BUILDING A NEWTONIAN TELESCOPE
Making your own Newtonian telescope is a most enjoyable project
both for the hours of enjoyment you're going to have using it and the pleasure
of making a scientific instrument of some considerable size. Where to start ? -
the major decision, obviously, is what size of mirror to buy. The larger the
better ? well, yes, but larger mirrors mean longer focal lengths, hence longer
tubes. Too big and it will soon become a chore to get it out the garage, set it
up, let it cool before use. Someone said the best telescope is the one you use
There is plenty of free advice on the Internet but the
consensus seems to be that a 6" (150mm) to 8" (200mm) diameter is
about the optimum for suburban use. Such a size is also capable of fitting
inside a car for more adventurous types. The typical focal ratio is f8 for
all-round performance giving focal lengths of 48" and 64"
respectively. These dimensions automatically define the length of the tube as we
shall see later.
The arguments for and against baffles can be
followed on the Internet; I chose to fit baffles mainly because they would act
as formers for construction. I can see that if a traditional circular tube
design was chosen (plastic pipe or cardboard tube), baffle fitting would be
quite exciting. I chose to make an octagonal tube from thin plywood, mainly for
lightness but also because I like woodworking.
Let's look at the overall plan. This is just a thumbnail
pic - click to load a larger, more readable version.
In essence, the construction consists of
eight identical boards of 1/8" ply glued around nine 1/8" octagonal formers
(baffles), each with a 7 1/4" diam cut-out - very much like the traditional
building of a model aeroplane or boat. The front view in the plan above shows the major
dimensions. I cut the baffles first as 8 1/4" squares and then cut the
central hole in groups of three using a router and home-brew circle cutting jig.
If you don't have a router - get one, they are immensely useful tools for all
sorts of jobs. All the baffles and the 1/2" ply back plate were then pinned
together and cut and sanded to the finished shape. This ensures they are all
identical in size.
The eight nominal 3 1/2" x 48" boards were
then cut with 22 1/2 deg. bevels (approx !)on the side edges and marked with the positions of the baffles. At this stage I had to
alter the position of two of them to allow them to clear the focuser holding
bolts (I don't think I altered the dimensions on the drawing!). One of the boards
was lightly pinned to the flat work-bench and the baffles glued to their marked
positions using woodworking hot-melt glue. They should all be mounted square and
vertical. One board was then carefully glued on ensuring that no twist was being
built into the construction. The opposite board was next, and then alternate
boards until one remained. TIME TO STOP ! The last board would not be put on
until the inside was painted. A quick dob round with some P.V.A. glue to
strengthen the joints and the assembly was left to set overnight.
There were now a few details to attend to. Thinking
ahead for once, I glued small pieces of ply inside the tube to reinforce stress points -
i.e. under the focuser, under the handle and halfway down the tube inside the bottom where I then
glued two pieces of dowel 3" apart to act as locating aids in the Dobsonian
cradle. Having marked the position of the mirror cell
inside the tube, four holes were drilled at the appropriate points to screw into
the 1/2" ply cell back plate. This was then removed for mirror
At this stage it looked like this
The inside was given a first coat of matt-black
blackboard paint and the inner side of the last board was painted. Doing it this
way ensured at all parts of the interior received at least one coat of paint, as
later, it would be difficult to reach every nook and cranny. The final board was
then glued on - the tube was essentially complete .
Now for the hard bit - I decided the interior was not as
black as it could be - so, another coat of paint, applied with a cut-down brush,
was brushed over as much as possible. Then, whilst the paint was still wet, I
threw a copious amount of sawdust inside attempting to cover all areas. The
whole thing was then left to dry and the loose sawdust shaken and vacuumed out.
The whole thing was then given another coat of paint to seal in the sawdust.
When dry the finish was very rough and very dark - I don't know what more can be
done in this direction. The baffles certainly help. Maybe you could stick black
flocked plastic or velvet in ?. The mirror end of the tube looked a bit flimsy,
so I wrapped a 1" wide band of fibre-glass tape and resin round the outside
to strengthen it. Finally, the surface was given a coat of sanding sealer,
rubbed down and then painted with two coats of hammer - finish gloss white - Luverley !.