Rebuilding a 12.5″ f/5 Truss Tube Dobsonian

12.5" Dobsonian
 
Some time ago my friend Bill Lofquist bought a dobsonian telescope from Roger Ceragioli. Roger had built the ‘scope to provide a home for a beautiful 12.5″ f/5 mirror he had made. The mirror is gorgeous, as is typical for Roger who is one of the best opticians I know. I have an APO triplet of Roger’s that is a prized possession.

Mechanically the scope had a few problems. The truss tubes were attached with separate hardware top and bottom, so that setup required over 20 minutes of sorting through screws and futzing with eight separate truss tubes waving around the whole time.

The elevation bearings had been set about 1/8th inch off from each other leading to a side to side twist when the scope was moved in elevation. This was not a major problem when using the scope visually but would make the use of digital setting circles impossible as DSC’s require orthogonal axis in the scope.

The ground board was a bit undersized, making the scope prone to tipping when used at low elevation.

The rebuilt scope is essentially finished with the usual tweaking and small adjustments remaining. Things are coming out very well and a few of the changes are worth passing along to the ATM community. In the sections below I will concentrate on practical details in hope of conveying some of the finer points in telescope making.

Mirror Box
 

Frustrated with the long setup times Bill asked me to rebuild sections of the telescope. The plan we came up with is simple… fasten the truss tubes into permanent triangles with new top and bottom hardware. Have no loose hardware needed for setup. While we were at it we would turn the secondary cage about 30° for more comfortable viewing. Replace the elevation bearings and do some general sanding and refinishing.

Thus a plan was formulated, and the scope dismantled and examined…

Mirror Box
 

A very nice feature of the telescope as originally built is the mirror cell. The cell is a simple but very effective arrangement with a plywood cell sitting on a central ball. Two collimation rods reach from the top of the mirror box to the back allowing convenient adjustment from the front.

Seen to the left is the inside of the mirror box with the mirror cell removed, allowing a clear view of the center pivot made of a round knob and the adjustment rods. The rods thread into a threaded insert underneath the bottom wooden blocks. Where the rod goes through the upper baffle there is only a bushing with no threads.

Mirror Post
 

We didn’t mess with the interior of the mirror box much beyond a new coat of Krylon ultra flat black paint.

One part of the mirror cell that has been rebuilt is the retaining posts. In place of the simple wooden posts originally used we installed an adapted version of the mirror posts from Berry and Kreige’s dobsonian construction manual. One inch Delrin rod is bored through off center to make a post that can be adjusted to position the mirror exactly in the center of the mirror box. Above this post a pin made from an eye-bolt will extend over the mirror to keep it from falling out of the cell during use or transport.

As shown in these photos the threads of the eye bolt have yet to be cut to length and covered with heatshrink tubing to pad them.

Because the Delrin scraps I had were too short I built up the spot with a couple layers of plywood cut with a hole saw. This is possible because of the double arch mirror has a thin high edge. The result is a very solid retaining post.

Lower Truss Clamp
 

To hold the bottom of each truss tube assembly in place Bill and I dreamed up a simple mounting arrangement that can be fastened quickly and securely. This consists of a base plate made of 1″ aluminum angle. This will set in place over two short pins and be clamped into position with a single knob at the center

The truss tubes will be mounted to either end of this base plate on machined pins that have had the ends machined to the needed angle. These truss tube pins are epoxied into the ends of the truss tubes.

Truss Alignment Pin
 

The alignment pins we made are stainless steel machined to 0.240″ diameter to fit neatly in 1/4″ holes drilled in the baseplate. 1/4″ hex standoffs were clamped in the lathe and machined down to dimension.

These pins could also be made by cutting the heads off of 1/4-20″ bolts and gluing the threads down in the holes. The clearance hole in the base plate would then need to be reamed out slightly larger than 1/4″ to give a good fit.

To allow these pins to set into a very precise position they are set into holes drilled to a slightly larger diameter, then epoxied while the base plate holds them in exactly the right location.

Threadsert
 

For the center knob a threaded insert is used. These inserts allow threads to be placed into wood by drilling an oversize hole that the insert can then be threaded into with a special tool.

While it looks like these can be set with a drill I always just attach a socket wrench instead and set them slowly by hand. This prevents stripping of the hole or the top of the insert.

Despite careful placement it was still necessary to slightly file several of the holes in the baseplates such that all four truss assemblies neatly fit on any of the four mounting points. The goal is that any truss assembly can be used interchangeably in any position

Encoder Pocket
 

At the center of the elevation bearing I have drilled out a recess for the elevation encoder. This was done before the two layers of the bearing were laminated giving a neat recess for the encoder. First a smaller hole saw is used to cut the recesses for the wiring and connector, then a 2-1/2″ hole saw cuts the recess for the encoder body by drilling right down the center hole leftover from routing the bearing.

To neatly finish out these holes a drum sanding attachment on a Dremel rotary tool was used to clean up the rough edges left from the hole saws.

Encoder Mount
 

The elevation encoder mount is completed with a simple cover plate made from 0.062″ aluminum. The encoder will be mounted behind this plate with the shaft protruding out to meet a tangent arm. The plate is mounted with four screws that fit in slightly oversized holes allowing a slight bit of adjustment to allow centering up the cover plate exactly on the axis of rotation.

Elevation Stop
 

In the past I have had trouble with the ends of the Ebony Star laminate beginning to peel off the elevation bearing with heavy use. With Deep Violet I machined little aluminum end caps to solve the problem permanently.

Here I was looking for a somewhat simpler solution using what was available in my local ACE’s hardware selection. What I found were nylon washers that could be used to cover the end of the laminate. These will also function as stops preventing the scope from skipping off the teflon pads if swung too far.

When cutting and drilling the laminate I drilled the hole for the stop a bit oversized as I could not be sure exactly where the end would come out when gluing.

Cooling Fan
 

To ventilate the mirror box a fan was installed on what will be the top side when the scope is in use. The fan is a standard 12VDC electronics cooling fan. The fan is positioned to exhaust air from the box, pulling it in through a number of ventilation holes and the front opening.

Note the 15ohm 1/2 watt power resistor soldered in series with the fan power, this slows the fan down, making it quieter and producing less vibration by reducing the voltage at the fan to about 9V when using a 12V battery. These type of fans will run reliably as low as 7V. A 15 or 20ohm resistor will do the trick, just experiment to see what works for you.

Cooling Fan
 

On the exterior a metal fan guard protects the fan and a standard 5.5mm DC jack provides power.

On the interior a plate made from 0.062″ aluminum covers the fan and blocks any light that might shine in through the fan onto the mirror. Probably not necessary for many scopes, but this telescope will get used for public viewing with bright lights around so a little extra work to light tight the scope may prove a good idea.

Upper Truss Clamp
 

For the upper truss clamps we ordered cam latches from AstroSystems. These allow quick connection of the cage with no loose hardware as we had wanted. To hold these clamps and make the upper end of the truss assembly a set of three triangles were machined from thick 0.082″ aluminum plate.

The mating part was made from the same 1″ angle used for the lower parts of the truss assembly, with a notch cut into it to accept the clamp. The corners of the triangular plate were cut off such that the depth is just right and the cage sits on all four truss assembly tops neatly before clamping.

Elevation Bearing
 

Another view of the elevation bearing showing the elevation encoder site as well as the “handle” cut into the elevation bearing to allow easy pickup and moving of the mirror box.

In practice these handles work very well, making the mirror box easy to move with primary in place. For testing Bill and I moved the fully assembled scope to an upper patio, I could pick up the full OTA with these handles and easily carry it.

The bearings are more than half circles, ensuring a wide range of movement without sliding off one of the teflon pads. The scope can go from just below the horizon to a bit behind the top through zenith.

Primary Mirror
 

A look down into the mirror box with the primary installed. The mirror retaining pins have been trimmed and covered with heatshrink. The collimation rods have be adjusted and the mirror is sitting in the correct position

When this photo was taken the new knobs for the lower truss mounts were not yet installed but were with a bunch of other parts waiting to go to the anodizing shop. The cap screws shown here have since been replaced with the aluminum knobs.

This has resulted in a telescope that is much easier to setup and use. It all looks good and assembly of the scope takes only a couple of minutes. First light on a rising full moon shows focus right in the center of the focuser travel. This scope has been heavily used in since completion with showings at Grand Canyon Star Party and many school star parties across Tucson.

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on Mauna Kea, Hawai'i.

Leave a Reply