ASTR 511 (O'Connell) Lecture Notes
PRINCIPAL UVOIR TELESCOPES
Summit of Mauna Kea, Hawaii
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
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.
The Mt. Wilson 100-in Reflector
II. AMERICAN OBSERVATORIES 1880-1970
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:
Optical Figuring Tolerance:
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.
standard reflector telescope designs
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.]
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:
The Yerkes Observatory 40-in refractor (Univ. of Chicago, 1897).
The largest refractor ever built (picture above right). Lens
originally intended by USC for a Mount Wilson site. Optics figured
by Alvan Clark; f/19; 63-ft length.
- The Mt. Wilson 60-in
reflector (1908), the first major reflector in the US. Fork mount. Optics
figured by George W. Ritchey. f/5 Newtonian, bent-Cassegrain. First
to have a coude focus. Early optical coatings were silver. Mt. Wilson
Observatory was operated by the Carnegie Institution.
The Mount Wilson 100-in reflector (1917), the most important
telescope of the 20th century (photo at beginning of this section). Optical figuring
by George Ritchey (with reluctance, because of bubbles in mirror blank below surface). English yoke
mount on mercury flotation bearings (exclusion zone near pole). Three
main foci: Newtonian
(f/5; reachable by dome-mounted platform), bent-Cassegrain, and
The Palomar Observatory 200-in (5-m) reflector (Caltech, 1948), the largest
working telescope until 1992. The 20-year process of planning &
building the 200-in is described in a photo-history here.
Placed at Palomar because, even by 1920, Mt. Wilson was suffering serious
light pollution from Los Angeles.
Above right is a photo of the 200-in dedication in 1948. A diagram of the
telescope is available here. Yoke-mounted, with a horseshoe-shaped,
oil pressure supported north bearing. First telescope with a prime-focus
(f/3.3) "cage" capable of carrying an astronomer. Other foci:
At the urging of Fritz Zwicky, among others, Caltech commissioned
the 48-in Schmidt
telescope as a survey instrument to support the 200-in.
Based on the design of
Bernhard Schmidt this catadioptric telescope uses a spherical
mirror together with a thin refractive corrector lens to eliminate
spherical aberration over a wide field of view (6o diameter
in this case). The 14-in square focal plane is inside the body of the
telescope; it is convex and requires that photographic plates be curved
under pressure to match. The 48-in Schmidt made the multiband
photographic "POSS" surveys.
- In the 1970's, NOAO developed a 4-m telescope design based on the 200-in, and
this has been reproduced, more or less closely, in multiple versions,
sizes 3.5-4.2m, around the world (e.g. KPNO 4-m, CTIO 4-m, AAT,
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.
- 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
- 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
- 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
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:
The US lead in state-of-the-art telescopes is now being challenged by
European and Japanese astronomers.
Site selection was recognized as critical. For best
transparency at infrared wavelengths high, dry sites, most over 12,000 ft, became
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
The European Southern Observatory Very
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.
- The European Southern Observatory 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), although the telescopes are normally operated separately.
Primary mirrors are spin-cast Shott Zerodur (very low CTE) in a
meniscus shape (46:1 aspect ratio). Shape is actively controlled with
150 actuators. The four telescopes can be combined to operate as an
interferometer and have well developed adaptive optics (AO) systems.
- The Keck 10-m
The largest individual telescopes,
based on a multiple-mirror design (picture at right).
36 stress-polished and cut 36" mirror segments.
The Keck mirror figure control system is a remarkable technical
achievement, although image quality is not quite as good as for a
monolithic mirror. With its AO system, Keck can deliver a resolution of 0.05" at
IR wavelengths. (Ground-based AO systems do not work well at
wavelengths below 1 micron or over fields larger than ~30".)
Principal foci: Naysmith, Cassegrain. Interferometric combination of
beams from the two telescopes is producing first results.
Telescope, operated by a consortium led by UTex and PennSt. An
"optical Arecibo" with a large (9.2-m) mirror made of spherical
segments. Fixed in altitude (55 degrees). Less successful figure control than
Keck. Intended for spectroscopy of faint sources. A twin is being
constructed in South Africa (SALT).
- The Gemini
Observatories: two telescopes (Mauna Kea & Paranal, Chile)
operated by an international consortium. 8.1-m, 20-cm thick Corning
ULE meniscus mirrors with 120 figure control actuators. IR-optimized,
using silver coatings. Total operations cost, about $33,000 per night.
- Subaru: 8.2-m optical/IR
telescope on Mauna Kea similar to Gemini.
- 6.5-meter class: MMT (Mt. Hopkins, AZ), Magellan I,
Magellan II (Las Campanas, Chile). All use Arizona Mirror Lab
spin-cast, borosilicate mirrors.
- The Hubble Space
First proposed by Spitzer in 1946 but launched in 1990. Long
lifetime (to 2010+). Orbits at 300 mi altitude. Can be serviced
by Space Shuttle crews (only such scientific satellite).
Small (2.4-m) mirror, but very high precision. Primary mirror
shape was inaccurate, however, owing to miscalibrated testing tools,
with edge about 2 µ too low. This produced large
spherical aberration (38 mm difference in focal length for inner
and outer mirror). This was correctable, however, with small additional
optical elements in each instrument. Servicing mission in 1993 carried
correcting optics, and HST achieved design goals thereafter.
Carries up to 6 instruments (imagers, spectrographs,
interferometers) covering the band 1100-22500 Å. Highest
UVOIR resolution images ever (0.05 arcsec). Deepest images m ~ 30 mag (4
billion times fainter than naked eye limit).
- Special Survey Telescopes: Sloan Digital Sky Survey (imaging &
spectroscopy), 2MASS All-Sky Infrared Survey (imaging): see Lecture 15.
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.
- The LBT consists of two
8.4-m diameter mirrors on a single alt-azimuth mount
- It can operate as two separate telescopes (pointing at the same
target), or it can combine the beams of the two mirrors to act as an
interferometer yielding the effective optical resolution of a 23-m
- Spin-cast, honeycombed, lightweight borosilicate mirrors, with
active ventilation thermal control system.
- Active control of secondary mirror for compensation of
mirror figure changes and suppression of atmospheric
- Click here for pictures of the LBT. Assembly
should be complete by 2004.
Additional References and Web links:
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.