ASTR 511 (O'Connell) Lecture Notes

PRINCIPAL UVOIR TELESCOPES


Mauna Kea

Summit of Mauna Kea, Hawaii


I. INTRODUCTION

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 in this course. That topic and the detailed properties of detectors and instruments are covered in Astronomy 512.


Mt. Wilson 100-in

The Mt. Wilson 100-in Reflector

II. AMERICAN OBSERVATORIES 1880-1970

A. BACKGROUND

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 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.

  • 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 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" (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.]


    • 40-in B. IMPORTANT MILESTONES


    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.

    III. NEW TECHNOLOGIES 1970-2000

    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.

    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 IR telescopes).
    • Multiple-mirror designs (modern implmentation by military; first large astronomical design: 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 directional control 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.


    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, most over 12,000 ft, became preferred.

    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 largest individual telescopes built to date, the 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 to date) secured through international treaties by the European Southern Observatory. Because of the rapidly escalating costs, US planning for telescopes in the 30-100 meter class over the next decade (e.g. the TMT) is based on hoped-for public/private partnerships.

    The US lead in state-of-the-art telescopes is now being challenged by European and Japanese astronomers.


    The European Southern Observatory Very Large Telescope.

    IV. STATE OF THE ART TELESCOPES

    There are now 10 ground-based telescopes operating with diameters of 6.5-m or larger, with three more expected in the next two years. A list is available here.


    V. THE LARGE BINOCULAR TELESCOPE

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


    Related pages:

    Additional References and Web links:

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    Last modified July 2007 by rwo

    Images from observatory public sites. Text copyright © 2000-2007 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.