Dobsonian Mount for 4.5” Newtonian Reflectors

From SkyInsight

---

Send this article to a friend

Copyright © 2005, Michael Hackney

Contents 1 Background 2 Building the Dobsonian Mount 3 Materials 4 Cutting and Assembly 4.1 Tube Box 4.2 Rocker Box 4.3 Foot / Rotating Platform 5 Final Assembly and Finishing 6 Enhancements 7 The Last Word

Background

Several weeks ago I attended a “Star Night�? with my youngest son’s 3rd grade class. I begrudgingly pulled out our Meade DS-114 4.5�? Newtonian Reflector and headed to the school. I say “begrudgingly�? because for the past several years I have not used the scope much - primarily because it is so difficult to set up. After spending several years struggling off and on with the German Equatorial mount, polar alignment, and “Go To�? computer control, I just stopped using the scope. Going to “all manual�? mode was also a pain because the manual movement of an EQ mount is non-intuitive. My kids would lose interest or “freeze to death�? before I could get things set up and ready to observe. So there it sat in the corner of my study gathering dust - until this particular Star Night.

During the night, I had the usual struggle with my scope. The science teacher running the event, Mr. Chaple, (who also writes a monthly column for Astronomy magazine) was there with his large Newtonian Reflector and a large crowd of kids and parents. It was dark enough that we could see the Andromeda Galaxy (M31) overhead in the cold November sky. I watched in amazement as he simply grabbed the scope and swung it into position to view the Andromeda Galaxy – no computers, no motors, and no batteries! When I got my turn to look, I was even more amazed at how stable the image was – virtually no vibration from the scope – even while I refocused. But the best part was when I tweaked the scope to recenter the image. The scope actually moved where I wanted it to, without resistance!

When the crowd thinned down later in the evening, I spent some time looking at the scope and mount and talking to Mr. Chaple. He told me the mount was a Dobsonian and explained how it worked. I was surprised at first to discover that such an experienced amateur astronomer was not using state of the art electronics on his scope. When asked about this, he simply replied, “This is much easier and faster.�? He got that right!

When I got home I started Googling “Dobsonian Mount�?. There is a wealth of information on the Web about them. I learned how John Dobson basically brought amateur astronomy “to the people�? with his simple but highly effective homebuilt telescope and mount that now bears his name. The Dobsonian Mount is a type of Altitude-Azimuth (Alt-Az) mount – a fancy way of saying that it swings up and down and left and right – very simple and intuitive! It is also very easy to set up and allows the scope and mount to be disassembled and reassembled in seconds, making transportation easier. The best part, however, is the Dobsonian Mount is very simple and inexpensive to construct. The only negative I discovered was that because these mounts are not equatorially aligned, astrophotography is more difficult. But even that limitation can be addressed with an equatorial platform or computer control.

So, armed with this information and photographs, descriptions and many plans published on the Web, I set off to build my own Dobsonian Mount.

Building the Dobsonian Mount

The basic Dobsonian Mount has 3 main components, described next from the ground up (see Figure 1 below):

1. A Rotating Platform, or “foot�? (the Azimuth bearing)
2. A Rocker Box that sits on the foot and cradles the tube box described next
3. A Tube Box fitted with circular side support bearings (the Altitude bearings) that holds the scope’s OTA and sits in cradles in the rocker box

All of these components can be made from 3/4�? plywood. The only “tricky�? part is the circular side bearings – but there are several options for these. The basic idea is that the bearings should be low enough friction to allow moving the telescope easily but have enough friction to hold it in place once the scope is in position. John Dobson highly recommends Teflon pads sliding against plastic laminate (Formica or similar). When I first read this, my engineering mind immediately thought, “Ah ha, I can use Lazy Susan bearings for the rotating platform and altitude bearings�?. A quick Google search turned up some references to folks using these, and a rather funny story about John Dobson himself saying “don’t do it�?. I decided to follow conventional wisdom and take the low tech approach. I am glad I did.

I started by measuring the length, balance point (with an eyepiece mounted), and diameter of my OTA. They were:

• length: 34�?
• balance point: 16�? from front
• diameter: 5�?

These measurements were used to develop the following design and should work for all commercial 4.5�? Newtonian Reflector telescopes.

Materials

I used 3/4�? HDO – basically plywood with a smooth phenolic resin impregnated fiber surface (it looks like brown paper) for the structural components. I used this material because it is more water resistant than normal plywood, has a smooth surface that paints nicely, and I had a sheet of it lying around! Use whatever you have. I also had a small piece of 1/2�? HDO that I used to make the bearing wheels. 2 thicknesses of 3/4�? would work just as well.

• 7 – 3/4�? round Teflon floor glides. These came 8 to a package. They have a plastic base that is tacked down with a small nail and the Teflon cap snaps in place. These are the bearing surfaces that the tube box rides against.
• 2 – 1/2�? x 2�? rectangular Teflon floor guides. These came 4 to a package and are self-stick. I had to peal the self-stick material off so they were not too thick. These were used to keep the tube box centered in the rocker box and are optional.
• 3 - adjustable leveling “feet�?. I had these laying around too. You can use anything that elevates the rotating platform about 1 1/2�? off the ground. Rubber furniture feet would be my first choice. I saw one plan that used 3 hockey pucks!
• A box of 1 1/4�? #8 wood screws
• 1- 3�? x 1/4�? carriage bolt
• 2 – washers for bolt
• 2- 1/4�? nuts for bolt
• 1 – 2’ x 4’ piece of plastic laminate (Formica or similar). Find some with a pebbly surface – this works better than a glossy smooth surface from what I read.
• Wood glue
• Epoxy
• Contact cement for the plastic laminate
• 1 can – grey spray primer
• 2 cans - flat black spray paint (Krylon works well)

Cutting and Assembly

I cut and assembled the Dobsonian Mount first and then spray painted it flat black after everything was complete. You could also paint the assemblies as you go. I was designing on the fly so painting at the end made the most sense.

Tube Box

I started by making the Tube Box and bearings for the OTA. I measured the diameter of the OTA and simply added 1 1/2�? (2 thicknesses of 3/4�? HDO). Using this measurement, I cut 2 squares from 3/4�? HDO. Mine were 7�? square. To make a square box, you will also need to cut 2 pieces 5 1/2�? by 7�?. These are assembled as shown in Figure 2. Screws are shown in blue.

Predrill the holes for the screws and make sure to countersink them. Do not use glue in the assembly! You need to be able to take one side off to insert the OTA. And, you might want to take it off in the future.

Next I made the round support bearings. Originally, I thought I would use 2 PVC 4�? pipe sections with a wooden disk cut to insert as a hub. Then I decided I would just follow John Dobson’s directions and make them from wood disks covered with plastic laminate. This had the advantage that I could make the disks larger – resulting in a more stable mount. These disks attach to the side of the Tube Box. The rule of thumb is to make them the same diameter as the OTA. I cut the disks from 3/4�? HDO using a router and circle-cutting jig. Then I glued on a piece of 1/2�? HDO with wood glue, and when the assembly was dry, I used the router with a pattern bit to trim the 1/2�? HDO to the circle. This gave me the 2 bearing disks 5 1/2�? in diameter and 1 1/4�? inch thick. I used contact cement and laminated the face of the disk with plastic laminate and trimmed with the router. Then I laminated the circumference of the disk with a premeasured strip of laminate. This strip was cut to the correct length but a little wider to allow trimming. I found that 2 6�? adjustable hose clamps worked great for clamping the laminate while the contact cement cured. Afterwards, I carefully trimmed the laminate to width. Finally, I used a washer and wood screw over the seam (see photo in Figure 3) to hold the ends down. I got this idea from Dobson’s instructions too. When you install these later, make sure this screw is positioned towards a corner of the tube box with both bearings mirroring each other.

Once the bearings were completed, I was a little concerned that the sides of the tube box might rub against the walls of the rocker box. So, I decided to cut some larger diameter circles from 1/2�? HDO and laminate the surface. These disks are 8�? in diameter. You could make them from 3/4�? plywood also – just make sure to adjust the width of the rocker box below. To assemble all of these, I screwed the 8�? disk to the back of the bearings with 2 countersunk wood screws. These were then centered on the 7�? x 7�? side of the tube box and screwed on from the back with 3 countersunk wood screws – I disassembled the tube box so I could do this. Don’t forget to position the screws that hold the laminate ends down properly (Figure 3).

Rocker Box

The rocker box was very straightforward to make. It is basically a 3-sided pillar. You need to know the distance from the back of the OTA to its balancing point so you can make the pillar tall enough so the scope can swing in to a vertical position for overhead viewing. The other important dimension is the width between the 2 cradle boards so the tube box can rotate freely but not so far apart that the side bearings don’t reach. I measured the distance from the outside of the large 8�? disks and added 1/2�? for the inside width. The extra 1/2�? is for the Teflon antifriction pads described later. Here are the dimensions for my Rocker Box:

• Side cradle boards: 10�? by 20�?
• Front board: 8 1/2�? by 15�?
• Back support: 8 1/2�? by 2�?
• Bottom: 10�? by 10�?

The front board needs to be shorter to allow the OTA to swing to view the horizon.

After cutting the boards, I cut the openings for the bearing cradle. I decided to use semi circles since I like the looks. John Dobson uses a straight 60 degree V cut. Both work fine, my choice was purely cosmetic. I added 1/2�? to the diameter of the side bearings to allow for the Teflon antifriction pads (which are about 1/4�? thick). Then I used a scroll saw to cut them out. I sanded the cut. Once these are complete, glue and screw (with countersunk heads) the rocker box together. Screw on the bottom at this point and make sure the screws are countersunk. Then laminate the bottom of the box with the plastic laminate using contact cement and trim the laminate when the cement is dry. Finally, drill a 1/4�? hole in the center of the bottom for the pivot bolt. This is the bearing surface for the rotating platform.

Foot / Rotating Platform

The final assembly is the foot or rotating platform. It is simply a plywood disk (3/4�? HDO) with a 1/4�? hole in its center for the pivot bolt, 3 Teflon antifriction pads, and 3 feet on the underside. My disk is 14�? in diameter but it could be any size from 10�? on up. 14�? seemed to be a good compromise between stability and portability. I cut the disk with my router. This part could even be a large square if you prefer. Once cut out, find the center and drill a 1/4�? hole. Attach the 3 legs to the underside. For my leveling legs, I had to drill 7/16�? holes 120 degrees apart and mount the leveling bases with screws. These are overkill in my opinion.

Note: now that I’ve been using the Dobsonian Mount for a few nights, I prefer it to be a bit taller so I don’t have to bend over as much. Turns out that a normal milk crate works great for this! The legs get in the way though so I took them off. If you use your scope with very young children, legs might be a good idea.

Final Assembly and Finishing

Mask off all of the plastic laminate surfaces with tape in preparation for painting. I sanded everything with 220 grit sandpaper and rounded over all of the exposed plywood edges with a 1/8�? round-over bit in the router. I painted all of the surfaces with 1 coat of primer first. Then I applied a second coat of primer to all of the exposed plywood edges since they really soak up the paint. Finally, I applied 2 coats of flat black Krylon.

Once the paint was dry, final assembly is very quick. Here are the steps:

1. Assemble the tube box around the scope’s OTA. I used 4 strips of craft foam (you could also use felt) to both pad and provide more friction to hold the tube in place. This way, I can move the tube if I need to and it will stay put. Dobson screws his tube box to the OTA. I put the center of the side bearings right at the OTA balance point.
2. Attach the Teflon antifriction pads to the cradle support cut outs – 2 on each side and about 120 degrees apart. The tube box side bearings rest on these.
3. Attach the 2 rectangular Teflon antifriction pads to the inside of the rocker box just below the bottom of the side bearing openings (see Figure 4). These were attached with epoxy and nailed with a small brad at each end. These pieces are optional; I don’t think I needed to do anything special to keep the tube box from rubbing the rocker box in retrospect.
4. Attach the 3 Teflon antifriction pads to the rotating platform top surface, 120 degrees apart and about 4�? from the center. You want to make sure that the rocker box base (coated with plastic laminate) completely rides on these. The further apart, the more stable the mount.
5. Insert the pivot bolt: use a 3�? x 1/4�? carriage bolt from underneath the foot. Tap it in to the wood with a hammer. Then slide the protruding bolt into the hole in the bottom of the rocker box bottom. Add 2 washers and 2 nuts. Adjust the nuts so the rocker box rotates easily but is not sloppy. Lock the nuts together with 2 wrenches to hold everything in place. Note: on my pivot bolt, I added 2 washers, a 1 1/2�? long compression spring, 2 more washers and then 2 nuts. I compressed the spring to about 1�? long and locked the nuts. Another unnecessary enhancement!

Enhancements

Like everything that I build, I always come up with enhancements after the fact! Here are several for this project.

1. Add carrying handles to the Rocker Box cradle boards. I am going to use round wire cabinet handles painted black.
2. Add a small shelf or 2 to the front side of the Rocker Box to hold eyepieces, Barlow lens, etc. I might even make this a box with a lid so I can just store stuff there.
3. On a 4 1/2�? Newtonian Reflector, the Dobsonian Mount is a little short. This is great for kids, not so great for adults. Simply adding longer legs (but make sure they are stable) or just setting the mount up on a milk crate works great. If you use the milk crate, you have the flexibility to use the scope without it too.

The Last Word

Although I’ve only used the new mount for a week of star gazing, it has been great. The scope is very stable and moving it to any position in the sky is very easy. Once positioned, it holds very well. I was even able to replace eyepieces without losing my target simply by pressing my thumb against the 8�? disk to lock the scope in place while I changed the lens.

As an added side benefit, this setup is a lot less cumbersome to transpor than the original aluminum tripod / OTA assembly. The scope comes apart in 2 easy to maneuver pieces and it takes up a lot less storage space too!

---

Copyright © 2005, Michael Hackney Published on Sky Insight with the author's permission.