Sky & Telescope - July 2022 - 60

Telescope Making
tile mat. Let it harden, dig out the channels between the
tiles, and there's your tile tool.
Hogging out can also be done with a small, round metal
tool on top of the mirror, often referred to as a ring tool. By
crossing the ring tool over the center of the blank with each
stroke, the tool spends more time in the center of the mirror
than on any given point on its edge, so the center wears
faster than the edge. An iron pipe flange works really well as
a ring tool.
How do you know when to stop hogging out? You decide
ahead of time what focal length you want your finished mirror
to be, and you calculate the depth of the required curve
(called the sagitta) with the following formula:
Sagitta = diameter
/(16 × focal ratio)
So, if you're making a 10-inch f/5 mirror, your sagitta is
10/(16 × 5) = 0.125 inch.
Seriously: You don't have to make a cereal bowl; an eighth
of an inch of depth in the very center, tapering off to nothing
at the edges, is all you need.
On the other hand, a little more complicated math reveals
that doing so removes almost five cubic inches of glass from
the blank. And you're doing that one microscopic chip at a
time. No wonder rough grinding takes several hours!
Fine Grinding
Hogging out is usually done with 80-grit carborundum
(silicon carbide) abrasive. When the pieces of grit tumble
around between the mirror blank and the tool, they gouge
out little bits of glass. You do this in a water slurry so the
grit can move around freely, and to minimize airborne glass
dust. (You really don't want to breathe glass dust.) The grit
particles quickly wear down, though - within just a few
minutes - so they need to be replaced. Each replacement of
the grit is called a wet.
Most telescope makers don't actually grind all the way to
their target sagitta with 80 grit. Such coarse grit leaves big
1
2
ONE-STOP PURCHASE Several online retailers offer mirror-grinding
kits. These packages come with all the grits you need, plus the tile mat
and the hydrostone to make the grinding and polishing tools. Not shown
here but included in a complete kit: a can of pitch.
pits in the glass, so you should stop a little short, say 80% to
90% of the way, and switch to 120 grit. (If you hogged out
with a ring tool, you'll need to make a tile tool at this stage.)
Grind for a couple of hours with that until you don't see any
of the 80-grit pits anymore, then switch to 220, and on to
finer and finer grits. With the medium grits, you can finetune
the focal length by flipping the mirror and tool over and
grinding with the tool on top. That will undo some of the
sagitta, flattening the curve if you went a little too far with
the coarse grits. The amount of correction you can make
becomes less and less with each successive grit, so you want to
be close to your final sagitta by the time you finish 220 or so.
In the old days, mirror makers used carborundum all the
way up to the end, but nowadays we switch to aluminum
3
WETTING THE TOOL The process of spreading grit and working the blank is called a wet. 1: Spreading grit on the tool is convenient using a
repurposed spice shaker - it doesn't take much for a single wet. 2: A spray bottle filled with water allows you to get the grit good and wet without
washing it away. 3: The back-and-forth motion helps the grit dig into the glass, removing it one tiny chip at a time.
60 JULY 2022 * SKY & TELESCOPE

Sky & Telescope - July 2022

Table of Contents for the Digital Edition of Sky & Telescope - July 2022

Contents
Sky & Telescope - July 2022 - Cover1
Sky & Telescope - July 2022 - Cover2
Sky & Telescope - July 2022 - 1
Sky & Telescope - July 2022 - Contents
Sky & Telescope - July 2022 - 3
Sky & Telescope - July 2022 - 4
Sky & Telescope - July 2022 - 5
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