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.