Amateur Telescope making
Go to An easy to build Secondary
Mirror Holder
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Since I am not equipped for metal work, I was looking for a
design that could be made of wood.
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I wanted to have a small diagonal. Thus the focuser had to be
low.
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Since this is the first telescope I built, I wanted as much
focuser travel as possible to allow for small errors in
determining the location of the focuser on the tube.
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The 100x190x10 mm focuser board is made of wood and rides on two
aluminium rails (only one is visible on the photograph). The
rails are 10 mm thick. The brass 58mm ID eyepiece tube is screwed
and epoxied to the focuser board.
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To accommodate 1 1/4" eyepieces, I made an adapter from a
wood ring and 33mm ID brass tubing. Together with the eyepiece
tube adapter, the focuser has a total height of 40mm. The 2
spider vanes of the
secondary holder are fixed to the 58mm brass tube.
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To allow lateral adjustment along the optical axis, the focuser
board can be tilted 5mm in each direction.
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The focuser is moved by pulling a centrally mounted M5 threaded
rod. The rod itself is pulled by turning a M5 T-Nut installed in
a wooden disk that is clamped between two L-shaped aluminium
supports. A small motor turns the wooden disk and allows for
focusing without introducing vibrations to the tube.
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The motor and the two 9V Batteries are housed in a plastic box to
protect them from dew. A potentiometer on the handpad allows for
fine adjustment of the focuser speed.
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Goto A "Sliding Focuser"
design
 
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For the "sliding focuser" design, the diagonal holder has
to be attached to the focuser carriage, since the focusing motion is
along the optical axis. Therefore it has to be supported by either
one or two spider vanes, which in my case are attached directly to
the 2" brass focuser tube.
On the images you can see the two holes on each vane for 4mm screws.
You can also see that the vanes had to be bent to fit the diameter of
the focuser tube. The vanes are made of 2cm broad aluminium strips
with a thickness of 1,5mm.
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The spider vanes are attached to the upper part of the secondary holder
via wood screws. The holder is adjustable towards and away from the
primary mirror, is rotatable and can be tilted in all directions via 4
push screws. I had to use 4 push screws instead of 3, because due to the
small area I could not place three push screws in a 120 degree angle.
This complication would not occur by making a holder for a bigger
secondary mirror. Mine is with 33mm minor axis rather small.
Parts listing:
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A few scrap pieces of 2cm plywood.
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2 aluminium strips, about 1mm thick and 20mm broad (depending on your
needs).
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2 big washers (OD ~ 25mm; ID ~ 6-8mm)
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3 screws M3 x 30mm
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1 screw M4 x 40mm + washer
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Some small wood screws
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1 T-Nut M4
Procedure:
First glue a few pieces of plywood together to make the 45 degree mirror
support for the diagonal mount. The thickness depends on the minor axis
of your secondary mirror. For my 33mm diagonal mirror, I needed 40mm
thickness, to be able to cut a 45 degree piece.
After the glue has dried, saw or bore a cylinder of the same diameter as
the diagonal minor axis. In case you bored out the cylinder, you will
have a central hole in it. Fill this hole with a wooden plug. Now cut the
cylinder to an angle of 45 degrees.
Drill a 1mm deep hole with the diameter of the M4 washer in the
top/center of your mirror holder. Now measure the head of your M4 x 40mm
screw and drill a hole 1mm larger than the screw head a few millimeters
deep in the center of the "washer hole". This will be the house
for your central pull screw of the diagonal holder.
Take one big washer, drill three equidistant (120 degree) 3mm holes and
countersink them. Now place the M4 x40 screw with the head forward in the
previously drilled hole. Next comes the M4 washer and then the big
washer. The big washer will be fixed to the wood via small wood screws
(3x16mm). I additionally glued it with epoxy to make sure that nothing
would get loose later. Now you should have the mirror holding part with
its central pull screw.
Now cut a rectangular piece of 20mm plywood. One side shall be as long as
the diagonal minor axis, the other side will have the length of diagonal
minor axis - thickness of the two spider vanes.
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Diagonal minor axis = 33mm
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Thickness of each spider vane = 1mm
Rectangle = 33mm x (33mm - 2mm) = 33mm x 31mm
Drill a straight 5mm hole in the exact center and install the M4 T-Nut in it. Now
epoxy the other big washer on the side opposite the T-Nut. Drill three
equidistant (120 degree) 2,5mm holes with a
metal drill through the wood and the washer. Cut a 3mm thread in the
washer and the wood. Round all the corners,
install the M3 push screws and attach the mirror-holding support.
Cut the aluminium vanes to the right size and attach them to the upper
part of the diagonal holder. Paint everything flat black and install the
holder in the tube.
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© 1996 Contact Berthold
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