Wood Truss Tube Newtonian Telescope 

Wood Truss Tube Telescope

This webpage documents one of my earlier telescope designs: a simple telescope constructed from wood boards and square pine dowel rods. The design goals for this telescope are: inexpensive and very simple to build. This project is a bit more complicated that just buying a solid telescope tube and installing the mirror holders, but it is still a beginner level project. To make things as simple as possible, the mirror boxes are simple squares and everything is just screwed or bolted together. Since pine boards and dowel rod are readily available and very inexpensive, this is a viable alternative if you can't locate or afford a solid telescope tube.

Truss vs Solid Tube

I preferred to use a solid tube for my telescope, since this is simpler and blocks more stray light, but couldn't find anything locally available. Out of necessity, I was forced to build a truss tube optical tube assembly (OTA). A truss tube telescope just replaces the telescope tube with a series of tubes, forming a cage assembly that holds the primary and secondary mirrors.

Primary and Secondary Mirror Boxes 

The primary mirror cell was purchased in 1981 for my Dobsonian construction and was reused in this project. I built two mirror boxes from 2.5 cm x 6.5 cm scrap pine (recycled from my son's bunk bed) with simple miter joints held together with woodworking glue and countersunk screws. I drilled three holes into the primary mirror box to accept the threaded rods on the original primary mirror cell. The secondary mirror is epoxyed to a piece of scrap pine (cut at a 45 degree angle). The prototype secondary mirror cell attached to a piece of iron rod that was epoxyed into the below-right secondary mirror box. I used a hole cutter to cut out 12 truss tube connectors (discs of laminate flooring) and attached them to the mirror boxes with wood screws. The 8 truss tubes screw into these disks to connect the primary and secondary mirror cells. 
 
 

Secondary Mirror Cell

The prototype secondary mirror cell was just a small wood block, cut at a 45 degree angle. I eventually fabricated a combination tube to improve my collimation; this showed that the prototype secondary mirror cell was too primitive, giving rather poor collimation.  The final secondary mirror cell was designed with adjustable features, allowing the secondary to be raised, lowered, rotated, and collimated.  The total fabrication cost was $1.25 for a large washer plus some left over parts from other projects (bolts and aluminum scraps).

The secondary mirror cell and support ring (below left and right, respectively). The secondary cell will be suspended from the three small (3 mm) holes in the support ring. The larger support ring holes (6.5 mm) are for the collimating screws.


The below photo shows the secondary mirror cell side and back views (left and right, respectively). In the event that the adjustment screws come loose, the central bolt with the large washer will prevent the secondary from falling into the primary mirror.


 
The finished secondary cell with collimation screws and the prototype secondary holder and diagonal (below left and right, respectively):



Since the secondary mirror box is too narrow to accommodate the new secondary cell, I had to retrofit an anchoring system. I inserted 3 threaded M6 rods into the secondary box (120 degrees separation).  The secondary holder was initially suspended on bicycle brake cable, which was narrower and stronger than my first design idea, bicycle spokes. In this test fit (below photos), the brake cable is clamped between M6 washers and lock nuts. Unfortunately the brake cable flexed too much and the secondary mirror vibrated as the OTA was elevated; I removed the brake cable and fabricated a more traditional secondary spider.



I had several scraps of 2 mm aluminum that I cut and attached to the secondary cell with M3 bolts (below left photo). I replaced the M6 threaded rods on the secondary mirror box with M8 threaded rods; these larger rods will flex less when the spider is tensioned. The spider is bolted onto 5 cm aluminum right angles (homemade) with M5 bolts. The slit in the spider vane allows it to slide around the M5 bolt for adjustment (below right photo). The right angle bends can also be raised, lowered, or rotated on the M8 bolts. The angle between the spider vanes can also be adjusted, allowing the secondary to be rotated. All of these adjustable features give the options to raise, lower, rotate, and adjust the secondary to center with minimal effort. Once adjusted, the bolts are tightened and the mirrors are collimated with the three wing nuts.



Truss Tube Length

The primary mirror focal length determines how far the eyepiece needs to be from the primary mirror. It's better to cut the truss tubes too long, since they can always be shortened. I decided to do a quick experiment to directly measure the correct distance between the mirror boxes rather than calculating it theoretically. I installed the optics in both mirror boxes and clamped the primary mirror box to a pine board on my portable workbench. I pointed the OTA at a microwave tower, inserted a low power eyepiece about mid position in the eyepiece draw tube, and slid the secondary box along the pine board until the system was in focus. The proper distance between both mirror boxes was measured and I repeated this process several times to check measurement accuracy. Note:  be sure to focus on an object in the far distance since the focal plane moves as objects become nearer. Using a close object will give too long a measurement and you will be out of focus on celestial objects (you will need to reduce truss tube length).        



Truss Tube Jig

After I determined the proper distance between the mirror boxes, I made a jig to hold the mirror boxes at the proper separation. The jig was just a long piece of scrap flooring with wood blocks screwed on to hold the mirror boxes at the proper separation distance. Once the mirror boxes were properly orientated in the jig, I dry fit the truss tubes (2 cm x 2 cm wood dowel), cut them to length, and screwed them onto the truss tube connectors.
 

 

Sector Box

I decided to build some added flexibility into the OTA by making the center of gravity adjustable. This means that the OTA can be slid forwards or backwards to adjust the center of gravity if I place a heavy object on the end of  the telescope (piggyback camera or guide scope, etc.).  The solution was to mount the sectors on a sector box that fit around the OTA and could be slid to adjust the center of gravity.  This eliminated the need for adjustable counterweights.

I routed out the truss tubes on the OTA top and bottom (the sides without sectors) and installed sections of aluminum channel bar (from an old curtain holder) into the truss tubes. I filed down several metric M6 bolts to fit inside the aluminum square bar. These bolts can slide inside the channel bar, but can't be rotated. This allows them to be slid to a desired position and be locked by tightening a nut.



I constructed a sector box around the OTA from the same square dowel used for the truss tubes. Stacking together the two sector boxes, with a spacer between them, created a channel around the M6 bolts protruding from the aluminum channel bar. As the sector box slides along the OTA, these bolts move outward or inward relative to the center of the sector box, due to the angle of the truss tubes. A router with a circle cutting attachment cut the 22.3 cm diameter sectors, which are connected to the sector box by two scraps of  old bed posts.




The Finished OTA and Current Status

This telescope performed very well and for several years I used it for visual observing (below photo). I eventually decided that I wanted to equatorially mount the optics for astrophotography. During the winter of 2010, I removed the optics and installed them in another truss tube telescope, built from aluminum rod and plywood rings (see the Ultralight Newtonian Webpage). The wood truss tube mount was eventually disassembled and the parts and hardware used in other projects.

The OTA and Autostar Based GoTo Friction Drive

 

                                                                                                                                                                
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