Raytracing makes the design easier, but the building is still as tricky as ever.

A 10″ Telescope, Because You Only Live Once

September 16, 2025 by Tyler August 21 Comments

Why build a telescope? YOLO, as the kids say. Having decided that, one must decide what type of far-seer one will construct. For his 10″ reflector, [Carl Anderson] once again said “Yolo”— this time not as a slogan, but in reference to a little-known type of reflecting telescope.

Telescope or sci-fi laser gun? YOLO, just try it.

The Yolo-pattern telescope was proposed by [Art Leonard] back in the 1960s, and was apparently named for a county in California. It differs from the standard Newtonian reflector in that it uses two concave spherical mirrors of very long radius to produce a light path with no obstructions. (This differs from the similar Schiefspiegler that uses a convex secondary.) The Yolo never caught on, in part because of the need to stretch the primary mirror in a warping rig to correct for coma and astigmatism.

[Carl] doesn’t bother with that, instead using modern techniques to precisely calculate and grind the required toric profile into the mirror. Grinding and polishing was done on motorized jigs [Carl] built, save for the very final polishing. (A quick demo video of the polishing machine is embedded below.)

The body of the telescope is a wooden truss, sheathed in plywood. Three-point mirror mounts alowed for the final adjustment. [Carl] seems to prefer observing by eye to astrophotography, as there are no photos through the telescope. Of course, an astrophotographer probably would not have built an F/15 (yes, fifteen) telescope to begin with. The view through the eyepiece on the rear end must be astounding.

If you’re inspired to spend your one life scratch-building a telescope, but want something more conventional, check out this comprehensive guide. You can go bit more modern with 3D printed parts, but you probably don’t want to try spin-casting resin mirrors. Or maybe you do: YOLO!

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Making Optical Glass From Ceran Stovetops

June 28, 2025 by Maya Posch 2 Comments

The Ceran discs, freshly cut from the old stovetop and awaiting polishing. (Credit: Huygens Optics)

Ceran is a name brand for a type of glass ceramic that has a very low coefficient of thermal expansion (CTE). This is useful for stovetops, but it is also a highly desirable property for optical glass. The natural question: Can an old ceramic stovetop be upcycled into something visually striking? This is the topic of the most recent video in [Huygens Optics]’s series on glass ceramics.

Interestingly, by baking sections of the Ceran glass ceramic for 10 minutes at 961 °C, the CTE can be lowered by another five times, from 0.5 ppm / °C to a mere 0.1 ppm / °C. Following baking, you need a lot of grinding and polishing to remove any warping, existing textures, and printing. After polishing with 220 grit by hand for a few minutes, most of these issues were fixed, but for subsequent polishing, you want to use a machine to get the required nanometer-level precision, as well as to survive the six to eight hours of polishing.

Following this final polishing, the discs were ground into mirrors for a Newtonian telescope. This raised a small issue of the Ceran being only 4 mm thick, which requires doubling up two of the discs using a very thin layer of epoxy. After careful drilling, dodging cracked glass, and more polishing, this produced the world’s first ceramic stovetop upcycled into a telescope. We think it was the first, anyway. All that’s left is to coat the discs with a more reflective coating and install them into a telescope frame, but even in their raw state, they show the potential of this kind of material.

If you decide to try this, and you’ve already cut up your stove, you might as well attack some kitchen bowls, too.

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A Giant Magellan Telescope Needs Giant Mirrors

November 24, 2017 by Gerrit Coetzee 47 Comments

The Giant Magellan Telescope doesn’t seem so giant in the renderings, until you see how the mirrors are made.

The telescope will require seven total mirrors each 27 feet (8.4 meters) in diameter for a total combined diameter of 24.5 meters. Half of an Olympic size pool’s length. A little over four times the diameter of the James Webb Space Telescope.

According to the website, the mirrors are cast at the University of Arizona mirror lab and take four years each to make. They’re made from blocks of Japanese glass laid out in a giant oven. The whole process of casting the glass takes a year, from laying it out to the months of cooling, it’s a painstaking process.

Once the cooling is done there’s another three years of polishing to get the mirror just right. If you’ve ever had to set up a metal block for precision machining on a mill, you might have an idea of why this takes so long. Especially if you make that block a few tons of glass and the surface has to be ground to micron tolerances. A lot of clever engineering went into this, including, no joke, a custom grinding tool full of silly putty. Though, at its core it’s not much different from smaller lens making processes.

The telescope is expected to be finished in 2024, for more information on the mirror process there’s a nice article here.