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SESK: Stuff Everyone Should Know · sesk.info/astro-chromatic-aberration

What lens flaw makes each color focus at a slightly different spot in a refracting telescope?

Chromatic aberration.

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  1. Spherical aberration
  2. Chromatic aberration ✔
  3. Atmospheric seeing

Why: Glass bends each wavelength by a slightly different amount, so colors focus at different distances and the image blurs. Mirrors reflect every color the same way, so reflectors avoid this problem.

Huh, didn't know that: Big lenses also sag under their own weight, because they can only be held at their edges.

The dad joke
My refractor gave every color its own focus. Talk about a lens with commitment issues.
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References

Answer: Chromatic aberration. Glass bends each wavelength by a slightly different amount, so colors focus at different distances and the image blurs. Mirrors reflect every color the same way, so reflectors avoid this problem.

  1. OpenStax Astronomy 2e, 6.1 Telescopes openstax.org/books/astronomy-2e/pages/6-1-telescopes
    known as chromatic aberration . Each wavelength focuses at a slightly different spot, causing the image to appear blurry.
    Checked 2026-10-10.

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