Mirror Making Myths
and Other Things to Know About Mirror Making
for the Curious, Inquisitive Non-Mirror Maker
For many years the Springfield Telescope Makers have held mirror making
demonstrations at the annual Stellafane Convention. Anyone who has staffed this
demonstration is peppered with hundreds of questions from the curious and inquisitive
visitors, many who have never made a mirror, and (sadly) many who never will. This
page attempts to answer those questions we have heard many times, to satisfy your
curiosity, and perhaps convince you that mirror making is not as hard, long, expensive or
incomprehensible as it may seem at first glance.
Mirror Making Myths
- Mirrors can only be made with Special Precision Equipment
in Expensive Factories or
Labs
Not True! Telescope mirrors can be made in your kitchen,
basement or garage with nothing more than the mirror blank and a second piece of glass
(called the tool) to rub the mirror against with an abrasive grit and water slurry.
You can do this on a wet towel on your kitchen counter, on a board with a few cleats
screwed to in on your lap, or the traditional way on a barrel or pedestal. You need no
special tools whatsoever to grind the mirror. Some sort depth gauge is helpful; a steel rule and
some feeler gauges (buy this at an automobile parts store) will work well.
Most professional and some amateurs do use machines, but these are simple, rotating
spindles to supply the energy for grinding from a motor and not your muscles. You may
notice that many commercial telescope advertisements proclaim they use "Hand Figured
Optics" - in fact this is one of the best and least expensive ways to get an accurate surface on a mirror -
most machines just can't hold the tolerances necessary, and any good telescope optic, including
the mirror you could make in your kitchen, should be hand figured.
- I need a Well Equipped Shop to Make a Mirror or Telescope at Home
Not True! As you have read above, you really need no tools to
make a mirror at home. If you want to build a telescope, you will need some simple hand
tools - a hand saw, a drill, ruler, hammer and screwdriver - but nothing exotic or
terribly expensive. Power tools are completely optional. Good, solid, useful telescopes
can be built by novice amateurs.
- Making a Mirror takes Hundreds of Hours of Work
Not True! A 6" or 8" mirror should take about 40 to
80 hours of effort. While this is a non-trivial amount of time, the mirror can be
completed in a few months, and certainly within a year. Most people work one to three
hours a day on their mirror about one or two times a week.
- Homemade Mirrors are of Inferior Optical Quality
Not True! In fact, many amateurs choose to make their own
mirrors to get Superior optical quality. Figuring optics is
labor intensive (which is why good optics cost so much), and some firms choose to stop
figuring at "good enough". The amateur can choose to spend a little more
time to make great optics.
Questions from the Curious
As we explain in our mirror making demonstrations at the Stellafane Convention, the
basic mirror making process involves grinding the mirror blank into a sphere with desired
radius of curvature for the target focal length. We then polish this sphere smooth, and
then slightly alter it into a parabolic curve.
- How do two Flat Pieces of Glass grind into Spheres?
With the Mirror on top, the tool on the bottom, and coarse grit in between, we start
grinding with the mirror overhanging the tool. We rub the center of mirror against the
edge of the tool, and this wears a depression in the center of the mirror (the edge of the
tool wears down also, so that the tool becomes convex (a "hill" in the center)
and the mirror becomes concave (a shallow "bowl").
As grinding progresses, we grind with more center-over-center strokes, which will cause
the mirror and tool to become matching spherical surfaces.
- How do you know you will get a Sphere when you Grind?
We know from geometry that the only shape that fits tightly together when moved
randomly in all directions is a sphere (note that a flat surface is just a sphere with
infinite radius). Because we work hard at rotating both the tool and mirror randomly, and
randomly varying the stroke length, we are very certain to get a spherical surface or be
very close to it when we finish grinding.
The random strokes and rotations assure that all the high spots will meet and
abrade over time, and the low spots will not grind because there is no pressure on them.
Making spherical surface is easy because you get them "automatically" with
random strokes.
- Why don't you grind the Parabola Directly and skip the Sphere?
Because this is very difficult to do. The deviation of the surface of the desired
parabola from that of the sphere is very slight, and it is much easier to work with a
sphere through most of the process because it is the "natural" surface shape
that you get easily with careful work. Parabolizing from a sphere as the last step is
quick and direct.
- Why do you want a finish with Parabola?
A parabola is the only surface that will bring rays of light that originate at
"infinity" (all astronomical objects are considered to be at optical infinity)
to a common focus. A sphere, or any other shape, will not. In order to get a well focused
view, you need to figure your mirror into a smooth parabola.
- How can you tell how good a mirror is and what shape its surface has?
While there are many ways to test optics, the most popular with amateurs is the
Foucault knife-edge test. A beam of light is shown on the mirrors surface, and the
reflected rays are returned to pass by a knife edge. Looking at the mirror past the knife
edge, one sees shadows and bright spots that indicate the shape and smoothness of the
mirrors surface.
A knife edge tester can be built at home using wood scraps and common hardware store
items with a hand saw, drill and screwdriver. Using threaded rod and a large cardboard
dial, one can measure the movement of the knife edge to about 1/1000 of an inch. Using
published formulas or one of many freely available computer programs, the knife edge
position numbers can be reduced to indicate the shape of the mirror's surface to within a
fraction of the wavelength of light, or a few millionths of an inch. Many people
find this hard to believe, but it is true, that with hand work and a simple tester you can
figure optical surface to these amazing accuracies.
- How do you make the Mirror Shinny?
It is possible to deposit a silver coating on your mirror at home, but this involves
some dangerous chemicals and silver coatings tarnish quickly. Most amateurs and
professionals today have a reflective aluminum coating deposited by a commercial coating
company. This can be done by mail in a few weeks.
- Why would grind a mirror when you can buy one just as cheaply?
When the North American amateur telescope making movement began in the 1920's,
there were few alternatives - the only commercial telescopes where extremely expensive and
often where individually commissioned. Making your own was the only way for most people.
Today, commercial telescopes are affordable, as well as optical components including
paraboloidal mirrors. All things considered, the cost difference between making and buying
is very small. There appear to be three main reasons people make their own:
- SATISFACTION: Many people derive great satisfaction and pride from
building something themselves. And building a great telescope that will will give years of
enjoyment will be a source of continuing pride and satisfaction.
- KNOWLEDGE: By making your own mirror and telescope, you will learn quite
a bit, including how to keep your telescope in tip-top shape.
- QUALITY: You have complete control over the quality of optics, and many
people build their own because they can get a much higher quality instrument than
otherwise.
For more detailed information, please read the rest of our
ATM Web Pages
or consult one of the Books listed in our Reference Section.
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