Sky & Telescope - September 2016 - 66

Jerry Oltion
Astronomer's Workbench

Contemplating Collimators
Make high-precision tools from plastic piping and underwear.
If you own a

Newtonian telescope, you need a collimator. It can be as simple as a focuser plug with a hole
in the center, or as complex as a combination Cheshire
sight tube with crosshairs and an angled reflecting plate.
It might even be a laser mounted so its beam shines
precisely down the optical axis of the scope. Most people
think precision tools like the latter two need to be manufactured under exacting standards in machine shops
with CNC milling machines accurate down to the gnat's
whisker, but that's not so. You can make a perfectly good
collimator in your home shop with little more than a saw
and a hand drill. Here's how:
Start with a 1¼-inch tube. If you're making a simple
collimation cap, cut the bottom off an old 35mm film
can, drill a 1/8″ hole in the middle of the lid, and you're
done. If you're going for a Cheshire or a laser collimator, buy a plastic drainpipe from the plumbing section
of your local hardware store. Make sure you get the
1¼″ outside-diameter pipe, not the 1½″. You might find
chrome ones, too, but they're a pain in the patootie to
work with. Get the plastic kind.

JERRY OLTION (2)

Homemade tools: a collimation
cap, two Cheshire sight tubes, and
a laser collimator.

66

September 2016 sky & telescope

Lining up the laser is the most finicky part, but with a
V-shaped notch and a little patience it's not all that difficult.

When making a laser collimator, find a small laser that
will fit inside the pipe, and cut the pipe so there's enough
length for the laser and three or four inches more.
To make a Cheshire eyepiece, cut the pipe to the same
f/ratio as your scope. (For an f/5 scope, make your tube
5× as long as it is wide.) If you do that, you'll be able to
see your entire primary in the secondary and also center
the secondary perfectly inside the Cheshire's outermost
circle. Don't obsess about the length, though; it'll work
fine if you're off a bit.
Both the Cheshire and the laser need a 90°° notch in
the tube, extending only halfway through. Again, don't
obsess about getting a perfect 90°. Anything close will
do. For the Cheshire, cut the notch close to the eyepiece
end. For a laser, leave room for the laser module.
Cut a white cardboard oval to fit inside the tube along
the eyepiece/laser side of the notch. A business card will
work perfectly. Make it 1.4 times as long as it is wide
and nibble away at the edges until it fits snug all the way
around. Then use a sharp knife or a hole punch to nibble
out another small oval at the center of the big oval. Make
it about ¼″ wide and 3/8″ long. Get this as centered as
you can, then glue the oval into place.
For the combination Cheshire sight tube, put a cap on
the eyepiece end with a 1/8″ hole in the center of it. Put
crosshairs at the other end.
Yeah, crosshairs. Crosshairs are easy! Don't mess
with wires or string; those always sag. Instead, pull a
piece of elastic out of the band of a pair of old underwear. You'll have to unwind the thread covering, but



Sky & Telescope - September 2016

Table of Contents for the Digital Edition of Sky & Telescope - September 2016

 Contents
Sky & Telescope - September 2016 - Cover1
Sky & Telescope - September 2016 - Cover2
Sky & Telescope - September 2016 - 1
Sky & Telescope - September 2016 -  Contents
Sky & Telescope - September 2016 - 3
Sky & Telescope - September 2016 - A
Sky & Telescope - September 2016 - B
Sky & Telescope - September 2016 - 4
Sky & Telescope - September 2016 - 5
Sky & Telescope - September 2016 - 6
Sky & Telescope - September 2016 - 7
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