10" Ultralight Newtonian
Truss Tube Telescope
I decided to rebuilt my wooden truss
to an ultralight telescope for use on a home built equatorial
astrophotography. This project had the following design goals:
- The OTA
should be as light as possible but very sturdy
- A design using 3 straight aluminum truss tubes
- The OTA attachment to the
slide along the trusses to adjust center of gravity
- The secondary
holder must slide along the trusses
accomodate a ccd
camera with a long RGB
filter holder; this
problems with my wooden OTA because the filter holder placed
camera too far away from the secondary mirror to bring to focus
All circular rings and the sectors
were cut from 1 cm thick
plywood using a router with a circle cutting attachment; this is the
to cut circular sectors, arcs, or large gear blanks (below photo).
All rings and sectors are 42 cm O.D., with the rings
being 6.25 cm wide . The below 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
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
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
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 photo). 30 cm diameter
holes are drilled through the rings and posts to accommodate the truss
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.
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
that reduces scratching the aluminum truss tubes.
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
found on the Wood Truss Tube Newtonian
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
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
the tab to the ruler edge is 2 cm, so this should be
interchangeable with the 2 cm wide straight vanes. The
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
The straight vanes were attached to small aluminum angle
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
truss tubes, it was very easy to adjust the mirror cell
The primary mirror focal
length determines how far the
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
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
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
All original graphics, photographs, content, and writings are
copywrited © 2007 by the author and all rights are reserved.
not copy or reproduce in any form without prior written consent.