Extremely large telescopes are becoming the standard in professional astronomy. This new class of telescopes has such an enormous capacity for collecting light that these mammoth “light buckets” will search the universe for embryonic galaxies, peer deep into star-forming regions, scrutinize the disks around black holes and examine distant solar systems for the planetary signatures of life.
Telescopes come in a variety of designs but all are variations of two basic types, the refractor and the reflector. A refracting telescope is the kind with a long tube and a lens at the front of the tube called the objective lens.
This lens bends the incoming light, which makes it converge to a focus. As the light begins to spread out from this focal point, an eyepiece directs it into the observer’s eye.
The world’s largest refracting telescope is located in Williams Bay, Wisconsin, in an ornate Yerkes Observatory built in the late 1890s. The Williams Bay refractor has an objective lens that is 40 inches in diameter and was used for many decades by the University of Chicago for astronomy research. In the 2000s, the operation of this historical site was transferred to a not-for-profit group primarily for astronomy education and public tours.
Such a large lens is a significant piece of glass and can only be supported around its edge; otherwise, the incoming light would be obstructed as it passed through. In addition, this weighty lens is at the top of a 50-foot long telescope tube, which makes the lens further difficult to support.
The other type of telescope, which forms the basic design for all professional telescopes today, is the reflector. This kind of telescope has its curved, primary mirror at the bottom of the telescope tube.
It reflects incoming light to a focus and redirects it to a smaller, secondary mirror. The secondary then reflects the light to an eyepiece mounted on the side of the telescope tube or to the instruments that will record the image.
In amateur astronomy, a beginner’s reflecting telescope will typically contain a mirror that is six inches in diameter, thus, a “six-inch” telescope. It is not unusual, however, for an amateur reflector to have a mirror in the eight- to twelve-inch range. Large telescope mirrors for amateur astronomers typically are from 18 to 36 inches across.
A primary mirror in a reflecting telescope has its reflection coating on the surface of a thick piece of curved glass. Since the “business” end of a telescope mirror is only its surface, telescope mirrors can be made as large as reasonably possible because the entire mirror weight can be supported from below.
New, innovative designs have come about to reduce the weight of large, one-piece or monolithic mirrors. Honeycombed support structures behind the reflecting surface or the use of multiple small mirrors in place of one large piece of glass have become commonplace in professional telescopes.
For several decades, the world’s largest reflecting telescope was the Palomar Telescope located in Southern California. This telescope contains a single, 5-meter (200-inch) diameter mirror and continues to be operated every clear night as “an iconic facility for scientific advancement.”
The first telescope to break the paradigm of the monolithic mirror was the Multiple Mirror Telescope located on Mount Hopkins in Arizona. From 1979 to 1998, it used six mirrors, each with a diameter of 1.8 meters (6 feet). These multiple mirrors were optically linked together to provide the equivalent light-gathering ability of a single 4.5-meter (15-foot) telescope mirror.
This new design was driven by the difficulty in producing large telescope mirrors. However, in the 1980s, the telescope mirror-making lab at the University of Arizona pioneered the casting of large monolithic mirrors backed with a honeycombed support structure and shaped in a large, rotating, mirror-making oven.
This innovative technique was so successful that in 1998, the six mirrors of the Multiple Mirror Telescope were replaced with a single 6.5-meter (21.3-foot) curved mirror.
Currently, the “most scientifically productive telescopes on Earth” are the twin telescopes of the Keck Observatory located at almost 14,000 feet on Mauna Kea in Hawaii. Each of the Keck telescopes contains a 10-meter (33-foot) diameter, multi-segmented mirror. Keck I came on-line in 1993, followed three years later by its sister telescope.
The mirrors in the Keck telescopes are composed of 36 sections that act together, mimicking a single telescope mirror. A system of sensors and actuators behind each segment adjusts the position of every mirror twice per second to within four billionths of a meter. This system of “active optics” is now commonly used with segmented mirrors at major observatories.
In 1996, the Large Binocular Telescope was built on Mount Graham in southeastern Arizona. As its name suggest, it consists of two large, monolithic mirrors, each 8.4-meters (27 feet) wide, giving it the effective light collecting area of a single 11.8-meter (39-foot) diameter mirror.
Each of the monolithic mirrors of the Large Binocular Telescope is ranked as the world’s largest. However, these two single mirrors come in second to the 10.4-meter (35-foot) diameter, segmented mirror in the Great Telescope of the Canary Islands, currently the world’s largest telescope.
The first true Extremely Large Telescope — a telescope with a mirror effectively greater than 20 meters (65-feet) in diameter — will be the Giant Magellan Telescope. Planned for completion in 2021, this telescope will have seven 8.4-meter, monolithic mirrors for the equivalent of a 25-meter (80-foot) diameter mirror. Construction was started in 2012 on the observatory for this telescope and four of the mirrors have already been cast.
Back in Mauna Kea, groundbreaking took place last October for the Thirty Meter Telescope. This telescope, slated for completion in 2022, will have a 98-foot mirror composed of 492 mirror segments.
Finally, the largest of the giants currently planned is the gargantuan European Extremely Large Telescope with an almost 40-meter (130-foot!) segmented mirror. It is anticipated that this telescope will be completed in 2024.
Richard Monda is an astronomer living in the Capital Region.
Categories: Life and Arts
