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Pushing Glass: How to Grind Mirrors
Grinding your own mirror can be great fun - and result in an excellent telescope.
The post Pushing Glass: How to Grind Mirrors appeared first on Sky & Telescope.
Equipment: Guides & Recommendations
Pushing Glass: How to Grind Mirrors
By:
February 20, 2026
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Grinding your own mirror can be great fun – and result in an excellent telescope.
[Jerry and Kathy Oltion using Hydrostone Tools, Colin Miller using ring tool | Sky & Telescope]
*DIFFERENT STROKES Jerry and Kathy Oltion (left and top) work on their mirror blanks using hydrostone tools. Colin Miller (right) hogs out his mirror using a ring tool.
Jerry Oltion; Glass disks: ohlee7481 / shutterstock.com*
I often tell people that making your own telescope mirror is easy: You just put two round pieces of glass together with grit in between them and push the top one back and forth over the bottom one about a million times, and voila, you’re done.
That turns out to be not entirely accurate.
The actual number of strokes is a lot fewer, probably only 300,000 for a typical mirror. And I left out a few steps.
In my article on whether it was cheaper to buy or to build a telescope (S&T: Nov. 2021, p. 66), I discussed mirror making as part of the process, but if you decide to actually do it, how do you proceed? Let’s get into the nitty-gritty — literally.
Here’s the short version: You really do put the soon-to-be mirror (called a blank) on top of the grinding tool and push it back and forth a lot. Whatever is on top slowly becomes concave, and whatever is on the bottom becomes convex. Once you dig it as deep as you want, you use finer and finer grits to undo the damage you did with the coarse grit, then switch to a polishing tool and continue pushing it back and forth until you’ve got a smooth surface again. Then you change your polishing technique to shape the surface into a parabola, coat the mirror with aluminum or silver, and put it in a telescope.
Now let’s take a closer look at each of those steps.
Rough Grinding
First off, how does pushing one piece of glass over another one make the top piece concave and the bottom one convex? Look at the illustration at the top-right. When the top piece is directly over the bottom piece, the grinding action is uniform across the entire surface, but when you push the top piece off-center, the action reduces to an ever-narrowing, football shaped area of contact. Because the grit has less area to work on, but the weight pushing the two pieces together is more or less constant, the pressure per unit of area increases dramatically as the two pieces slide more and more off-center.
HOMEMADE TOOLS Top: To make a tile tool, lay a tile mat on top of plastic wrap over the mirror blank, add a dam of paper or tape around the edge, and pour waterproof plaster (hydrostone or similar) over it. Bottom: *When the hydrostone has set enough to remove but is still soft, separate the tool from the mirror, dig out the channels between the tiles, and you’re ready to start.
Jerry Oltion*
Because you’re pushing down evenly on the top piece, most of that extra pressure is directed toward the center of the top piece and toward the edge of the bottom piece. That means the center of the top piece wears faster than its edge, and the edge of the bottom piece wears faster than its center. You rotate the mirror and the tool in opposite directions every few strokes in order to keep the mirror’s curve symmetrical. (Grinding with mirror and tool in the same orientation creates a saddle shape — not what you want!) If there’s room, progressively walk around the mirror as you work it. A 55-gallon drum full of water makes a great workstation, which led to the term “once around the barrel” to refer to a complete revolution of tool and mirror with respect to each other.
[Hogging Illustration | Sky & Telescope]
*HOGGING OUT When the mirror is centered over the tool (left), abrasion is even across the entire surface. When the center of the mirror is over the edge of the tool (right), gravity and the pressure of your hand cause more wear on the center of the mirror and the edge of the tool.
Gregg Dinderman / S&T*
The illustration above is a bit exaggerated; the actual stroke you use only moves the mirror 1/6 of its diameter off-center in either direction, for a total stroke length of 1/3 the diameter. This is called the “One-third center-over-center stroke,” and you’ll become very, very familiar with it by the time you’re done.
This first phase is known as the rough grinding or “hogging out” stage of mirror making. You can simply use another blank as your tool, but nowadays most mirror makers use a tool made of ceramic tile rather than a second piece of glass for the bottom piece. Either way, the method is the same.
Tile tools are simple to make: Take a mat of one-inch bathtub tile and cut it into a circle the size of your mirror. (You can either cut the tiles with a tile saw, bust them with pliers, or even simply cut away any tiles that stick out over the edge of the mirror.) Next, place the tool face-down on a sheet of plastic wrap over the mirror, add a dam of tape around the edge of the mirror, and pour an inch or so of liquid hydrostone or dental stone (waterproof types of plaster) over the 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.
[Mirror-Grinding Kits DIY | Sky & Telescope]
*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.
Jerry Oltion*
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