Refractors and Reflectors: Two Telescope Designs

Point a long brass tube at Jupiter on a clear night and you are using, in essence, the instrument Galileo turned skyward in 1609. Light from the planet enters a lens at the far end, bends as it crosses the glass, and converges to a sharp image you magnify with an eyepiece. This is a refracting telescope. Now imagine, instead, that the far end of the tube is sealed by a curved mirror. Light travels down the tube, bounces off the mirror, and is brought to a focus near the front, where a small secondary mirror redirects it to your eye. This is a reflecting telescope, the design Isaac Newton built in 1668 to escape a problem that had begun to haunt the refractor: chromatic aberration.

Chromatic aberration is the consequence of a basic fact about glass. Different wavelengths of light bend by different amounts as they pass through it — blue light bends more sharply than red — so a simple lens cannot bring all colors to the same focus. The result is a faint colored halo around bright stars and planets, a smearing of detail that worsens as the lens gets larger. Eighteenth-century opticians partially tamed the problem with the achromatic doublet, pairing two glasses with different dispersions so their color errors largely cancel. Modern apochromatic refractors, using exotic fluorite or extra-low-dispersion glass, push the residual color almost to nothing — but at considerable expense.

Mirrors sidestep the problem entirely. A reflective surface bends all wavelengths by the same angle, so a well-figured mirror produces an image free of chromatic aberration regardless of color. That single fact, combined with cost, has made reflectors the dominant design for serious astronomy. A glass blank can be ground and aluminized on its front surface; only one optical face needs to be perfect, and the glass behind it need not even be transparent. Lenses, by contrast, must be flawless throughout their volume and figured precisely on both surfaces, and they can only be supported around their edges. A meter-wide lens sags under its own weight; a meter-wide mirror can be braced from behind. Every research telescope larger than about a meter — and all of them larger than about 1.2 meters, the practical ceiling for refractors set by the Yerkes Observatory's 40-inch lens in 1897 — is a reflector.

The two designs trade other characteristics as well. Refractors have sealed tubes, so their optics stay clean and their internal air currents settle quickly, giving steady high-contrast views of the Moon, planets, and double stars. Reflectors are open to the air, and their secondary mirror sits in the light path on a small support called a spider, which diffracts starlight into the four-pointed spikes familiar from Hubble images. That diffraction reduces contrast slightly, which is why a good apochromatic refractor of modest aperture can still outperform a larger reflector on planetary detail. Reflectors, however, gather far more light per dollar, making them the natural choice for faint deep-sky objects: nebulae, galaxies, distant clusters.

Maintenance differs too. A refractor's lens, once aligned at the factory, generally stays aligned for life. A reflector's mirrors must be collimated — adjusted so their optical axes coincide — and the primary's aluminum coating eventually tarnishes and needs renewing every decade or two. The choice between designs is therefore not a ranking but a matching of instrument to task. A planetary observer who wants a low-maintenance, high-contrast view of Saturn picks a refractor and accepts the price. A deep-sky observer who wants photons from a distant galaxy picks a reflector and accepts the upkeep. Professional astronomy, where every available photon matters and apertures are measured in meters, has made its peace with the upkeep and not looked back since the nineteenth century.