# Astronomical Instruments

## The Future of Large Telescopes


### Learning Objectives

By the end of this section, you will be able to:
1. Describe the next generation of ground- and space-based observatories
2. Explain some of the challenges involved in building these observatories
If you’ve ever gone on a hike, you have probably been eager to see what lies just around the next bend in the path. Researchers are no different, and astronomers and engineers are working on the technologies that will allow us to explore even more distant parts of the universe and to see them more clearly.
As we saw earlier, the premier space facility for the coming decade will be the James Webb Space Telescope (). Its work is just beginning, but its flawless deployment in the first half of 2022 has given astronomers a strong sense of optimism about its ability to probe the infrared universe. The smaller Hubble Space Telescope is still functioning after more than 30 years in space. NASA is also looking at launching (around 2027) the Nancy Grace Roman Space Telescope, an infrared instrument which will have a smaller mirror but a wider field of view than the Webb.
On the ground, astronomers have started building the Vera Rubin ObservatoryThe observatory is named after the American astronomer whose work led us to the understanding that much of the universe is made of a mysterious substance that scientists call dark matter (which we explain in , which has an 8.4-meter telescope with a significantly larger field of view than any existing telescopes. It will rapidly scan the sky to find transients, phenomena that change quickly, such as exploding stars and chunks of rock that orbit near Earth. It is expected to see first light in 2022.
The international gamma-ray community is planning the Cherenkov Telescope Array (CTA), two arrays of telescopes, one in each hemisphere, which will indirectly measure gamma rays from the ground. The CTA will measure gamma-ray energies a thousand times as great as the Fermi telescope can detect.
Several groups of astronomers around the globe interested in studying visible light and the infrared are exploring the feasibility of building ground-based telescopes with mirrors larger than 30 meters across. Stop and think what this means: 30 meters is one-third the length of a football field. It is technically impossible to build and transport a single astronomical mirror that is 30 meters or larger in diameter. The primary mirror of these giant telescopes will consist of smaller mirrors, all aligned so that they act as a very large mirror in combination.
The most ambitious of these projects is the European Extremely Large Telescope (ELT) (). (Astronomers try to outdo each other not only with the size of these telescopes, but also their names!) The design of the European ELT calls for a 39.3-meter primary mirror, which will follow the Keck design and be made up of 798 hexagonal mirrors, each 1.4 meters in diameter and all held precisely in position so that they form a continuous surface. Construction on the site in the Atacama Desert in Northern Chile started in 2014, and operations are expected to begin in about 2025.

International consortia with major contributions from U.S. astronomers have developed plans for the construction of two large new telescopes. One is a Thirty-Meter Telescope (TMT) for which the preferred site is Maunakea in Hawaii. The design of this telescope is similar to that of the European ELT and will make use of 492 hexagonal elements. Each segment is about 1.44 meters (56.6 inches) across corners. The segments are closely spaced, with gaps between the segments only 2.5 mm (0.1 inch) wide.


The Giant Magellan Telescope (GMT) is the second ELT project with major participation by U.S. astronomers. The GMT is also a segmented mirror telescope that employs seven stiff monolith 8.4-meter mirrors as segments. Construction has started at the selected site, which is near the Las Campanas Observatory on the southern edge of the Atacama Desert.

These giant telescopes will combine light-gathering power with high-resolution imaging. These powerful new instruments will enable astronomers to tackle many important astronomical problems. As just one example, they provide us images and spectra of planets around other stars and thus, perhaps, give us the first real evidence (from the chemistry of these planets’ atmospheres) that life exists elsewhere.
The largest space telescope yet, the 6.5-meter James Webb Space Telescope, was launched in December 2021 and is sending back a flood of useful information about the infrared universe. New and even larger telescopes are on the drawing boards. Gamma-ray astronomers are planning to build the CTA to measure very energetic gamma rays. Astronomers are building the LSST to observe with an unprecedented field of view and a new generation of visible-light/infrared telescopes with apertures of 24.5 to 39 meters in diameter.

### For Further Exploration


### Articles

Blades, J. C. “Fixing the Hubble One Last Time.” Sky & Telescope (October 2008): 26. On the last Shuttle service mission and what the Hubble was then capable of doing.
Brown, A. “How Gaia will Map a Billion Stars.” Astronomy (December 2014): 32. Nice review of the mission to do photometry and spectroscopy of all stars above a certain brightness.
Irion, R. “Prime Time.” Astronomy (February 2001): 46. On how time is allotted on the major research telescopes.
Jedicke, Peter & Robert. “The Coming Giant Sky Patrols.” Sky & Telescope (September 2008): 30. About giant telescopes to survey the sky continuously.
Lazio, Joseph, et al. “Tuning in to the Universe: 21st Century Radio Astronomy.” Sky & Telescope (July 2008): 21. About ALMA and the Square Kilometer Array.
Lowe, Jonathan. “Mirror, Mirror.” Sky & Telescope (December 2007): 22. On the Large Binocular Telescope in Arizona.
Lowe, Jonathan. “Next Light: Tomorrow’s Monster Telescopes.” Sky & Telescope (April 2008): 20. About plans for extremely large telescopes on the ground.
Mason, Todd & Robin. “Palomar’s Big Eye.” Sky & Telescope (December 2008): 36. On the Hale 200-inch telescope.
Subinsky, Raymond. “Who Really Invented the Telescope.” Astronomy (August 2008): 84. Brief historical introduction, focusing on Hans Lippershey.
Young, Monica “The New Space Race (and Problem).” Sky & Telescope (March 2020): 14. A good discussion of the danger that huge numbers of new reflective satellites (to provide Wi-Fi) poses to the darkness of the night sky for astronomers.
James Webb Space Telescope
Geithner, P. “Building the James Webb Space Telescope.” Sky & Telescope (November 2021): 20. The instruments and engineering challenges of building the new space telescope.
Mather, J. “The James Webb Space Telescope Lives.” Astronomy (October 2021): 14. The senior project scientist previews the giant infrared telescope NASA is launching.

### Websites

Websites for major telescopes are given in , , , and .
Information on Swarms of Satellites on Low-Earth Orbit



### Videos

Astronomy from the Stratosphere: SOFIA: https://www.youtube.com/watch?v=NV98BcBBA9c. A talk by Dr. Dana Backman (1:15:32)
Galaxies Viewed in Full Spectrum of Light: https://www.youtube.com/watch?v=368K0iQv8nE. Scientists with the Spitzer Observatory show how a galaxy looks different at different wavelengths (6:22)
Lifting the Cosmic Veil: Highlights from a Decade of the Spitzer Space Telescope: https://www.youtube.com/watch?v=nkrNQcwkY78. A talk by Dr. Michael Bicay (1:42:44)

### Collaborative Group Activities

1. Most large telescopes get many more proposals for observing projects than there is night observing time available in a year. Suppose your group is the telescope time allocation committee reporting to an observatory director. What criteria would you use in deciding how to give out time on the telescope? What steps could you take to make sure all your colleagues thought the process was fair and people would still talk to you at future astronomy meetings?
2. Your group is a committee of nervous astronomers about to make a proposal to the government ministers of your small European country to chip in with other countries to build the world’s largest telescope in the high, dry desert of the Chilean Andes Mountains. You expect the government ministers to be very skeptical about supporting this project. What arguments would you make to convince them to participate?
3. The same government ministers we met in the previous activity ask you to draw up a list of the pros and cons of having the world’s largest telescope in the mountains of Chile (instead of a mountain in Europe). What would your group list in each column?
4. Your group should discuss and make a list of all the ways in which an observing session at a large visible-light telescope and a large radio telescope might differ. (Hint: Bear in mind that because the Sun is not especially bright at many radio wavelengths, observations with radio telescopes can often be done during the day.)
5. Another “environmental threat” to astronomy (besides light pollution) comes from the spilling of terrestrial communications into the “channels”—wavelengths and frequencies—previously reserved for radio astronomy. For example, the demand for cellular phones means that more and more radio channels will be used for this purpose. The faint signals from cosmic radio sources could be drowned in a sea of earthly conversation (translated and sent as radio waves). Assume your group is a congressional committee being lobbied by both radio astronomers, who want to save some clear channels for doing astronomy, and the companies that stand to make a lot of money from expanding cellular phone use. What arguments would sway you to each side?
6. When the site for the new Thirty-Meter Telescope on Hawaii’s Maunakea was dedicated, a group of native Hawaiians announced opposition to the project because astronomers were building too many telescopes on a mountain that native Hawaiians consider a sacred site. You can read more about this controversy at http://www.nytimes.com/2015/12/04/science/space/hawaii-court-rescinds-permit-to-build-thirty-meter-telescope.html?_r=0 and at http://www.nature.com/news/the-mountain-top-battle-over-the-thirty-meter-telescope-1.18446. Once your group has the facts, discuss the claims of each side in the controversy. How do you think it should be resolved?
7. If you could propose to use a large modern telescope, what would you want to find out? What telescope would you use and why?
8. Light pollution (spilled light in the night sky making it difficult to see the planets and stars) used to be an issue that concerned mostly astronomers. Now spilled light at night is also of concern to environmentalists and those worrying about global warming. Can your group come up with some non-astronomical reasons to be opposed to light pollution?

### Review Questions


### Thought Questions


### Figuring for Yourself
