hole with a 5/8" Forstner bit to recess the bolt heads.

I epoxied a ¾"-wide strip of Ebony Star laminate to the bearing surface. I have found that this step is easier if I first introduce a set into the laminate by wrapping it into a circle and letting it sit for a while. The pair of  altitude bearings weighs 5.4 lb.

On the 12.5" scope, only two clamps hold each of the bearings in place and permit the scope to be balanced. In practice, I've found that the bearings are always in the same position once I've decided the location of my finder. To provide greater support for the 17.5" scope, each bearing uses two clamps plus the screw holding it to the side support. The side support screw also ensures that each bearing is placed and/or maintained in the same location. The finished bearings ride on Teflon pads screwed into the rocker box. The lip of each bearing also keeps the OTA centered in the rocker box.

I have found that the bottom of the rocker box is very important for controlling twist. So, it needs to be thicker than other elements. It has a finished thickness of 1" and is 24" square. The bottom is constructed of a sandwich of two ¼" plywood sheets around a ½" thick core. A substantial area has been removed form the ½" core to minimize weight.

Because the sides are short, and reinforced by the front and back, ¾" provides sufficient stiffness.

The eyepiece shelf within the rocker box serves two purposes: it increases stiffness and provides a place to hold eyepieces. I cut the eyepiece shelf from 1/4" Finnish Birch plywood and glued it into grooves routed into the inside of the rocker box. It has space for two 2", and two 1-1/4" eyepieces.

All joints are tongue-and-groove and glued together using Gorilla Glue. This adhesive expands as it dries, forcing itself into the grain, making a very strong bond. After it dries, it is also waterproof.

Like other scopes, Ebony Star laminate, glued to the bottom of the rocker box, rides on three Teflon pads at the ends of a triangular ground board. The rocker box pivots on a ½ " diameter stainless steel bolt that passes through a brass bushing. The rocker box weighs 28.5 lb.

Although I designed the scope to have minimal weight, I wanted to minimize how often I had to lift that weight. In addition, I wanted to be able to leave the scope set up in my garage, and be able to wheel it out for quick observing sessions. Wheelbarrow wheels and handles are typically used to move large scopes. However, those I’ve seen take too long to install and remove. In addition, the side door opening in my van is too narrow to allow wheelbarrow handles to be attached to the sides (figure 17). The solution was to attach wheels to the front of the mirror box (figure 18) and to build the attachment means for the handles into the rocker box (figure 19). I can insert the cylindrical wheelbarrow handles (aluminum tubes) into the holes in the lower corners of the rocker box, wheel the scope outside, and remove the handles in 10 seconds! No clamps or screws are needed. Once the struts have been inserted, applying inward pressure at the ends of the tubes holds them in place. Furthermore, the two wheelbarrow handles have different diameters and both can be nested within the struts during transport.

Figure 17 - Rolling the scope into the van is a tight fit. The means to hold the wheelbarrow handles (the aluminum tubes) is built into the rocker box.

Figure 18 - The wheelbarrow wheels are normally left on the permanent aluminum brackets.

Like the focuser shelf, I made the brackets that hold the wheels from aluminum channel (figure 20). I use the aluminum cutting carbide blade in my mitre saw to make the 45° and 90°cuts. I mill down the small raised section and then drill and tap the holes. 6" diameter wheels ride on 1/2" bolts that serve as axles. Each bracket is attached to the front of the rocker box with a pair of 1/4"-20 screws that screw into T-nuts inserted on the inside of the rocker box. When the scope is in use the wheels can usually be left on since there is 1/2" of clearance between them and the ground. On rough terrain, I often have to remove the wheels. The clamp knobs permit rapid removal in this case.

Figure 20 - Rough cut parts from aluminum channel: focuser shelf on the left and wheelbarrow wheel brackets on the right.

When observing, I place a second Kydex baffle over the top of the primary mirror enclosure (figure 21). This provides a light baffle for the primary mirror and minimizes reflections from the top of the mirror box, improving contrast.

It is important to me to be comfortably seated during long observing sessions. I have more stamina and can see more. In particular I like to have my star charts on one side of me and my finder on the other. So the 80mm right angle finder is positioned at eye level while I’m seated.

I made a mounting bracket out of aluminum C-channel that attaches to the top strut using an aluminum "U" bolt (figure 21). The C-channel aligns itself with the cylindrical strut, maintaining alignment upon re-assembly. Also, the finder makes a good adjustable counterweight. When using heavy eyepieces, I can simply loosen the wingnuts and slide the finder down a couple inches. This does not interfere with its alignment.

Overall, I am thrilled with the performance of my new scope.  I can see the laser spot move a bit on the inside of the collimator face when I move the scope from 0 to 90 degrees. However, this doesn't seem to affect the image during viewing. And I very much enjoy the brighter views vs. my 12.5" scope.

The telescope cools down quickly and I see little or no tube currents. Surprisingly, the open structure and exposed optics resist dew formation. Although, I must confess that most of my observing is in rather dry conditions. 

I can assemble or disassemble the telescope in the dark using no tools within 15 minutes.  I designed the scope so that the heaviest component, the mirror enclosure with primary, weighs only 45lbs. However, in practice, I never lift anything heavier than 10lbs. The scope also maintains collimation well after re-assembly. I only need to tweak the adjustment screws. 

The disassembled scope is compact (figure 17) and is easily transported. Short lengths of each diameter aluminum tube substitute for the long struts and securely support the upper ring. The three long struts and the two wheelbarrow handles nest within one another during transport (figure 22). Each end is protected with 2" diameter plastic caps.

Figure 22 - The three struts and two wheelbarrow handles nest within one another during transport.

The built in wheelbarrow wheels and quick-install wheelbarrow handles performed well beyond my expectations. I can insert the handles, move the scope, and remove the handles within seconds. There is plenty of friction to hold the handles in place when I apply inward pressure on them. This is a nice feature for any moderate to large scope

In addition, the nesting struts is an approach that would work out particularly well for a travel scope. All the struts could fit in a relatively small diameter cardboard tube.

I received numerous requests from friends, people I meet at star parties, and people who stumble across my website, to make them a scope or to send them plans so they can use or copy my designs. Since I received so many requests, I built a limited number of the 18" ultralight dobsonian (the Mark III) telescopes. I am no longer building these. However, If you think you would like to buy one, please contact Custom Telescopes by Plettstone. I am not planning to build any of my other designs at this time.

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Copyright © 2002-2006 by Albert Highe, unless otherwise noted. All rights reserved.
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