ASTR 1230 (O'Connell) Supplementary Lecture Notes


HST on Orbit

The Hubble Space Telescope in orbit.


The human imagination has never been a match for the universe. That is why astronomy, more than any other science, has been regularly revolutionized by new observational discoveries. Since 1610, these have depended on telescopes. When telescope technology has developed slowly, as in the early 19th century, progress was slow. When technology surged, as in the late 20th century, progress was explosive.

This lecture surveys the state of observational astronomy today, with some background on how we got here. In Lecture 2 we have already discussed the basics of telescope design. Here we discuss some of the milestone developments in telescopes.

A key theme: to build an instrument at the frontier of performance is always costly in terms of brains and money. Thus, progress has coupled new technology and visionary astronomical pioneers with the generosity of wealthy private donors or the financial strength of governments.

Wilson 100-in
100-in reflector on Mt. Wilson. Click for enlargement.


Initially, telescopes were built adjacent to universities, usually in or near cities. However, the growth of artificial light pollution drove telescope construction to ever more remote sites, first in the southwestern US (e.g. California, Texas, Arizona), and ultimately to high mountains like Mauna Kea in Hawaii or the northern deserts of Chile.

40-in Optical and mechanical technology in the last few decades of the 19th century had advanced to the point that the construction of large telescopes was feasible. Most of these were associated with universities. They were costly and required substantial private donations. Important examples: (click on the links for more information):

George Ellery Hale was the premier American telescope founder. He planned, successively, the four largest telescopes of their era and lived to build the first three of these. He had a great facility for obtaining private financing, from Carnegie and Rockefeller, among others. The three major Hale telescopes were



Until about 1975, despite many technical improvements, big telescope design was based largely on the concepts used for the Mt. Wilson 100-in telescope (designed ca. 1907). Unfortunately, the cost of extending such designs to sizes larger than 200-in was prohibitive.

In the early 1980's a series of innovations was introduced that made yet larger telescopes affordable, mainly by reducing the total weight per unit optical collecting area. These included: Keck 10-m Another key innovation was the development of spin-cast mirrors, which are pre-shaped to reduce the major effort needed to polish a large mirror. Click the link for a video showing how this is done.

These technologies have now been employed to build a number of telescopes in the 6-m to 10-m class.

The largest individual US telescopes built to date are the twin Keck 10-m telescopes (see picture above right). These have an innovative multiple-mirror design.

8-m Mirror
Polished & coated 8-m (315-in) mirror for the Gemini project, 1999.


Big telescopes are costly. Both public and private funding is now involved in building them. The experience of World War II, in which physical science and mathematics provided the key technologies leading to victory, convinced the government that broad-based federal support for basic science and technology was essential. This included astronomy, and since 1950 the federal government has become the largest source of support for research in astronomy. The two dominant sources of funds for astronomy are

Initially, public funding almost completely replaced the private financing responsible for the large telescopes through 1950. But NSF's budget failed to keep pace with the rapidly increasing number of astronomers and the expanding observational opportunities enabled by the new technologies.

By 1985, astronomers began turning again to private benefactors to finance large ground-based telescopes. The Keck Telescopes, for instance, were supported by a private gift of $120 million to Caltech. In the next decade, public/private partnerships will help create telescopes in the 20-30 meter class, including the Giant Magellan Telescope a multiple-mirror design with 7 8.4-meter spin-cast mirrors and an equivalent collecting area of a single 22-meter mirror.

The US lead in state-of-the-art telescopes has been challenged by European astronomers. The European Very Large Telescope, four 8.2-m telescopes on a very dry site in northern Chile, now has the largest total collecting area in the world (326,000 square inches). And the European Southern Observatory is presently constructing the Extremely Large Telescope, a multiple-mirror design with a 39-m equivalent collecting area.

The European Southern Observatory Very Large Telescope.


Astronomers today have access to almost the entire cosmic electromagnetic spectrum described in Lecture 2 and ranging from radio waves at the long wavelength end to gamma rays at the short wavelength end. All of the devices for detecting EM waves are called "telescopes," even though some (e.g. radio antennas) look nothing like classical optical telescopes.

The first steps outside the confines of the optical band were taken in the 1930's and 40's when natural radio waves were first detected from cosmic objects. Radio astronomy developed rapidly in the 1950's, followed by infrared, ultraviolet, X-ray, and gamma ray astronomy. Because of absorption by the Earth's atmosphere, observations of most cosmic EM radiation other than optical and radio require a telescope in space (see below).

You can find compilations of information on telescopes at the following websites:


Access to powerful telescopes is provided through a competitive proposal review process, in which an astronomer, or group of astronomers, submits a detailed proposal which is reviewed in competition with other proposals by a "time allocation committee."

There are many more proposals than can be accommodated. There is always intense competition for "dark of the Moon" time (only two weeks out of each month), which is required for work on very faint objects at ground-based sites. One out of two proposals will be successful on "under-subscribed" telescopes, while only one out of 5-10 will succeed for more cutting-edge facilities like HST or the Chandra X-ray Observatory. It typically takes astronomers 2-4 weeks to write a competitive proposal. This is why they seem busy most of the time.

For more background on how astronomers use large telescope facilities and the resources needed to maintain their scientific productivity, see this article on the Space Telescope Science Institute.


The Large Binocular Telescope is a good example of current telescope building technology. UVa is a member of the consortium of universities operating the LBT in southern Arizona.

Astro on orbit
Astro-2 UV observatory in Shuttle payload bay.


1. Why telescopes in space?

2. Why do space telescopes cost up to several 100x as much as equivalent sized ground-based facilities?

3. Examples:

Web links: links are embedded in the text above.

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Last modified December 2020 by rwo

Images from observatory public sites. Text copyright © 1998-2020 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 1230 at the University of Virginia.