Frequently asked questions

From atop Mount Wilson, one has a wonderful bird's eye view of the lights of Los Angeles and the surrounding cities. It's a beautiful scene, but it also shows how much light is being wasted by the citizens of the cities below. Much of the light meant to illuminate streets is shining up into the sky and into space where it is seen only by astronauts. From within large cities, the Milky Way is no longer visible and stars seem to disappear altogether in the most densely populated regions. Because Mount Wilson is so near the growing city below, the sky above the observatory is not as dark as when it was founded almost 100 years ago.

While the mountain location is much darker than the cities below, Mount Wilson still suffers from light pollution compared to more remote locations far from any city lights. Fortunately, while casual stargazers and amateur astronomers are severely impacted by light pollution, many professional research projects are affected little, if at all, by the dazzling lights of civilization below.

There are many criteria for choosing a site for astronomical research and none can meet the requirements of every research project. From the first time Mount Wilson was visited by astronomers during the 19th century the mountaintop has been known for having unusually steady air above it. The motion of the air above us is what causes the apparent twinkling of starlight that you see when you gaze at the night sky. Stars don't actually twinkle -- photographs taken from space show the stars as steady points of light. Only in the last few dozen miles of the starlight's long journey to Earth is the light distorted as it traverses the dynamic ocean of air that blankets the Earth.

At Mount Wilson, the air is steadier on average than at any other location in North America. This allows projects that require high resolution observations (detecting very fine detail on the object being studied) to operate more often -- and with better results -- than at most other observing locations. With the introduction of adaptive optics -- a system designed to correct for the distortion in starlight caused by the atmosphere's motion -- it would seem that the problem has been solved. But relatively steady air is still required for adaptive optics to function properly, and Mount Wilson has achieved among the best results with its adaptive optics system on the Hooker 100" telescope. In addition, by using adaptive optics to concentrate a star's light into a smaller area by removing the quivering that usually smears the image, stars stand out in greater contrast to the background light of the sky. The effect of light pollution is thus diminished when an adaptive optics system is used.

Interferometry is another field of study that requires steady air. Mount Wilson has been home to the earliest and the best of these specialized instruments. Designed to use multiple telescopes to detect finer details than can be seen with a single telescope, steady air is needed in order to obtain the best results these systems can produce. The world's largest optical interferometer -- the CHARA array -- has been built on Mount Wilson recently to take advantage of these superb conditions.

Most research programs that study stars are affected very little by ambient light. Light pollution puts a limit on the faintest stars that can be observed but many projects don't need to observe very faint stars. The H-K project -- a 50-year program begun at Mount Wilson in the early 1970's to study activity cycles on the surface of other stars similar to our Sun -- is virtually unaffected by the increase in light pollution in the last 30 years. And projects that study the infrared radiation from stars are virtually unaffected by the glow of the city lights. The city lights that appear so bright to us actually emit very little infrared radiation so the sky still appears quite dark as seen by the infrared "eye" of a telescope. Solar research also continues to play a large role in the observatory's activities, a daytime pursuit that is not affected at all by the nighttime city lights.

The programs that are most affected by the brightness of the sky are those that study extremely distant, diffuse objects such as galaxies and nebulae. These projects are generally carried out at isolated, dark observing sites during the dark phases of the Moon. Some deep-sky projects have been conducted at Mount Wilson in the past -- most notably the work of Edwin Hubble and Milton Humason on the expansion of the Universe . But this type of research has never been the primary thrust of the observatory's efforts, nor was it intended to be. While it would be nice for the lights of the cities to stop shining into the sky (thus saving energy and money for Southern Californians), the work at Mount Wilson Observatory continues despite the limitations imposed on some types of research. And when the "marine layer" rolls in -- a low-lying layer of thick clouds from the ocean that covers the Los Angeles area -- the sky can get quite a bit darker. This weather phenomenon isn't predictable enough to include in observing plans so deep sky projects still go elsewhere, but it's nice for the mountain staff to occasionally elude the lights of the cities below and see the sky in all its glory.