So you have figured out where to put your giant telescope and how to pay for it. Now what are you going to do with it?

For astro-political reasons, I’ve been reviewing the science cases for the three extremely large telescopes (ELTs, of course) currently somewhere in the process of being built. In alphabetical order they are:

  • the European Extremely Large Telescope (E-ELT)
  • the Giant Magellan Telescope (GMT)
  • The Thirty Meter Telescope (TMT)

All three have documents describing the science questions that the telescopes are designed to answer. The EELT science case is the least technical of the three and mentions exoplanets, fundamental physics, black holes, star formation and evolution, and distant galaxies. The other two telescopes mention all of these topics and add solar system objects (planets, dwarf planets, moons) and compact objects (white dwarfs, neutron stars). The TMT detailed science case is indeed very detailed and not for the faint of heart; the GMT case is somewhere in between the other two. (For astronomers, I recommend the TMT science requirements document as also enlightening, particularly Section 1.4 on “general comments regarding large telescope capabilities”.)

It’s not surprising that the science cases for the three telescopes are similar. All three are general-purpose facilities, designed to be able to perform a wide variety of science. Astronomy is a world-wide enterprise, so the topics of interest to one national or international science community are not hugely different from those of interest to another. All three telescopes are to be sited on the surface of the Earth, and the atmosphere imposes fundamental limitations on what a ground-based telescope can observe. All three telescopes will use adaptive optics to compensate for the blurring effects of the atmosphere, but AO can’t change the absorption of light by the atmosphere at most wavelengths.

Where the three telescopes do differ slightly is in the instruments that they will use to detect and analyze the light that they collect. The instruments are very sophisticated pieces of technology costing tens to hundreds of millions of dollars; a telescope can’t do anything without them. The instruments are closely linked to the science goals in that certain types of science naturally go with particular instruments. There are only about half a dozen broad categories of instruments used on ground-based telescopes like these. All three telescopes plan to eventually have one of each kind of instrument, but they differ in which ones they’re planning to start with.

The E-ELT will have a near-infrared camera and an integral field unit (IFU) spectrograph; GMT will have an IFU, a multi-object spectrograph and two high-resolution spectrographs, one each for visible and near-infrared light. TMT will have an IFU and two multi-object spectrographs – one visible, and one near-infrared. The three telescopes differ in their plans for deploying their adaptive optics systems - with TMT aiming to have the AO system ready somewhat earlier, as I understand it. All three have plans for future “extreme AO” instruments to be used to study exoplanets, but these are such complex instruments that they will take longer to build.

To summarize: what you are going to do with your giant telescope, at least at first, is probably similar to what you would do with your non-giant telescope. The giant telescopes will collect more light and see finer detail, allowing fainter and further objects to be studied. But the most interesting things will happen if the ELTs discover entirely new classes of objects in the universe — things that we didn’t know about at all before. Then you’ll get to use all of the incredible instruments to dissect the light of these new objects and figure out what they are, and the next generation of astronomers will have a new set of justifications for building their new telescopes!