SESKStuff Everyone Should Know
SESK: Stuff Everyone Should Know · sesk.info/astro-observatory-site-rules

What three things make the best sites for big optical telescopes?

High, dark and dry.

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  1. Low, dark and humid
  2. High, bright and wet
  3. High, dark and dry ✔

Why: Altitude puts the telescope above much of the air and water vapor, darkness avoids light pollution, and dry air absorbs less infrared. That is why giant telescopes sit in Chile, Hawaii and the Canary Islands.

Huh, didn't know that: Observatories are best placed at least 100 miles from the nearest large city.

The dad joke
Astronomers love a good night out, as long as it's dark, dry and up a mountain.
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References

Answer: High, dark and dry. Altitude puts the telescope above much of the air and water vapor, darkness avoids light pollution, and dry air absorbs less infrared. That is why giant telescopes sit in Chile, Hawaii and the Canary Islands.

  1. OpenStax Astronomy 2e, 6.2 Telescopes Today openstax.org/books/astronomy-2e/pages/6-2-telescopes-today
    The best observatory sites are therefore high, dark, and dry.
    Checked 2026-10-10.

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