12-inch F6 Truss Dobsonian

Background:

The Dobsonian telescope was introduced in the 1960's by John Dobson. He invented a functional means of mounting a large aperture newtonian telescope so as to allow for altitude/azimuth movement with common materials that could be purchased and assembled very cheaply. His innovative use of teflon pads and countertop laminate to create low-friction bearings for both axes worked very effectively. The utility of his design became apparent to the amateur astronomy community and gained popularity through the 1970's and 1980's. Today the vast majority of large-aperture telescopes owned and built by amateurs and professionals alike are Dobsonians.

Overview:

This project involves the design and construction of a 12.5" f/6 truss dobsonian. The truss is one variation on the original Dobsonian telescope (which featured a traditional closed tube) in which a truss conisisting of eight aluminum poles serves as the structural backbone of the telescope. The truss offers several advantages over a closed tube. It is lightweight and inexpensive when compared to the cost of a similarly sized closed tube made of aluminum. It also allows for faster cool-down times. Lastly, a truss-dobsonian can break down into three main pieces for transport and does not require an amount of space equal to the assembled length. The truss collapses down into a small stack of poles, leaving only the bottom and upper portions of the telescope which can be transported in a car trunk.

Design & Components:

The entire telescope was modeled in 3D using TurboCAD 14 Deluxe. All wooden parts were cut from baltic birch HVHC plywood in three thickness on a CNC router. Many components were purchased elsewhere and are outlined throughout the project stages.

Application:

Upon completion this instrument will serve as my main visual telescope. It will be used on many of the same nights that I conduct imaging sessions in the background. I have several observing goals centered around the Astronomical League's observing clubs, such as observing 100 binaries, the entire Messier Catalog, a good portion of the NGC and Herschel's Catalog, and others.

Thanks for checking out my project!


Rough Draft	An early version of my 3D model used moon-shaped altitude bearings. The torques for this particular arrangement left a deficit below the balance point. This is partly due to the rather long focal length of my primary mirror. Most 12" or 12.5" truss-dobs are f/5 or f/4, mine being a full foot longer than the former. I was forced to redesign the bearings and correct the imbalance.

This new bearing design solved several problems: 1. The balance point is raised three inches providing some much needed torque below and reducing torque above. 2. The bearings themselves are more massive and added some additional weight below the balance point, again increasing torque where it was needed. 3. A 'flare' was added that reaches up to cover the imaginary axis line. This point will hode an optical encoder linked to a set of Digital Setting Circles (allowing me to access a computer holding thousands of cataloged objects, then aim the telescope accordingly).	Revised Altitude Bearings

Wooden Parts via the CNC Router	I owe Jeremy Hudson and the staff of Prestige Casework a great debt of thanks for their assistance in cutting these parts. CAD drawings were uploaded and used to drive a very large CNC router. The results were exceptional.

The alt bearings are 1.25" inches thick. Two pieces were cut to form each bearing, from 0.5" and 0.75" respectively. Here two halves are clamped and glued together to form a single bearing.	Laminating the Altitude Bearings

Assembling the Mirror Box	The primary mirror is held in the bottom of a wooden box, which will then have the altitude bearings fastened to either side. I spent a couple of hours scribing and cutting out the finger joint corners (round fillets were left by the router bit in each joint). The prepped sides went together smoothly. Material for this box is all 0.75" plywood.

The completed box with light baffle installed and lid.	Completed Mirror Box

Completed Mirror Box	Inside view of the mirror box. Corner gussets are installed underneath the light baffle to strengthen the box. Some additional braces will be added later.

This smaller box will support the mirror box and allow the entire telescope to tip up and down via the altitude bearings. The bottom of this box serves as one half of the azimuth bearing, allowing the scope to be aimed left and right.	Completed Rocker Box

Supporting the Mirror	The first truss dobsonians featured a hinged hatch on the bottom of the mirror box called a tailgate. This component supports the mirror cell, adds necessary ballast, and strengthens the mirror box. In modern truss-dobs the tailgate no longer swings open, but rather serves it's purpose in a fixed position. For my telescope I sketched a traditional steel-frame tailgate and designed an upper cell plate to support the conical primary mirror.

My tailgate is composed of three sections of 1.0" x 1.0" square tube (0.125" wall) and two sections of 1.5" x 0.25" flat bar. The metal pictured here cost $13 at my local supplier.	Building the Tailgate

Building the Tailgate	As I have learned the hard way, it is necessary to build a jig that securely holds each piece in the exact position desired BEFORE welding. This jig was taken to a local welder with the metal pieces already seated.

For budgetting reasons I abandoned my original plans to have the mirror cell water-jetted from aluminum. Instead I've improvised a low-tech method of cutting the desired shape and hole pattern using tools in my home shop. Here I've printed out the CAD drawing at 1:1 to transfer the pattern onto the aluminum. Lines were digitized between hole centers for the sole purpose of scribing those points into the metal.	Fabricating the Mirror Cell

Fabricating the Mirror Cell	The outline was cut on a table saw. I drilled three additional holes to bolt the aluminum to a sacrificial piece of plywood that was ripped to be the same width as the template (measured from broad side to narrow side). The hardest part of setup was finding a fence that was thnner than the plywood jig so the metal could pass over it on the non-cutting side.

This made the whole process very easy. I ripped the metal along both sides of the plywood, then rotated the metal 60º (or one hole over) and repeated the cuts.	Fabricating the Mirror Cell

Fabricating the Mirror Cell	The holes for mounting the conical primary and collimation bolts were an easy job on my drill press.

Top view of completed (but unpainted) mirror cell.	Fabricating the Mirror Cell

Fabricating the Mirror Cell	Bottom view of completed (but unpainted) mirror cell.

Side view of completed (but unpainted) mirror cell with 12.5" f/6 Royce conical primary mirror installed. This combination tailgate/cell is currently a candidate for production and may be available to order from Optical Supports	Fabricating the Mirror Cell

First Light	First light took place on November 12 at the 2009 Nova Sedis Star Party in Chiefland, Florida. The telescope is pictured here is about 90% complete and fully functional with a few minor quirks.

A descrete counterweight system will be added to the rear of the mirror box (pictured here) to help the scope stay balanced when aimed at objects low to the horizon. A simple computer will also be added to give "Push-To" convenience of entering an object and having the display prompt the user one where to aim the scope.	Planned Upgrades

Planned Upgrades	Here the UTA is pictured with a Telrad finder (far left). I'm also going to add a 50mm finderscope which will mount between the Telrad and the focuser. The interior of the rings will be lined with Kydex which is currently being shipped to me. Last but not least, wiring will be installed to run the dew heater moutned behind the secondary mirror.

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