10" Ultralight Newtonian Truss Tube Telescope

Design Goals

I decided to rebuilt my wooden truss tube telescope to an ultralight telescope for use on a home built equatorial mount for astrophotography. This project had the following design goals:
  1. The OTA should be as light as possible but very sturdy
  2. A design using 3 straight aluminum truss tubes
  3. The OTA attachment to the mount must slide along the trusses to adjust center of gravity
  4. The secondary holder must slide along the trusses to accomodate a ccd camera with a long RGB filter holder; this gave problems with my wooden OTA because the filter holder placed the camera too far away from the secondary mirror to bring to focus

Plywood Parts 

All circular rings and the sectors were cut from 1 cm thick plywood using a router with a circle cutting attachment; this is the easiest way to cut circular sectors, arcs, or large gear blanks (below left photo). All rings and sectors are 42 cm O.D., with the rings being 6.25 cm wide . The below right photo shows the primary holder sector (left) and the two rings (center and right) that combine to form the central truss support.

 
 


The below photo shows all the wood components:  two rings and three 5.25 cm square posts that form the central truss support (left), the solid sector that supports the primary mirror cell (right), a 10" commercial mirror cell (black metal, right), the secondary mirror cell components (a full ring, a partial ring, and three 5.25 cm square posts), and 60 cm O.D. plywood truss tube connectors (discs).





Primary Mirror Cell 

The primary mirror cell is a solid sector that supports a commercial 10" mirror holder (below photo). The mirror holder rests on three springs that can be loosened or compressed by adjusting wing nuts on the sector underside.


Truss Tube Connectors 

The truss tube connectors (below photo) are made by gluing three 1 cm thick x 60 cm O.D. plywood discs into a cylinder and drilling a 30 cm diameter center hole through both the truss tube holder and the solid mirror holder sector. A solid 60 cm O.D. plywood disk is placed underneath the solid mirror holder to cover each 60 cm hole (not shown). The solid disk is attached with screws that protrude through the solid mirror holder sector and into the truss tube connectors; this locks the truss tube connectors to the solid mirror holder section. 



The truss tubes (30 cm O.D. x 2 cm wall thickness x 140 cm long aluminum tube) insert into the truss tube connectors and are secured with wood screws (below photo). I inserted small sections of wood dowel inside the truss tubes (the ends inside the truss tube connectors); this gives a better material for the wood screws to grip.



Central Truss Support 

The central truss support is an assembly that slides along the truss tubes and can be locked in place with set screws. This component serves several functions: it stiffens the truss assembly, it allows the center of gravity to be adjustable, and it is where the telescope mount attaches to the OTA. The central truss tube support is composed of two rings with an 11 cm separation, attached to three 5.25 cm square posts (below left photo).  30 cm diameter holes are drilled through the rings and posts to accommodate the truss tubes. The posts contain M8 lock washers to accommodate the set screws (below left photo).  The M8 washers are hammered into the posts and secured with large M8 washers and small screws (below right photo).

 

Epoxy Tipped Set Screws 

The M8 set screws have clear polycarbonate grips and are tipped with epoxy resin to give a smooth surface that will not scratch the aluminum truss tubes. The polycarbonate grips are discs cut from polycarbonate hobby plate, tapped with a M8 central hole. I epoxied two disks together to form each set screw grip. I cut sections of M8 threaded rod to form the screws and epoxied the polycarbonate disks to the threaded rod.

The epoxy tipped screws are molded from standard two component epoxy glue. I created a mold by drilling and tapping a M8 threaded hole in a scrap of nylon kitchen cutting board and inserted a set screw from below (below left photo). 
Two component epoxy was added to the mold and allowed to partially set (below center photo). Before the epoxy was totally set, the screw was removed from the mold. The epoxy was formed to the desired shape and allowed to harden (below right photo). The epoxy tip gives a hard surface that reduces scratching the aluminum truss tubes.


Secondary Mirror Cell

The secondary mirror cell is constructed identical to the central truss support, except that one ring is only a partial ring (below photo).  The secondary mirror holder and spider was reused from my wood truss tube Newtonian and construction details can be found on the Wood Truss Tube Newtonian webpage (Secondary Mirror Cell section).



The 7 cm separation between the top and bottom (partial) rings is the same as the width of the eyepiece holder (below photo).  The entire secondary mirror cell slides on the truss tubes and can be locked in place with epoxy tipped set screws. The Telrad holder can also be slid along the truss tube and locks in place by rotating the wood disks against the truss.



Curved Vane Spider

After completing the ultralight telescope, I retrofitted a curved vane spider. This was a relatively simple project that only required three 24 inch stainless steel rulers and a couple of hours. The curved spider vanes are constructed from 24 inch stainless steel rulers. I removed the cork backing with a utility knife and used petroleum solvent to remove the adhesive. The simplest retrofit was to retain the secondary holder and just replace the straight vanes with curved vanes. The straight vanes used 2 cm wide aluminum, but the steel rules were about 3 cm wide. I cut a 1 cm long slit into the steel rulers, heated them to red hot with a blow torch, and then bent a tab (below left photo). The distance from the underside of the tab to the ruler edge is 2 cm, so this should be interchangeable with the 2 cm wide straight vanes. The tab bolts into the secondary holder in the same manner as the straight vanes (below center photo); I drilled 4 mm holes in the tabs and attached them to the secondary holder with 4 mm machine bolts. I added a second 3 mm bolt to each vane to prevent vane rotation when they are bent (below right photo).

             

The straight vanes were attached to small aluminum angle brackets on top of the secondary mirror cell ring (below left photo). I replaced these angle brackets with small arcs of plywood (below center photo). The steel rulers bend into place and are only held in place by wood clamps because they still require some fine adjustments. The final installation is shown in the below right photo. The temporary plywood arcs were replaced with hardwood wedges, which still require final painting.
 

Adjusting Mirror Separation

Because the secondary mirror cell was designed to slide along the truss tubes, it was very easy to adjust the mirror cell separation. The primary mirror focal length determines how far the eyepiece needs to be from the primary mirror.  I clamped the OTA to 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 mirror cell along the trusses until the system was in focus.  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).  


Completed Ultralight Truss Tube Telescope

All wood parts were sanded, and treated with homemade wood sealer (woodworking glue diluted in water) between each sanding. Successive sanding used finer grain sandpaper. After final sanding, all wood was given two coats of exterior oil based paint. The final ultralight telescope, before adding a curved vane spider, is shown below.


All original graphics, photographs, content, and writings are copywrited © 2007-2014 by the author and all rights are reserved. Do not copy or reproduce in any form without prior written consent.