Frequently asked questions

Are the telescope domes cooled inside?

The air inside the telescope domes must be kept close to the expected nighttime temperature during the day. If the air inside the dome warms during the day, it will rise and escape out of the open shutter of the dome at night when the outside air is cooler -- directly in front of the telescope. This would cause distortions of the light that enters the dome before it can be collected by the telescope, seriously degrading the image of the distant objects under study. In addition, warm air inside the dome can heat the telescope itself, including the mirror it uses to gather the light. While a few degrees may not seem like much of a temperature difference, telescope mirrors are polished to a perfect shape to within a few millionths of an inch across their entire surfaces. Heating the glass of a telescope's precision optics by even a few degrees can seriously distort the figure and render the resulting images unusable.

The domes of the Mount Wilson telescopes don't have active air conditioning, however. The largest domes -- those of the 60-inch and 100-inch telescopes -- are constructed of two walls separated by a large space. The outer wall of the dome acts to shield the inner dome from sunshine and the space between the domes acts to insulate the inside of the dome from warmer outside air. The domes are surprisingly cool to visitors that enter them at mid-day. In the late afternoon, the domes are opened toward the east (away from the setting Sun) to allow the inside air to equalize in temperature with the outside air. Large temperature changes from one night to the next can require extra ventilation time. Large fans are placed in the walls of the domes to expel warm air and speed the ventilation process, when necessary. Some other large observatory telescopes do have active air conditioning systems controlling the temperature inside their enclosures but so far it has not been necessary at Mount Wilson.