# Cooldown

Also called: thermal equilibrium, tube currents, cool-down.

Letting a telescope reach the outdoor temperature, so warm air inside it doesn't blur the view.

A telescope brought out of a warm house carries heat in its mirror or lenses. As it cools, warm air rises off the optics in shimmering currents that blur high-power views. Small refractors are ready in minutes; a big Dobsonian's thick mirror can take an hour or two — a fan behind the mirror speeds it up.

**At the eyepiece:** Put the telescope out at sunset and observe low-power objects first; save the planets for later in the night.

**Related:** [Seeing](https://astropilot.space/glossary/seeing.md), [Dew](https://astropilot.space/glossary/dew.md), [Dobsonian](https://astropilot.space/glossary/dobsonian.md)

From the AstroPilot astronomy glossary: https://astropilot.space/glossary/cooldown