Designing a Dobson (Newtonian Reflector) Telescope for Optimal Performance

Starting Point

This was my new project: a decrepit old OTA that housed an interesting primary mirror. Beside it, the SkyWatcher 8" that I had bought a few months previously. Like many of their customers, I spent hours talking to Harout and Bryce about collimation and other such topics. Bryce lent me Vic Menard's guide on the subject. Finally, given my interest in technical aspects of the hobby, Harout proposed that I buy this old OTA and start a "project".

Old Spider and Vanes

This is a view down the old OTA (primary mirror removed)

Old focuser

This was the original focuser which was quite tall. I purchased a new Feathertouch so that it would be shorter thus allowing me to reduce the distance between the focal plane and the secondary mirror which in turn would allow me to lengthen the distance between the primary and the secondary such that either a smaller flat can be used or the fully illuminated field increases (or a combination of both). I used the same size flat but increased the fully illuminated field from essentially zero to ~9 mm.

The primary mirror

This was the reason for purchasing the old OTA. This 10" primary mirror is quite thick. It was hand-figured to a 55.5" focal length. The mirror mount also appears to be quite robust and I used it in my new scope. (This photo was taken with flash; notice that an image of the mirror was projected onto the wall behind it.)

PVC Drain pipe to Reinforce OTA

I purchased a length of PVC drain pipe and had them plastic weld two lathed PVC blocks, one on each side. The two blocks act as a gimble that serves as the altitudinal axis. I do not have a lathe, nor could I have plastic welded the blocks on ... not to mention getting things "square" on a tube. The PVC drain pipe is to add some rigidity to the OTA which is critical for maintaining collimation. (In later steps, I will drill holes in the OTA which will weaken it)

Painted Sonotube in the PVC Pipe

There is a very snug fit between the Sonotube and the PVC drain pipe

Drilled holes in OTA

Using a hole saw in a hand drill, I drilled evenly spaced holes around the OTA at the end where the primary mirror would be mounted. Then I painted with primer (Zinsser Bull's Eye 1-2-3). The old Meade equatorial mount made a handy painting jig.

Two Rings to Support Light Shield

Using a jigsaw and a DIY jig made of a piece of sheet metal and a screw, I cut circles in 3/4" plywood to generate two rings. These rings fit snugly around the OTA and were screwed in place. They allow about a 3/4" air gap between the OTA and the light shield that will be wrapped around it to block light from entering via the perforations. When the shield is on, the air gap allows better air flow. Notice that I also drilled small holes in the rings themselves to allow airflow between air gap and the "exterior".

Light Shield

The first shield (blue) looked great but splintered quite easily. I made it out of a sheet of wood veneer. I replaced it with a sheet of plastic ... some sort of construction waste, possibly ABS (bottom right corner). Both designs were a "wrap" that closed via a Velcro strip at each end. The image on the top left shows that the perforations extend above and below the primary mirror.

Baffled Ring (PVC)

A ring was added to the end of the Sonotube to help maintain the circular form of the OTA at the front aperture. This ring was a section of thicker-walled PVC drain pipe. I had wide grooves cut in with a lathe so as to create baffles. Afterwards, I scribed two circular baffles (not shown here) out of a thin sheet of black ABS using a box cutter. Then I epoxied one to each end of the ring. The one on the interior side just resulted in one extra plastic baffle (before the metals baffles start). The one on the exterior side serves as the entrance pupil.

Metal Baffles

Using a Dremel tool and a scribe, I cut circles out of sheet aluminum. This was a painful process as the scribe was not reliable enough and the Dremel bites into the metal at times The top left shows the circles; top right the circles are aligned on the jig for gluing (used epoxy as well as perpendicular side braces with tabs and slots); bottom left, the baffle cage; bottom right, the primered baffle cage with a hole cut out for the focuser draw tube.

Versions of Secondary Holders

Version 1 (top left) was abandoned quickly. It was a minimalist design with good points of attachment for the strings but it was missing a rotational axis. I thought I could get away without the rotation, but relying solely on manipulating the six wires (via tuning pegs) proved too challenging. Version 2 performed well and I used the scope like this at first. But because it relied on nuts and washers, there was sometimes slippage as the wires were tightened so positioning the secondary was problematic. Version 3 was a single piece of aluminum turned on a lathe. Although it had a similar diameter to version 2, it has increased torsional vibration. I believe the "play" allowed by the slipping washers in version 2 dampened those vibrations At the suggestion of Jerry Oltion, I made a final version with a larger diameter (bottom right corner). Notice on the final version (4) that the top flanges have a slightly smaller diameter than the bottom two flanges. The larger flanges go right up against the secondary mirror and the smaller ones face the entrance. This was to allow for the largest diameter (to reduce torsional vibration) while ensuring that the holder stays well within the "shadow" of the secondary mirror itself.

Baffled Aperture with Secondary

Here you can see the completed, painted baffles. In the photo, I am just installing version 2 of my secondary. You can see the guitar strings as well as the gold-coloured tuning pegs in the black housings on the exterior of the OTA. Also, the first (closest) ABS baffle that serves as the entrance pupil is plainly seen. The paint used inside the OTA is called "Scenic Black" and is specially formulated for the film industry by Cloverdale Paint. They use it on set to prevent light reflections etc. I think it does the trick.

Secondary Installed (Final Version)

This is the final version of the secondary, installed.

Dobson Mount

The Dobson mount was made with 3/4" plywood. I used a "lazy Susan" bearing for the azimuthal axis. I used 4 roller bearings (two on each side) to support the PVC gimble on the OTA; this served as the altitudinal axis. I had been frustrated with the teflon sliders used on my Skywatcher; when you want to nudge the scope to track an object, you feel resistance and then a sudden release and you can go too far. I wanted my mount to be slippery smooth. Both axes are. in fact, I had to add a three nylon bolts to the platform so that I can add more friction in a controlled manner (azimuth). The altitude was a dream to move ... however, it wouldn't stay put! I realized I needed a really practical counter-weight system.

Counter-weight System

I wanted one simple weight that could slide along as needed. I hack-sawed off the loops of two finder bracket mounts and smoothed the remaining piece with a file, and drilled some holes. I bought a steel rod and had it threaded at each end. I bolted two furbs to the scope, one on the OTA and one on the panel that cover the end of the OTA (behind the primary). The result was just what I was after. It looks good and it's perfectly functional. Importantly, thanks to the furbs, it comes off the OTA for transport.

Finally finished

Now all I need is clear skies!

Downtown Vancouver Stray light Test

A large, bright, skylight spews out light pollution as do the lights from nearby buildings. This photo was taken from the position of the telescope, looking towards the south east where Jupiter was (a little out of frame on the top-side of the photo, dead-centre). I observed both Jupiter and the Great Nebula in Orion and was happily surprised with the views given the amount of stray light present.