Rotating Telescopes & Collaboration: A Tour of the Multiple Mirror Telescope

Atop Mount Hopkins near Tucson, Arizona, stands the Multiple Mirror Telescope. At the time of its establishment in 1979, it was the third largest telescope in the world – even President Carter sent congratulations on its dedication.

Primary mirror constructed at UA’s Mirror Lab

In 1998, the telescope was converted from having six smaller mirrors to one large mirror, but the name remained the same. This new mirror was created at the Richard F. Caris Mirror Lab at the University of Arizona and was one of the largest mirrors ever created at that time.

Since its construction, technology has significantly advanced and the MMT is no longer the largest telescope in the world. That title goes to the Large Binocular Telescope, and in a few years will belong to the Giant Magellan Telescope. 

VERITAS telescope

The journey to the Multiple Mirror Telescope (MMT) is curved and arduous. Visitors and astronomers first meet VERITAS, an array of gamma ray telescopes, at basecamp and then begin the drive to the top of Mt. Hopkins.

At Mt. Hopkins’s summit, the MMT rises above all other telescopes on the mountain. Other smaller, Smithsonian-operated telescopes occupy the area below.

A characteristic of MMT that was unique during its construction is that during observation, the entire structure, the telescope and dome, rotates 360 degrees. Both the telescope and the building rotate together as one, instead of the dome and telescope rotating separately, which is standard for many telescopes. This technology has since been applied to other telescopes after it was successful at MMT.

MMT dome

“For the MMT, in order to keep costs down, there was a decision made to make the whole enclosure rotate with the telescope,” said Grant Williams, director of the MMT. “Everything rotates with the telescope.”

Visible: Original blue anchors

Williams began his journey at the MMT as a postdoctoral researcher in 2002 and was hired on staff in 2004 as a researcher. In 2011, Williams was promoted to director of the observatory and he traded in much of his time previously used for his research for administrative tasks like budgets, staff management, and public outreach.

The MMT is constructed of several old and new pieces of equipment and technology. The oldest pieces that are still in operation from the original telescope are light blue anchors that keep the telescope standing. Besides the anchors, nearly everything else has been updated.

Smaller interchangeable secondary mirror

The telescope itself is operated with two mirrors: one large primary mirror and and a secondary mirror. There are three smaller and interchangeable secondary mirrors, named F-5, F-9 and F-15; these mirrors are named after their focal ratios, which dictate the width of the field of view in the sky. The two mirrors that go into operation on the telescope depend on the science goals and instrument currently used on the telescope.

ARIES (purple) & interchangeable rotator flange attached to bottom

Instruments attach to the bottom of the telescope and are screwed into an interchangeable rotator flange. These instruments allow the telescope to collect data for scientific observations. One of the most recent instruments utilized on the MMT is the ARIES system. ARIES is a near-infrared imager and spectrograph; a spectrograph breaks down light into a spectrum and makes identifying objects from space easier. Williams explains these processes as “taking the twinkle out of the stars to make the images more clear.”

The latest instrument that is going to be installed on the MMT is binospec. Binospec is similar to the hectospec, which is a multi-object fiber optic system that uses two robots to control fibers and allows telescopes and observers to view up to 300 objects at once.

“Binospec,” Williams said, “is more powerful than hectospec because it’s more sensitive. You can study fainter objects with it in shorter amounts of time but you can also get many objects all at once.”

In addition to the instruments and equipment, the MMT has a control room located in the basement. The control room consists of a series of computers that monitor the various aspects of observing at night. Some monitor weather, including cloud cover and wind; others control the telescope and the various instruments attached to it.

Control room & Director Williams

“This is where all the action happens at night,” Williams said.

The control room also houses the telescope and instrument operators, who are on staff at the MMT, and guest astronomers. Astronomers come from the University of Arizona (UA) and Smithsonian Astrophysical Observatory, as well as the occasional national or international guest astronomer from outside of these organizations.

“Right from the original MMT telescope, the MMT Observatory has been a joint operation between the University of Arizona and the Smithsonian Astrophysical Observatory,” Williams said. “That joint operation allows both of the institutions to send observers from their institutions to observe at the MMT.”

Open dome during observation

Being a joint operation, both the UA and Smithsonian split funding for the MMT and have priority for observing. Outside funding for individual astronomers may come from the National Science Foundation (NSF) or National Aeronautics and Space Administration (NASA), who must pay for their time on the telescope.

The MMT, like any other telescope or piece of technology, is ever-changing and expanding. From ARIES and Hectospec to the cooperation between the Smithsonian and UA, astronomy at the MMT enraptures the mind and expands humanity’s knowledge of the universe.

Taking time to visit the MMT is worth the trip – even if it’s just for the view.


To learn more about the Multiple Mirror Telescope and its observatory, visit their website.

To learn more about UA’s Mirror lab, visit their website.

For more information about the Giant Magellan Telescope and its progress, visit their website.

Enjoy reading this article? Check out our story on the Large Binocular Telescope, currently the largest telescope in the world, here!


  • Hello there,

    It’s a great informative article indeed. I have a little interest in astronomical works and I always wish that if I could be an astronaut. We all dreams never come true always. From this article, I learn much information about a rotating multiple mirror telescope. I appreciate the effort you took to share it with us. Hope, will get more from you soon.

  • Hi, thank you very much for this info!

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