magnilux - Astronomy Technology Today

THE “HOME DEPOT” SCOPE
Image 3a-3c: A movable battery box serves as both power source for the cooling fan and adjustable counterweight for balancing the
heaviest eyepieces. Details of the primary-mirror cell are available at www.rfroyce.com.
stronger adhesive than glue.
Next, I built the mirror mount using
guidance from Bob Royce’s web site.
Shower curtain rods were then cut to size,
adding about two extra inches to allow for
possible errors in my tube length calculations.
They were placed in the holes of the
mirror tube section end-rings and secured
with screws. At this point, I coated both
cardboard tube sections, the mirror
mount and end plate, and the end rings
on the mirror tube section with epoxy
resin. End-rings on the front tube section
were left uncoated. When the resin was
dry, I installed the mirror, diagonal assembly,
and focuser. The front tube section
was then slid into position on the
rods but was not secured, allowing for adjustment
of the OTA length.
With mirror, diagonal, and an eyepiece
in place, the telescope was pointed at
Polaris. The front tube section was then
moved back and forth until the focal
plane was just where I wanted it in the focuser
tube. Then it was back to the garage,
to lock the forward tube section in place
with screws, and apply epoxy to the end
rings. This pretty much completed the
assembly of the OTA, except for two
additional small aluminum tubes to provide
extra stiffness in the altitude plane.
That may have been overkill, but it
seemed worthwhile to invest an additional
five dollars to add more strength to the
structure. There has been no detectable
flexure of the tube, regardless of eyepiece
weight.
Design and construction of the
mount was “Dob-basic,” with two exceptions:
where the heavier 3/4-inch plywood
was used, I made as many weight-saving
cut-outs as I could without sacrificing
much stiffness; and support boards for the
altitude bearings are separate and removable
from the box. This allowed me to use
lighter-weight 1/2-inch plywood for the
box, and gave me the option of changing
the size or configuration of the altitude
bearings if I wanted to, for any reason. In
retrospect, this change from a basic box
was probably unnecessary. But I’m pleased
that the result is a very steady mount that
weighs only fifteen pounds. And the largediameter
altitude bearings provide smooth
motion with relative insensitivity to
balance.
Some Accessories
A recent addition to the mount
(Image 2) is a set of simple and cheap
wheelbarrow handles and wheel sets that
allow the scope to be moved as a single
unit. Materials included two 7-inch lawnmower
wheels with mounting bolts, two
1-inch by 2-inch pine boards, two small
pieces of scrap 1/4-inch plywood, four
hooks that screw into the handles, and
four eyes that screw into the sides of the
box. By mounting the wheels on the outside
of the handles and the hooks on the
underside of the handles, torque keeps the
handles pushing firmly against the side of
the box. Works well, was easy to make,
and cost about twenty dollars. The 1-inch
by 2-inch boards may seem a little small,
Astronomy TECHNOLOGY TODAY 65