Sky & Telescope - December 2020 - 74

ASTRONOMER'S WORKBENCH by Jerry Oltion

Ceramic Mirrors
Build a telescope from the ground up.

devil is in the details.
First, you have to choose which
kind of ceramic to use. Earthenware?
Stoneware? Porcelain? Each has its
advantages and disadvantages, although
porcelain comes out the clear winner
due to the fineness of its grain. When
fired hot enough, porcelain becomes
almost glass-like in its smoothness
and lack of internal voids. Plain white
pottery slip works, though, as Mark's
mirror demonstrates.
Then comes the mold. You could
just pour a flat, round slab and hog it
out like a normal mirror blank, or you
could honeycomb the back to reduce
weight and improve cool-down time.
Mark chose the latter, placing hexagonal blocks of plaster of Paris inside the
mold before he poured the slip over it,
then removing them once the slip had
stiffened, but not hardened.
A sufficiently gadget-happy person
could spin-cast a perfect parabola into
the surface while it's a liquid, but hogging out "greenware," as the dried-but-

T An aluminized ceramic mirror looks pretty much like any other from its reflective side.

74

DECEMBER 2 02 0 * SK Y & TELESCOPE

S The back of the ceramic mirror is honeycombed to reduce both its weight and cooldown time.

not-fired ceramic is called, is so simple
that spin-casting is hardly worth the
effort. Just use another greenware disk
as the tool and you'll have your curve
within minutes, no grit required.
Ceramic shrinks when it dries, and it
shrinks some more when it's fired. That
can distort its figure, so you have to
expect some fine-tuning of the curve in
the "bisque" (fired) stage. So, Mark fired
the greenware tool, too.
After he'd fine-ground the mirror
to the right sagitta (he went with a
10-inch f/4.8), Mark glazed the surface
and fired it again to melt the glaze. That
provided a smooth, glassy surface that
he could then fine-grind, polish, and
parabolize like a normal mirror.
A point worth making: At this stage,
Mark had put about $5 into the project.
The cost of the blank is insignificant!
You can make half a dozen of them and
choose the best one to finish out.
The mirror wasn't beautiful. The
glaze cracked, as large spans often do,
and those cracks affected the figure.
Mark continued to polish and parabolize it, and he got it to maybe half-wave
precision before he decided that was
good enough for a proof of concept and
sent it off to be coated.
He built a basic Dobsonian scope
to house the mirror and tried it out. It
wasn't the sharpest mirror ever made,
but it served pretty well, splitting doubles down to maybe four arcseconds and
providing pleasing views of galaxies and

JERRY OLTION (3)

ANYONE WHO'S GROUND MORE than
a couple of primary mirrors begins to
look at pretty much anything round
and hard as potential mirror material. I've seen mirrors made out of glass
tabletops, chunks of storefront door,
and even a granite paving stone.
And now, thanks to Oregon ATM
Mark Yonker, I've seen one made out of
ceramic. Not the glass-like Black Vitrified Ceramic that's been popular for
several years, nor Zerodur, which is a
"fully densified ceramic," but plain old
pottery-style fired clay.
Mark got the idea from Pat Cannon, his high school physics instructor,
but the idea goes back at least as far as
the 1970s (S&T: Oct. 1975, p. 259 and
March 1976, p. 202). Mark freely admits
he's not the inventor of ceramic mirrors
but is instead a pursuer of them.
The idea is fairly simple: Pour liquid
ceramic (called slip) into a mold, fire it
in a kiln, glaze it, and finish out the mirror's parabola on the thin glaze layer.
As with many simple concepts, the



Sky & Telescope - December 2020

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Sky & Telescope - December 2020 - Cover2
Sky & Telescope - December 2020 - 1
Sky & Telescope - December 2020 - Contents
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