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SESK: Stuff Everyone Should Know · sesk.info/astro-largest-refractor-yerkes

How big is the largest working refracting telescope, at Yerkes Observatory?

40 inches (about 1 meter), completed in 1897.

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  1. 40 inches ✔
  2. 200 inches
  3. 10 meters

Why: Big lenses must be flawless all the way through and can only be held at their edges, so they sag. That limit near 40 inches is why every giant telescope since is a reflector.

Huh, didn't know that: A 49-inch refractor was built for the Paris 1900 Exposition, but it was dismantled afterward.

The dad joke
The Yerkes refractor has been the biggest since 1897. It really knows how to hold a lens.
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References

Answer: 40 inches. Big lenses must be flawless all the way through and can only be held at their edges, so they sag. That limit near 40 inches is why every giant telescope since is a reflector.

  1. OpenStax Astronomy 2e, 6.1 Telescopes openstax.org/books/astronomy-2e/pages/6-1-telescopes
    Currently, the largest refracting telescope is the 40-inch refractor at Yerkes Observatory in Wisconsin.
    Checked 2026-10-10.

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