Frequently asked questions

The two largest domes you see at Mount Wilson house telescopes that were completed in the early part of the twentieth century. Though they were the largest of their kind at the time, larger telescopes have been built in the intervening years. The telescopes at Mount Wilson continue to be very productive scientific instruments, however. While different materials and technologies are used in current telescope designs and construction, the early telescopes at Mount Wilson were designed to perform the same function as most of today's new telescopes -- to gather the light from distant objects and present it to the science instruments that analyze that light. Despite looking their age to the casual observer, these telescopes continue to do the job they were designed for, and new science instruments compliment the old workhorses to keep them at the forefront of astronomical research.

The massively built 60-inch and 100-inch reflectors -- completed in 1908 and 1917, respectively -- were fashioned of thick steel girders and castings riveted and bolted together, a construction method that gives them the appearance of battleships (the 100-inch was actually built by a ship yard). The mirrors were made from very thick and heavy solid discs of glass. Both of these choices in materials were meant to eliminate internal stresses and strains that might cause the telescope structure or mirror to flex as the telescope is pointed at different positions in the sky. The thick mirrors are also meant to decrease the known effect of changes in temperature on their precision reflective front surfaces; mirrors will swell or shrink non-uniformly with changes in the temperature of the air around them. Both of these telescopes broke new ground when they were built. As they surpassed the size and difficulty of construction of any existing telescopes, each represented a push into unknown areas of engineering. The designers and builders of the telescopes did their jobs well -- the structures and mirrors are rigid enough to hold their finely tuned shapes under the most demanding of circumstances. Although their function is as good as any modern-day astronomer could ask for, telescopes today depend on far different designs and construction methods.

Simply put, steel is a very heavy material in this day of new lightweight materials and computer-aided design. Rather than heavy steel girders to ensure rigidity, today's telescopes take advantage of more rigid designs that use fewer and thinner members. New technologies, both in optical design and materials and computer conrol, have also allowed for the design of much shorter telescopes with less massive mountings used to track the telescope during the night to follow the slow motion of the stars and the Earth turns on its axis. All of this reduction in material reduces the "thermal mass" of the telescope -- the amount of heat that the telescope structure can absorb during the day and release during the night into the path of starlight entering the telescope tube. Shorter telescopes also means smaller enclosures. Note the size of the 60-inch telescope's dome compared to that of one of the nearby 40-inch CHARA array telescopes. And all of this adds up to improved ease of construction and, perhaps most important of all, reduced cost. Organizations that could never have afforded such massive telescopes decades ago now find large instruments within their grasp. And while decades would pass between increases in the size of the world's largest telescope during most of the twentieth century, a flurry of design and construction of huge telescopes in the past 10 years surpasses even the last century's most optimistic astronomer's expectations.

Along with advancements in structural design and materials, the mirrors of new, large telescopes have undergone a true revolution. Lighter and more rigid materials have been developed (leading to much lighter finished mirrors requiring less massive support structures). Engineering advances allow the manufacture of optical quality "blanks" for mirror making that were beyond the capabilities of twentieth century materials engineers. New mirror making techniques allow grinding and polishing these mirrors more quickly than ever before. And mirrors can now be assembled from segments to reduce the problems associated with maintaining microscopic precision on optical surfaces measuring 10 meters across. Lasers, computers and high-speed motors constantly monitor and maintain the mirror segments in their proper positions. Telescopes of up to 100 meters (4000 inches) are on the drawing boards. We are currently in the greatest astronomical construction boom in history.

But where does that leave the old telescopes of Mount Wilson? These telescopes continue to collect starlight as well as they ever did, despite the older techniques used to build them. What has been surpassed is the techniques used to gather and analyze the faint glow from distant objects. One of the limitations that affects all telescopes on the Earth's surface is the constantly moving air between us and the steady light of the stars as it approaches through space. In the last few miles of its long travel, starlight is disturbed by this ocean of air, leaving astronomers with an imperfect picture of the objects they study. Adaptive optics, which senses and compensates for the distortions of starlight imposed by our atmosphere, have allowed great advancements in astronomical research. Indeed, without adaptive optics it is unlikely that many of the world's great new telescopes would be built. With adaptive optics in place, these great telescopes can often reach their full potential without atmospheric limitations. The 60-inch telescope at Mount Wilson was a test bed for one of the first adaptive optics systems used purely for astronomy after it was declassified as part of the Strategic Defense Initiative. The Hooker 100-inch telescope has a next-generation adaptive optics system, known as ADOPT, that allows it to perform research that was never possible during its first 75 years of life. With the ADOPT system, the 100-inch telescope has recorded finer detail than even the 94-inch Hubble Space Telescope can above the Earth's atmosphere.

Other modern instruments also aid astronomers in their nightly observations. CCD cameras designed specifically for different types of astronomical research allow images and spectra to be recorded digitally directly into computers in a fraction of the time that photographic plates used to take. Computer control systems allow faster pointing and more accurate tracking than ever before. With modern-day instrumentation to augment the grand dames of astronomy, the telescopes on Mount Wilson are more productive than ever before.