Popular Science's 2016 article The silliest names scientists have given very serious telescopes... Ranked! says:

Arizona’s MMT Observatory used to house the Multiple Mirror Telescope. In 1998, however, it upgraded to the superiorly-named Massive Monolithic Telescope, which replaced the multiple mirrors of its predecessor with a single 21-foot-wide reflector.

While the article suggests that the name is superior, this isn't a sufficient reason to get rid of the old mirrors and associated positioning instrumentation and buy a new, monolithic mirror.

Question: Why the change, and was it really an "upgrade" in performance?

The Telescope formerly known as Multiple Mirror:

Massively Monolithic Telescope, formerly known as MMT Source

  • $\begingroup$ You mention "positioning instrumentation". Is there something that described that as also being replaced? $\endgroup$
    – BowlOfRed
    Jun 10, 2021 at 0:37
  • $\begingroup$ @BowlOfRed I am assuming that the original telescope had some instrumentation necessary to measure and maintain correct alignment between all of the mirrors, and that that was one of the feats that the original telescope was known for. With a single mirror, that stuff would no longer be necessary. $\endgroup$
    – uhoh
    Jun 10, 2021 at 1:29
  • $\begingroup$ The top of the link in my answer says the mirrors were all mounted to the same structure, so I assumed a single steering mechanism that could be used by the new mirror as well. The previous mirrors were not independently steerable. $\endgroup$
    – BowlOfRed
    Jun 10, 2021 at 1:34
  • $\begingroup$ @BowlOfRed in order to form a useful image, the separate mirrors must have their surfaces matching to within a fraction of a wavelength of light. It is not necessary to have a reference to know this optical fact, and with the coefficient of thermal expansion of metals in the 1E-06 to 1E-05 range we can know beyond any doubt that there had to be some system to actively maintain alignment. One way around that would be to make the mounting system out of the same kind of glass as the individual mirrors, but then again, that's what a single mirror is. $\endgroup$
    – uhoh
    Jun 10, 2021 at 1:45

1 Answer 1


From http://www.mmto.org/history-of-the-telescope/

After nineteen years of productive operations, progress in the production of large mirrors (pioneered at the University of Arizona’s Mirror Lab), and new instrument technologies drove the desire to upgrade the telescope to utilize a single 6.5-m mirror in place of the smaller six-mirror array. The conversion would more than double the light-gathering power of the telescope and increase the area of sky the telescope could observe at one time (called the “field-of-view”) by a factor of more than 300.

Although that page disagrees with the name of the telescope. They don't seem to use the "massive monolithic telescope" name anywhere on their website. I wonder if everyone called it that during construction but some board nixed it as the official name?

The new telescope was simply renamed the “MMT,” which is no longer an acronym.

  • $\begingroup$ It's been deacronymized, wow! Thanks for the speedy answer. It's hard to understand how the FOV could increase unless they've changed the type of telescope, or if the original mirrors were all spherical or at least axisymmetric (as opposed to off-axis paraboloids) and the new one is some optimized shape. $\endgroup$
    – uhoh
    Jun 10, 2021 at 1:27

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