Sky & Telescope - July 2022 - 61

oxide after 220. The size of the particles
is measured differently, so you move on
to 25-micron, then 15-micron, and then
9-micron aluminum oxide. (Some people
proceed all the way to 5-micron, but that's
not necessary.) Alox, as it's sometimes
called, is a gentler abrasive. The particles
are more plate-shaped, shearing off high
spots rather than gouging out pits. It gets
you to the final stages of smoothness a lot
faster than carborundum will.
WEAR AND TEAR Silicon carbide grit starts out
sharp and pointy (left) but quickly wears down into
rounded particles (below).
Polishing the Mirror
When you get to 9-micron grit, the mirror
floats back and forth across the tool with
just a whisper of resistance from the abrasive.
The surface looks smooth even under a
magnifying glass. But the glass still appears
frosted, and in terms of wavelengths of
light, the surface is still a cratered disaster.
It needs to be polished to crystal clarity
before it can be used in a telescope.
That step is done with a pitch tool.
Right: pitch, as in boiled tree sap. There's
also synthetic pitch (called AccuLap),
which has essentially the same characteristics as the natural
stuff. We either pour a quarter-inch or so of it over the
grinding tool, or we make a new hydrostone disk to pour it
on (the preferred method in case you need to go back to the
grinding tool later to remove a scratch).
The pitch tool is used with a very fine abrasive, cerium
oxide, that embeds itself in the pitch and both wears away
and pushes around glass molecules to smooth the surface.
Channels cut or molded into the pitch lap let the polishing
compound (a mixture of cerium oxide and water) sluice
around during polishing.
Parabolizing
At the end of the polishing stage, you'll
have a nearly perfect section of a sphere
with a radius of curvature (the distance
from a curved surface to its imaginary
center point) that's twice the focal length
of the mirror. Why a sphere? Because that's
the only geometrical figure that allows you
to slide one surface over another without
high and low spots interfering with one
another. If anything sticks out, it quickly
gets worn down. So, as you do the fine
grinding and polishing, you're wearing
down all the high spots until your mirror is
nearly perfectly spherical.
When I say " nearly perfect, " I really
mean it. If you're careful with your technique,
you'll be within a tenth of a wavelength
of light or so of perfection. All done
by hand with tools and techniques that
have changed very little in 350 years.
Alas, a spherical surface makes a poor telescope mirror.
Incoming light that hits the outer part of the mirror focuses
much closer to the surface than light that hits near the center.
The focal point is smeared out into a focal line. In order
to bring all the incoming light to a focus at the same place,
the surface has to be changed from a spherical section to a
paraboloid.
The problem is, the geometry of pushing one disk back and
forth over another doesn't make paraboloids. Well, okay, it
sort of does, because that's how we do it, but at this stage we're
not making the paraboloid as the inevitable result of geometry
SLIGHT CHANGE One cubic inch of glass
is a lot to remove by abrasion. You'll remove
five of these if you make a 10-inch f/5 mirror. A
30-inch f/3? 220 of 'em.
ROUGH SURFACE Coarse grit leaves deep
pits in the glass that require successive wets of
finer abrasives to smooth out.
FINE POLISH Aluminum oxide grit particles
are more plate-like, so they shear off high spots
rather than gouging into the mirror's surface.
sk yand tele scope .o r g * JULY 2022 61
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Sky & Telescope - July 2022

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