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8. Materials and Fastenings.
I have some experience working in wood, so I used it extensively. I
got a moderately good grade of half-inch plywood from a lumber yard, and
for the small quantities of thinner plywood I needed, a hobby shop had
plenty of variety in good-quality stock. Local hardware stores provided
small pieces of "project" wood -- poplar or oak, finished on four sides,
in handy sizes. I used 5/8-inch hardwood dowels for truss poles. I had
to pick through a whole bin of dowels to find enough straight ones.
I have had occasion to use good cements, so I made permanent
assemblies with glue alone -- no fasteners. The hobby shop had clear
epoxies with pot lives of 30 and 120 minutes, in four-ounce bottles.
The 30-minute stuff penetrates far better than the usual quick-setting,
"5-minute" kind. The 120-minute version flows nearly like water, easily
fills cracks and voids, and makes nice fillets. I selected Weldwood
phenolic/formaldehyde wood glue for large areas with no end grains, and
used contact cement to wrap Ebony Star around the side bearings. Contact
cement works best when you follow the directions: Let it dry smooth and
hard before joining the pieces. Don't ask me how I know.
After a few experiments and some visits to hardware stores, I picked
threaded inserts and T-nuts for joining pieces that come apart for
transportation. I was able to put all of them in places where they
would not pull out when the machine screws that went through them were
torqued down.
I used stainless fittings and fasteners whenever I could find them.
Marine hardware stores offered a particularly large assortment, and also
supplied the rubber grommets used to attach bungee cords to the shroud.
9. Construction -- A Few Details.
I won't bore you with conventional details of a truss-tube Dobson --
Kriege and Berry is a wonderful source for those -- but there are a few
places where I did things differently, and it may be noteworthy that the
construction is very light. Let's take it top-down.
The telescope does not have a sky-end "cage" as such -- the focuser
attaches between two truss poles, four inches below an octagonal wooden
"ring" at the upper end of the truss. I built the octagon by gluing
four strips of 3/16-inch plywood, 1.5 inches wide, into a square whose
width was a bit less than the inside dimension of the mirror box. This
piece looked like a square cake pan with the bottom cut out. Then I
glued braces of 3/4 by 1.5 inch oak, with ends mitered to 45 degrees,
inside the corners of the square, with dimensions such that the inside
of the glued assembly was a regular octagon. Finally, I cut off the
corners, outside the braces, creating an octagonal structure whose wall
thickness alternated between 3/16 and 3/4 inch. This part nests inside
the box for transport, but when I rotate it 45 degrees from the nested
position, the 3/4 oak sides provide a surface for attaching the upper
ends of the truss poles.
With careful use of a spade bit, I could drill a 5/8-inch hole half
an inch or more into the end of 3/4-inch square stock without breaking
through. Thus I made square cross-section "rod ends" for my 5/8-inch
truss poles, and epoxied them in place. The ends are drilled through
7/32, for 10-32 machine screws that hold the truss poles to threaded
inserts embedded in the top-end octagon or in the structure of the
mirror box. I made two sets of poles, one cut for the beeswax trick and
one not. If the beeswax joints had worked poorly, I would have made a
case for the long poles and traveled with them as excess baggage. The
cut poles fit into a small suitcase or duffle with plenty of room left.
I was very careful to make the truss poles interchangeable -- I
worked hard to make sure that the spacing between attach points at the
opposite ends of the pole was the same for each one. When I cut the
poles for the beeswax trick, I was also careful to make the cuts at the
same distance from the attach points, for each pole; thus any upper pole
piece can be used with any lower pole piece.
The focuser is mounted on a thin piece of plywood that presses
against two truss poles from within. The focus tube protrudes between
the poles. Wooden strips glued to the edges of the plywood help located
it precisely. A piece of thin brass with a hole for the focus tube fits
outside, and machine screws with spacers go through plywood and brass
alike, to hold all in place. The focuser thus extends minimally out
from the tube. The focuser is far enough down from the octagon ring
that there is no need for an additional light baffle that extends
skyward from the ring.
A friend did me a huge favor by doing nearly all of the fabrication
of a beautiful shroud from black rip-stop nylon. She has worked
professionally creating women's lingerie -- we thought about trimming
the shroud with black lace, but decided not to. She modestly demanded
that I not show the shroud to anyone with fashion or costuming
experience, but telescope owners ogle it no end -- even without lace.
It has an edged hole for the focus tube. The bottom bungees go all the
way around the mirror box -- the rod-ends on the truss poles are a bit
small for a bungee to get a grip on.
The mirror box is five sides of a cube. There is no detachable
cell; the mirror side of the box is glued in place. It is half an inch
thick; the other sides are quarter-inch. I glued long pieces of "2x2"
(actual dimensions, 1 3/8 by 1 3/8 inch) inside the corners, where the
quarter-inch sides join together, as braces. The truss poles attach to
the outside of the box.
I didn't happen to have any half-inch plywood when I was ready to
make the mirror box, but I had extra quarter-inch, so I laminated two
pieces of quarter-inch together for the mirror side. It was rainy that
day, so I could not use the usual trick of weighting the glued assembly
by jacking up my car and setting it down with one wheel on the plywood.
I glued it indoors, with 90 pounds of GEM counterweights and 75 pounds
of unused, bagged cat litter as weights. I had about 10 pounds of used
cat litter standing by just in case, but I am not convinced I could have
kept the cat away from it long enough for the glue to set.
The mirror sits on three nylon collimation bolts inserted through
the bottom via T-nuts. I also installed extra T-nuts at the right place
for 9-point support pivots, in case I need them. After the mirror is in
place, I attach two wood assemblies inside the box to keep it there.
Each assembly has two nylon furniture glides, to define the mirror's
position laterally: It sits touching two glides; the others don't let it
slide more than a quarter inch if I should tip the tube backward. The
assemblies also have cork-tipped fingers that protrude a little way in
front of the edges of the mirror, so it can't fall out if the tube gets
upside down. The cork clears the mirror by a quarter or half an inch,
depending on how far in I turn the nylon bolts. Each assembly is a
piece of oak, 2.5 by 6 inches, with one face against the inside of the
box and the 6-inch length running crosswise; it has two wooden wedges
glued on, at the right place and angle to hold the furniture glides, and
two wooden fingers glued on, carrying the cork pads.
The side bearings are semicircles of 5/8-inch plywood, with
"piece-of-pie" sectors for bracing across the clearance cuts for the
truss poles. They detach from the mirror box for transportation. The
threaded inserts that hold them in place are embedded in either the
"2x2" corner braces, or in extra pieces of half-inch oak that I glued to
the inside of the box, just to carry the inserts. I put washers between
the threaded inserts and the box sides as I was gluing everything
together: Thus the inserts are kept from pulling out through the thin
plywood. I could not find any 10-32 T-nuts; they would have worked
better than threaded inserts and washers.
Rocker and ground board are conventional, of half-inch stock, with
Ebony Star and DuPont Teflon for bearing surfaces. There was a thread
on sci.astro.amateur when I was building the rocker, about how difficult
it is to cut Ebony Star. Don't believe everything you read on
sci.astro.amateur: I scored it with a hobbyist knife and then cut it
with scissors, all just fine.
I had some 1/16-inch sheet Teflon for bearings: I made convex
surfaces wherever I needed Teflon, bent strips of the slippery plastic
over them, and fastened the ends with wide-head screws whose heads were
thin enough to clear the bearing surface, given the convexity. The
curved surfaces were the top edges of the rocker, for side bearings, and
convex blocks of wood epoxyed to the ground board, for the bottom
bearing. Several telescope makers have seemed delighted by such a quick
and easy way to attach Teflon. I have made bearings of thicker Teflon
with countersunk screws, but this way was simpler, because Teflon tends
to deform when you torque down the countersunk heads, and that can be a
pain to deal with. It was also more compact, and used less material.