Super-large space telescopes

        Throughout history, there have been four things that have determined just how much information we can glean about the Universe through astronomy:

1. The size of your telescope, which determines both how much light you can gather in a given amount of time and also your resolution.
2. The quality of your optical systems and cameras/CCDs, which allow you to maximize the amount of light that becomes usable data.
3. The “seeing” through the telescope, which can be distorted by the atmosphere but minimized by high altitudes, still air, cloudless nights and adaptive optics technology.
4. And your techniques of data analysis, which can ideally make the most of every single photon of light that comes through.

        There have been tremendous advances in ground-based astronomy over the past 25 years, but they’ve occurred almost exclusively through improvements of numbers 2 and 4. The largest telescope in the world in 1990 was the Keck 10-meter telescope, and while there are a number of 8-to-10 meter class telescopes today, 10 meters is still the largest class of telescopes in existence. Moreover, we’ve really reached the limits of what improvements in those areas can achieve without going to larger apertures. This isn’t intended to minimize the gains in these other areas; they’ve been tremendous. But it’s important to realize how far we’ve come. The charge-coupled devices (CCDs) that are mounted to telescopes can focus on either wide-field or very narrow areas of the sky, gathering all the photons in a particular band over the entire field-of-view or performing spectroscopy — breaking up the light into its individual wavelengths — for up to hundreds of objects at once. We can cram more megapixels into a given surface area. Quite simply, we’re at the point where practically every photon that comes in through a telescope’s mirror of the right wavelength can be utilized, and where we can observe for longer and longer periods of time to go deeper and deeper into the Universe if we have to.

        And finally, computational power and data analysis technique have improved tremendously, where more useful information can be recorded and extracted from the same data that we can take. These are tremendous advances, but just like a generation ago, we’re still using the same size telescopes. If we want to go deeper into the Universe, to higher resolution, and to greater sensitivities, we have to go to larger apertures: we need a bigger telescope. There are currently three major projects that are competing to be first: the Thirty-Meter Telescope atop Mauna Kea, the (39 meter) European Extremely Large Telescope in Chile, and the (25 meter) Giant Magellan Telescope (GMT), also in Chile. These represent the next giant leap forward in ground based astronomy, and the Giant Magellan Telescope is probably going to be first, having broken ground at the end of last year and with early operations planned to begin in just 2021, and becoming fully operational by 2025.


491