TMT versus Space Based Alternatives

With the controversy again raging over the TMT we are again seeing complete mis-representation of astronomy in the press and social media. These postings and comments make interesting reading. These show the wide range of feelings in the community. There also seems to be a misunderstanding about the roles of large telescopes in general.

TMT Rendering
An overhead view of the proposed Thirty Meter Telescope, credit TMT Observatory Corporation
The last two decades has seen an enormous advance in our understanding of the universe and its history. We now know the age of the universe, we have discovered planets around other stars, we have learned that ordinary matter makes up only a small fraction of the universe. These represent huge leaps in our understanding and these revelations have come, to the greater degree, from ground based astronomy. Many of these discoveries have been made, or greatly assisted, by the telescopes atop Mauna Kea.

Note: This post was originally written and posted to the old DarkerView blog in August 2008. Re-posted again as it answers on of the common comments in the current episode of the TMT debate. A few edits thrown in to update for the current context seven years after the first publication.

One of the themes that popped up in the comments several times was the idea that space telescopes are where to put the money and the belief that ground telescopes were “obsolete’, thus projects like TMT are not needed. This could not be further from the truth. Yes there are advantages to space telescopes with respect to interference from the atmosphere. But space telescopes face several severe and inherent disadvantages.


Space telescopes are hugely expensive for what you get. Hubble is a mere 2 meter class telescope. Very small in the terms of modern telescopes. This limits the performance drastically. Hubble can only collect a little more light than a single segment of the current Keck telescope, and Keck has 36 segments in it’s 10 meter mirror. Work out the math and you find that Keck gathers 25 times more light than than Hubble. The new James Webb Space Telescope will be a 6.5 meter telescope. The Thirty Meter Telescope will be 30 meters with 492 segments, thus will gather 28 times more light than JWST.


Competition for time on the Hubble telescope is intense, there are only so many hours available and the number of applications to use that time is vastly greater than the time available. This tends to eliminate programs that take large amounts of telescope time, and some of those programs have proven to be the most productive from a scientific standpoint. Several ground based telescopes can be built for the same cost as a single space telescope, this means more astronomers can get time and far more science can be accomplished. Keck is scientifically the most productive telescope in the world, or off the world. More science papers are written based on Keck data than any other telescope in operation.

James Webb Space Telescope
An artist’s concept of the James Webb Space Telescope


When something broke on Hubble the telescope was without that capability, or even completely crippled until a service mission could be planned and launched at a cost of millions of dollars. With the retirement of the shuttles even this is no longer possible… It stays broken. When something breaks on a ground based telescope a technician hops in a pickup truck and drives up the mountain at the cost of a tank of gas.

It will not be possible to service the James Webb Space Telescope, as it will be located in an orbit a million miles from Earth, four times the distance of the Moon and far beyond the reach of any currently available manned spacecraft.


Science changes, we learn what works or does not work and adapt the telescope to fit. With a space telescope you are stuck with what you launch until a service mission can be launched. You can not change or reconfigure the instrument to a new role or to exploit any new discoveries. Ground based telescope are reconfigured daily and instruments are continuously updated or replaced.

Adaptive Optics

Modern ground based telescopes have overcome many of the limitations of peering through the atmosphere through the use of adaptive optics systems. Keck, Gemini, Subaru and similarly equipped large telescopes can see more sharply than Hubble using the combination of a larger mirror and adaptive optics. And these systems are currently being upgraded to even higher levels of performance. TMT will utilize such a system as part of its basic design from initial conception.


The result is that the current Webb telescope, or any other seriously proposed space telescope can not come close to replacing what TMT will do for our understanding of the universe. A good space based telescope can be very valuable to science, but in any subjective measure of cost effectiveness, flexibility or capability for maintenance and upgrade a large ground based telescope wins. In practical use both types of telescopes complement each other, allowing each telescope to do what it does best. Often data from several types of telescope is used together to penetrate the riddles of our universe.

Author: Andrew

An electrical engineer, amateur astronomer, and diver, living and working on the island of Hawaiʻi.

6 thoughts on “TMT versus Space Based Alternatives”

  1. I’ve heard teeth grind more than once when a layperson assumed that any fantastic image of space was taken by a Hubble. I wonder if this is a case of a little knowledge being a dangerous thing, since many people have heard, at some point, that the lack of atmosphere is a great advantage to space-based instruments (which, to be sure, it *is*, but there’s also something to be said to being located at the top of the road).

  2. I think you can put much blame on the well funded NASA/STScI public relations department. They really have captured the media attention for many years,

  3. I got this response I assume as a reaction to a post suggesting Hubble rather than TMT. I have to ask what’s the point since my post very clearly was to abandon Maunakea as another land grab for limited interest poor social benefit science. Read your post Larry as if you were another reader and try to make sense out of “…that the lack of atmosphere is a great advantage to space-based instruments (which to be sure, it is…”but”)…(the use of selective deletion of ideas in order to give your own more precedence or validity). Hubble was mentioned as an alternative to keeping TMT and the several other non-working units off of our mountain.

  4. Why do you boast SIZE and amount of light FROM HERE ON EARTH – without addressing the fact – GETTING CLOSER – does not require BIGGER BEING BETTER – when in fact ASTRONOMY AND MODERN TECHNOLOGY IS GOING DIGITAL AND MICRO?

    And as you have stated – WHY NOT UPGRADE what you have – instead of DYNAMITING OUR SACRED MAUNA – for THE BIGGEST DEEPEST – when smallest is BEST?

    As an UPDATE on May 30, 2015 – Why did you NOT MENTION THE LATEST – THE LSST – TO THIS DISCUSSION?


    1. Thank you for being a great example of why an article like this is necessary. There are so many very basic misunderstandings about science and how it is done in our society. These myths tend to get amplified in discussions like this, repeated back and forth in social media, until many believe what they want to believe, regardless of the truth in the matter.

      Size does matter in astronomy, in very basic ways dictated by physics. Both the resolution and light gathering power of the telescope depend on size. This telescope would not be supported by the astronomy community if we did not need it to push forward. Telescopes must get larger to beat the performance issues and limitations we are currently faced with.

      The telescope is actually quite simple in design, the real technology is in the instruments mounted to the telescope. There astronomers take full advantage of the latest and greatest in new technologies, often driving the development of new tech.

      I am quite familiar with LSST, the Large Synoptic Survey Telescope, in both the design and mission. I watched it’s 8.4 meter primary mirror being manufactured by friends of mine who work in the Steward Mirror Lab. A telescope like LSST neatly complements the bigger ‘scopes like Keck or TMT. Telescopes like LSST and Pan-STARRS are designed to discover, scanning the whole sky, while bigger telescopes will be needed to study what they find.

  5. I’m writing a novel in which the TMT controversy is part of the background. I’ve got good sources of information and ppl to guide me on the Hawaiian “protectors” side, even including one activist relative. I’d like to get additional information, i.e., help, from the astronomy perspective. Your post on TMT vs. JWST is a great example. Would it be possible to arrange a brief Zoom session or phone call to ask some questions to help me understand better? BTW the first novel in this series, “Bones of Hilo,” was published in June 2021. In that one, destruction of a heiau provides the “McGuffin” and background for the plot. I want to use TMT this time. Mahalo!

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