Sky & Telescope - April 2020 - 39

when you do so will wedge tightly into the front of the tube.
Glue the secondary mirror to the angled face, aim it at the
focuser hole, and adjust the collimation by pushing and pulling on the shingles where they meet the tube.
I'm really not kidding - it works like a charm!
The focuser is equally simple. It's just a cardboard tube
sticking out the side of the main telescope tube. You glue it to
a rectangle of hardboard that's screwed into place so you can
adjust its aim with shims. Your eyepiece goes in another tube
that ﬁts inside the ﬁrst one, which you push inward and pull
outward to focus.
As for eyepieces: You don't buy them. Well, you do, but
not the way you think. What you buy is an old pair of cheap
binoculars at a thrift store or garage sale and take them apart.
Instant eyepieces! They'll probably be too small in diameter
to ﬁt snugly inside the focuser tube, so wrap some cereal box
cardboard around the barrel until it's just right.
The ends of the Sonotube get pretty badly beat up just
from resting the scope on the ground before setting it up,
and from banging it into door frames on the way in and out
of the house. I don't know what, if anything, Dobson did to
protect the ends of the tube, but I discovered that ¼-inch gas
hose split lengthwise makes a dandy protective trim. Once I
was sure of its ﬁt, I glued it into place with Shoe Goo.
A lot of early Dobsonians didn't have ﬁnders. Users simply
sighted down the tube or along the edge of the tube box. But
I built a simple peep-sight ﬁnder for mine, reasoning that it
was consistent with the simple-and-cheap theme of Dobson's
design. A shower cap for a dust cover also ﬁts that theme.

People love it.

This scope gets the longest
line of any of them, õrst
to look at it, then to look
through it, again and again.

t A phonograph record
on Teflon pads
provides an excellent azimuth
bearing. Alas,
it's not Holst's
The Planets, but
it is a classic.

Constructing the Rocker Box
At this point you have a functioning telescope, but you
haven't yet built the actual Dobsonian mount.
The OTA mounts inside the rocker box by way of a plywood "tube box" that ﬁts snugly around the cardboard tube.
Dobson was a fan of scrap plywood, and this scope was built
to spec: ¾-inch thick and fairly smooth on one side, but
rough as a cob on the other. That helped immensely with
gripping the tube and holding it ﬁrmly in place.
The altitude bearings are sewer pipe end caps resting on felt
pads. No form-ﬁt circular cutouts here, just a V-shaped notch.
The rest of the mount is built of the same rough plywood.
Lots of it. And its design is as brilliant as the OTA. For one
thing, seven of the nine plywood slabs that comprise the
mount are the same size, making cut-out and construction
as simple as you could ask for. Two of those slabs glued and
screwed together make up the ground board, while two more
make up the bottom of the rocker box. (Can you say "Rigid"?)
Two more make up the side boards and another makes the
front board. Add two more narrower, tall "cradle boards"
with V-shaped notches for the altitude bearings to rest in,
and you're done with the rocker box.
One major advantage to this design is that you can
customize the cradle boards all day long without having to
rebuild the rocker box. If you need more room for the bottom

t The rocker
box and ground
board are made
of seven identical rectangular
slabs of plywood, plus two
upright cradle
boards.

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Sky & Telescope - April 2020

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