ASTR 511 (O'Connell) Lecture Notes


Mauna Kea

Summit of Mauna Kea, Hawaii

Estonian translation


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 developed slowly, as in the early 19th century, progress was slow. When technology surged, as in the late 20th century, progress was explosive.

This page surveys the principal UVOIR telescopes available in this decade together with a review of the milestones of the last 100 years. A hallmark of the major telescopes in this era is the remarkable variety of clever innovations, many of which have even more distant historical roots. There is very little in current telescope design that was not thought of long ago, though converting good ideas into realizable technologies is a different matter.

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

Note: we will not discuss telescope optics except briefly in this course (see this page.) That topic and the detailed properties of detectors and instruments are covered in Astronomy 512.

Wilson 100-in

The Mt. Wilson 100-in Reflector



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. Success with large telescopes demands that a large set of disparate requirements be met simultaneously: quality glass for optical elements, high precision shaping/polishing of optical surfaces, precision mechanical support systems, excellent control systems, excellent instrumentation, and good observing sites. Any such project is a major engineering undertaking.

Most of the large telescopes through 1960 were associated with universities. They were costly and required substantial private donations. Because of an abundance of industrial expertise, excellent observing sites, and wealthy contributors, the US became the world's leader in building large telescopes.

Refractors vs Reflectors:

The large telescopes of the late 1800's were mainly refractors. These were simple optically and featured good stability for astrometry, for instance. Through the mid-1800's, most reflectors had used metal mirrors and were of generally poor optical quality. However, the invention of high reflectivity thin metallic coatings for glass (initially silver) around 1850 made possible the use of glass mirrors. These were immediately competitive with refractors in terms of quality.

This, together with a host of other reasons dictated that instruments larger than the Yerkes 40-in refractor were all reflectors:

  1. Lenses (even achromats) produce chromatic aberration, limiting the bandwidth usable for imaging & spectroscopy.
  2. Lenses must be figured on two sides (per element), whereas mirrors need be figured only on one.
  3. Mirrors are easy to support accurately from behind, whereas lenses require support at their edges and will sag; it is harder to support heavy lenses mechanically at the top end of a telescope tube than a mirror at bottom end.
  4. The beam-folding action of primary and secondary mirrors means that reflector tubes are much shorter than in a "straight through" refracting design, easing mechanical design and reducing dome size.

  • Brief description of standard reflector telescope designs

  • Optical Figuring Tolerance:

    To maintain a good image, a single reflecting surface must be figured to within 1/4 wavelength of its intended design. For optical telescopes, this is 10-5 cm---very demanding. Good polishing/test techniques capable of reaching this precision were not developed until the late 19th century. When there are several reflecting surfaces, the tolerances must be tighter. Specifications for state-of-the-art telescopes are for 1/10-1/20 wave optics. The most precise large mirror yet made was the HST 2.4-m, which was figured to about 1/50 wave (of its test wavelength of 6328 Å).

    Scale comparison: if a 320-in (8-m) diameter telescope mirror were scaled up to the size of the continental United States, i.e. about 3000 miles diameter, then the maximum size of a ripple allowed in its polishing would be less than 2 inches! [You should be asking yourself how it is possible to determine the figure of a large mirror to that precision without the use of very expensive metrology equipment.]



    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 also built several major solar telescopes. Hale had a great facility for obtaining private financing, from Carnegie and Rockefeller, among others. The four major Hale telescopes were

    Also of note:

    8-m Mirror

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


    Telescope technologies steadily improved throughout the first half of the 20th century, with much progress in mechanical design (e.g. the oil pressure bearing of the 200-in), structural materials, optical figuring, electrical control systems (e.g. analog computers), and astronomical instruments to attach to telescopes. However, until about 1975, big telescope design was still based largely on the concepts used for the Mt. Wilson and Palomar telescopes (designed 1900-30). Unfortunately, the cost of extending such designs to sizes larger than 200-in was prohibitive (for a given design, cost scales roughly as the mass of the telescope or the cube of the diameter).

    In the early 1980's a series of innovations was introduced that made yet larger telescopes affordable, mainly by reducing the total weight, including the dome, per unit optical collecting area. These included:

    • Shorter focal length optics, < f/2 (permitting smaller domes)
    • Lightweight structural materials
    • Lightweight monolithic mirrors (thinner designs and/or honeycombed)
    • Spin-cast glass mirrors (Roger Angel, UAz; method originally developed by Robert Leighton, Caltech, for mid-size epoxy IR mirrors).
    • Multiple-mirror or segmented-mirror designs (modern implmentation by military; first large astronomical design by Jerry Nelson, UCal)
    • Alt-azimuth mounts (simpler weight-bearing design is less costly than equatorial)
    • Naysmith foci (light beam exits along altitude axis) allow use of massive instruments without stress on telescope tube
    • Common azimuth bearing for both dome and telescope; dome & telescope move together
    • High performance computer control for active figure correction of thin mirrors and precision orientation of alt-az mounts
    • Thorough and rapid ventilation of domes and mirror cells to keep nighttime temperatures uniform (within ~1o C) and therefore improve seeing.
    • Sophisticated active and adaptive optics systems for obtaining near-diffraction-limited performance in the near infrared.

    Various combinations of these innovations were first incorporated in a number of 4-m class telescopes (e.g. ESO NTT, WIYN, ARC), but their main impact was on 6-m and larger telescopes.

    Important related issues:

    Site selection was recognized as critical. For best transparency at infrared wavelengths, high, dry sites, over 8,000 ft, became preferred. The summit of Mauna Kea, on the big island of Hawaii, is at 13,800 feet and is the premier telescope site in the northern hemisphere.

    The financing yo-yo:

      After 1950, public funding from NSF had almost completely replaced the private financing responsible for the large telescopes prior to World War II. 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, US astronomers began turning again to private benefactors to finance large ground-based telescopes.

      The twin Keck 10-m telescopes were supported by a private gift of $120 million to Caltech, with a comparable contribution of state funds in the form of operating costs from the University of California. Other large facilities with a significant private component include the Magellan telescopes, the MMT, and the LBT. By contrast, the European VLT was financed with public funds (about $800 million) secured through international treaties by the European Southern Observatory. Because of the rapidly escalating costs, US planning for the next generation of telescopes in the 20-50 meter class (e.g. the TMT) is based on public/private partnerships.

    The European Southern Observatory Very Large Telescope.


    There are now (2017) 16 ground-based telescopes operating with diameters of 6.5-m or larger. A list with links is available here, and a nice graphical comparison of the collecting areas of large telescopes through 2025 is shown here.

    The Large Binocular Telescope


    The Large Binocular Telescope currently has the largest collecting area of any single telescope. UVa is a member of the LBT consortium of universities. First binocular light was achieved in March, 2008.


    Two very large telescopes based on the segmented-mirror concept of Keck have been designed: the Thirty Meter Telescope and the European Extremely Large Telescope (39-meters or 1530-in). The ELT is now under construction in Chile, but the TMT is embroiled in a dispute over environmental and cultural impacts at its preferred Mauna Kea site. The Giant Magellan Telescope (at right), also under construction in Chile, is a multiple-mirror design with 7 8.4-meter spin-cast mirrors and an equivalent collecting area of a single 22-meter mirror. The combined primary mirror surface is parabolic, and the six off-axis segments are asymmetrically aspheric and difficult to figure.

    Another large telescope with a very different design and operations mode is the Large Synoptic Survey Telescope. To achieve a wide field of view (3.5o) it employs a unique 3-mirror design in which the primary and tertiary mirrors have been figured on a single piece of spin-cast glass 8.4-meters in diameter. The telescope is intended to repeatedly image the entire usable sky every three nights, searching for transient or moving targets while building up an ultra-deep combined image of the sky. Continuous output from its 189 imaging CCDs will generate an unprecedented data volume (15 TB/night). Under construction in Chile, LSST should be completed by 2019.

    The James Webb Space Telescope, expected to be launched in 2018, is the follow-on to HST. It features a 6.5-m diameter primary mirror (a 25 square meter collecting area, 5.5 times that of HST) composed of 18 hexagonal segments that must be deployed on-orbit. JWST carries four imaging and spectroscopic instruments and is optimized for the near-infrared (1-30 µ). A large sunshield permits a low overall structural operating temperature through cooling to space. JWST will orbit around the Earth-Sun L2 point, about 900,000 km from Earth.

    Related pages

    Additional References and Web links

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    Last modified June 2017 by rwo

    Images from observatory public sites. Text copyright © 2000-2017 Robert W. O'Connell. All rights reserved. These notes are intended for the private, noncommercial use of students enrolled in Astronomy 511 at the University of Virginia.