Reflections

My Uncle Arch was a serial hobbyist. He would get deeply interested in one thing for a while, then move on to something else. Arch pursued ham radio, violin, square dancing, RC airplanes, sports cars, real airplanes, and long-distance motorcycle touring. Also astronomy.

Arch once explained the difference between refractive and reflective telescopes. The former works like binoculars in that you peer through a lens that captures and focuses light directly to an eyepiece. The power of a refractive telescope is limited by the size of its lens and, therefore, the amount of light it can capture. Reflective telescopes use mirrors to capture light over a much larger area and bounce it (via secondary mirrors) to an observer. The primary mirrors can be big, and they can be clustered to form an even larger area.

“Why”, you may be asking yourself, “for the love of God, is he telling me this?”

Because while visiting Tucson, we toured the epicenter of large mirror research and technology, the Richard F. Caris Mirror Lab, found at the University of Arizona. The largest mirrors ever created are made under the bleachers at the football stadium (Go, Wildcats!).

Our visit began with a one-hour talk on mirror technology and the history of the program. It was much better than it sounds.

Modern reflective telescopes use multiple mirrors. The Lab is currently building seven for the Giant Magellan Telescope, each about 28 feet in diameter. That’s an illustration of the GMT at the top of this post. Sorry for the crappy photo – light reflections on the glass display case.

The mirrors my Uncle Arch made by hand for his telescopes were small, solid glass, and under a foot in diameter. There’s no way you could make a 28-foot mirror out of solid glass. It would be immensely heavy and would self-destruct. (One cubic foot of optical glass weighs about 140 pounds, or about 64 kilograms.)

Dr. Roger Angel, while an ASU grad student, figured out how to build a lighter mirror based on the structure of honeycombs. It takes over four years to produce a single mirror.

The big red thing is a heater. Water-soluble honeycomb forms are placed in the frame, then blocks of glass are added. The whole assembly is brought to 1100 degrees Fahrenheit and rotated at between three and eleven RPM. Why rotated? Rotation causes the molten glass to form a parabola (higher around the edges, lower in the middle.) The faster the rotation, the deeper the dish. Forming a mirror requires a continuous million watts of electricity.

After a very long cooling period (the temperature is lowered about 5 degrees per day), the mirror is moved to this station, where it is lifted upright to expose the underside. High-pressure water is used to blast away the honeycomb forms.

The flat red platform in the middle distance is used to move the mirror from station to station. It rides on a cushion of air that raises it about an inch off the ground. Like playing air hockey with a multi-ton puck.

The Neff Test Tower does not, despite its name, test for Neffs. You can’t see all of it because it’s about 100 feet tall. Lasers are used to look for imperfections – high and low spots – on the mirror surface. The final product will conform to its desired shape within one-millionth of an inch.

Want to know how large a millionth of an inch is?

Take a human hair and use a magic knife to split it in half lengthwise. Now take one of the halves and split it again. Now repeat this process 2,498 more times. The final cut gives you a hair fragment that is one-millionth of an inch wide.

The folks running the operation.

The white disc is a mirror, partially polished. The blue machine and its companion beyond the mirror do the work.

All of the devices you’ve seen – the furnace, the mirror-moving platform, the test tower, and the polishers were invented and built at the University of Arizona. If you’d like more information about the process, check out their FAQ.

Someday, this mirror and its six siblings will be placed in carriers that will be welded into place aboard ships bound for Chile. Why welded? To eliminate the possibility of shifting during transport.

Should you find yourself wandering the Chilean high desert in a few years, you may find this mirror exploring the universe as a part of the Great Magellan Telescope.

Refractive vs. Reflective
Refractive and reflective telescopes both bring distant objects into view, but they differ in how they collect and focus light: refractive telescopes use lenses, while reflective telescopes use mirrors.

How refractive telescopes work
A refractor gathers light through a large front lens (the objective lens), which bends (refracts) the light toward the back of the tube, where a smaller eyepiece lens magnifies the focused image for your eye or camera. Because the optical path is straight through clear glass, the tube is usually sealed, making refractors robust, low‑maintenance, and thermally stable.
Refractors are known for sharp, high‑contrast views and are especially good for lunar and planetary observing, as well as wide‑field astrophotography. However, they can suffer from chromatic aberration in cheaper designs (color fringes around bright objects), and large‑aperture refractors are expensive and long, which limits their portability.

How reflective telescopes work
A reflector uses a curved primary mirror at the back of the tube to collect and reflect light toward a smaller secondary mirror near the front, which then bounces the light sideways to the eyepiece (Newtonian design) or back through a hole in the primary (Cassegrain‑type designs). Because they rely on mirrors rather than lenses, reflectors avoid chromatic aberration and can be built with much larger apertures for the same cost.
Reflectors excel at gathering light, so they are popular for deep‑sky objects such as galaxies and nebulae. However, they typically have an open tube, which can collect dust and require more frequent cleaning; the mirrors also need periodic collimation (alignment) to keep the optics optimized.

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