Telescopes from the Ground Up
Image of a technician polishing a mirror blank during the era of great reflectors.

The glass ceiling

Yet by the end of the 1800s, the refractors had reached their peak. The lenses had reached 40 inches and could grow no larger for three main reasons:

First, the casting process used to create even bigger lenses introduced imperfections such as bubbles into larger pieces of glass, making them unusable in telescopes. Second, larger lenses had to be so thick that they absorbed much of the light they were collecting. That meant the image wasn’t any brighter than the one produced by the smaller mirrors of reflecting telescopes, and those mirrors were both less expensive and easier to build than glass lenses.

Finally, because light has to pass through the whole lens for a refracting telescope to be effective, the lens can only be supported by its thin edges. Since there is no support at the center, where the glass is thickest, the glass sags in the middle, deforming the lens. In any lens larger than 40 inches, the sagging would have been so great that the lens would have been useless for astronomy.

Astronomers stopped building large refractors in the 1890s. But before then and for some time afterward, refractors were the most popular instruments around. By the mid-1800s, 40 of 48 British observatories used refractors. Even today, the Yerkes Observatory continues to use its large refractor for astronomical studies.

In a nutshell...

New technology allowed astronomers to create larger lenses that produced bright, clear images. For a while, refracting telescopes became more popular than reflecting telescopes.

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