April 16, 2009
Title: What is the World’s Largest Telescope?
Podcaster: Rob Sparks
Link: http://halfastro.wordpress.com
Description: People are hooked on superlatives such as the “World’s Largest Telescope” or the “World’s Most Powerful Telescope”. It is a common question asked at star parties. When asked by the public, they are usually referring to ground based telescopes that observe in the optical and near infrared. Many years ago, the question could be answered easily. With advances in telescope construction, the answer has become less clear. There are many different observatories that claim to be the world’s largest telescope based on a particular criteria. This podcast will explore some of the world’s largest telescopes as well as interferometers, arrays of telescopes whose light is combined to give the resolution of one large telescope.
Bio: Rob Sparks is a Science Education Specialist at the National Optical Astronomy Observatory. A lifelong astronomy enthusiast, he earned a B.A. in physics at Grinnell College and his M.S. at Michigan State University. He taught high school physics, math and astronomy for 11 years at schools on St. Croix, Florida and Wisconsin. He spent the 2001-2002 school year working on the Sloan Digital Sky Survey as a recipient of the Fermilab Teacher Fellowship. He spent the summer of 2003 at the National Radio Astronomy Observatory as part of the Research Experience for teachers. He has been working as a NASA Astrophysics Ambassador since 2002. He is a member of the Galileoscope Working Group for the International Year of Astronomy.
Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by Astrocamp Summer Mission of Idyllwild, California. Help introduce a child to the world of Astronomy. Learn more at www.astrocamp.org.
Transcript:
Hi, this is Rob Sparks from the Half-Astrophysicist blog (at halfastro.wordpress.com). I am an astronomy educator at the National Optical Astronomy Observatory in Tucson, Arizona. I enjoy setting up my telescope at local star parties and answering questions from the public. Today, I will address one of the most common questions asked at star parties.
What is the worldʼs largest telescope? When most people ask this question, they are usually referring to ground based telescopes that observe in the visible and near infrared part of he spectrum and that is where I will focus today. This question was simple when I was a child. The 200 inch (5 meter) Hale Reflector on Mount Palomar reigned supreme as the worldʼs largest telescope for almost three decades. When I was a child, I received Patrick Mooreʼs Book “A Picture History of Astronomy” for Christmas. The pictures of the Hale Telescope were among my favorites and I was convinced there was not a better job in the entire world than riding in the prime focus cage all night imaging the sky.
Advances in mirror fabrication and telescope design has ushered in a new era of telescopes. Single mirrors can now be cast up to 8.4 meters in diameter. Larger telescopes can be constructed by using many small mirrors to create a single large mirror. We can combine the light from several small telescopes to give the collecting area and resolution of a much larger telescope. Several new telescopes lay claim to superlatives such as the worldʼs largest telescope or the worldʼs most powerful telescope.
Before we try to answer the question of what is the worldʼs largest telescope, letʼs back up and look at what telescopes actually do. First, telescopes collect a lot of light. They are sometimes called light buckets. The larger the surface area, the more light a telescope can collect. The more light, the fainter objects we can see with the telescope. Doubling the diameter of a telescope increases its light gathering ability by a factor of 4.
The second function of a telescope is to allow us to see fine detail in small objects. This property is called resolution. Doubling the diameter of a telescope lets us smaller details. Another way to increase the resolution of a telescope is to combine the light from two separate telescopes in a process called interferometry which I will discuss later in more detail.
Now we will look at some of the different telescopes that lay claim to the title of the worldʼs largest. Letʼs start with an easy one: the worldʼs largest refractor. A refractor uses an objective lens to focus the light. The largest refractor is the 40 inch telescope at Yerkes observatory in Williams Bay, Wisconsin. It was the largest telescope in the world when it was commissioned in 1897. It is difficult to make a useful lens over 40 inches in diameter. The lens starts to sag in the middle under its own weight. You also have to deal with a phenomena called chromatic aberration in large lenses. Chromatic aberration is due to red light and blue light bending different amounts when they pass through a lens. The result is annoying color fringes in your image. Therefore, we have never built a larger refractor.
Now letʼs move onto reflectors. Reflectors use mirrors to focus light. Making extremely large mirrors is difficult, so many large telescope use segmented mirrors. Segmented mirrors use lots of smaller mirrors, usually in the shape of hexagons, to create one large mirror.
At this point, the question gets tricky. The Hobby-Eberly Telescope (or HET) at McDonald Observatory near Fort Davis, Texas, uses 92 small hexagonal segments to create a mirror that measures 11 meters by 9.8 meters which makes it the telescope with the largest mirror. However, the HET uses what is known as a constant elevation mount. The telescope cannot tip up or down and can only rotate around a vertical axis. Due to its unique design, the camera cannot collect light from the entire mirror. The telescope collects light from an area equivalent to a telescope with a 9.2 meter mirror. The South African Large Telescope uses a similar design so we will call it a tie with the HET.
The telescope with the largest effective diameter is the Grand Telescope of the Canaries. Its mirror, composed of 36 hexagonal segments, has a diameter of 10.4 meters, slightly larger the the 10 meter Keck telescopes on Mauna Kea in Hawaii. There are other players in running as well. The recently completed Large Binocular Telescope, or LBT, claims the title of the worldʼs largest telescope on its website. The LBT uses two 8.4 meter mirrors on a single mount. These two mirrors give the same light collecting area as a single 11.8 meter mirror. The telescope will be operated as an interferometer in the future operating a a single instrument with the resolution of a 22.8 meter telescope.
If we decide to focus on telescopes with the highest resolution, we move to the realm of interferometers. Interferometers combine the light from several small telescopes to achieve the resolution of one large telescope of the same diameter. Interferometry typically is only useful for relatively bright targets. The resolution you achieve in the end depends on the separation of the telescopes as well as the wavelength of your observations. Therefore, it is difficult to pick one interferometer as having the highest resolution as different interferometers may operate at different wavelengths. I will briefly mention some of the more prominent interferometers.
The two 10 meter Keck telescopes on Mauna Kea can operate as an interferometer with a baseline of 85 meters. The Very Large Telescope Interferometer in Chili can be used in a variety of configurations. The VLT consists of four 8.2 meter telescopes and four 1.8 meter auxillary telescopes. The auxillary telescopes are on tracks and can be moved to change their spacing up to a maximum of 200 meters.The Center For High Angular Resolution Astronomy, or CHARA, operates an array on Mount Wilson in California. The CHARA array uses six telescopes each with a diameter of one meter.
These telescopes are spread out over 330 meters yielding even higher resolution, although with less light gathering ability than the Keck or VLT interferometers. That is the current state of the art, but we are already planning larger telescopes. The Giant Magellan Telescope will use seven 8.4 meter mirrors to create a telescope with a diameter of 24.5 meters. The Thirty Meter Telescope will use 492 segments to create its giant mirror.
As you can see it is difficult to say which telescope is the largest. Each of these amazing instruments has its own unique set of abilities that contribute to our knowledge of the universe. So perhaps it is time to retire the title of “Worldʼs Largest Telescope”.
Thanks for listening to this episode of the 365 Days of Astronomy podcast. Visit my blog at halfastro.wordpress.com.
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365 Days of Astronomy
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