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 except briefly in
this course (see this page.)
That topic and the detailed properties of detectors and instruments
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:
Brief description of
standard reflector telescope designs
- Lenses (even achromats)
produce chromatic aberration, limiting the bandwidth usable for
imaging & spectroscopy.
- Lenses must be figured on two sides (per
element), whereas mirrors need be figured only on one.
- 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
- 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.
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
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.]
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
the mirror blank below the 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 coude.
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
shown 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: Cassegrain,
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 required that photographic plates be
curved under pressure to match. The 48-in Schmidt made the multiband
photographic "POSS" surveys and today, with the substitution of a
large-format CCD mosaic camera, is undertaking the
- 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 1970-2020
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
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.
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:
- Shorter focal length optics, < f/2 (permitting smaller domes)
- Lightweight structural materials
- Lightweight monolithic mirrors (thinner designs and/or honeycombed)
- Spin-cast glass mirrors
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
- 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 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
and adaptive optics systems for obtaining near-diffraction-limited
performance in the near infrared.
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
The European Southern Observatory Very
IV. STATE OF THE ART TELESCOPES
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
- 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 (211 square meters),
although the telescopes are normally operated separately. The
monolithic primary mirrors are spin-cast Schott 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)
- The Keck Observatory
Twin 10-m diameter telescopes based on a
segmented primary design (picture at
right). Each contains 36 stress-polished and cut 36-in hexagonal mirror
segments, with a total collecting area of 76 square meters and
a focal ratio of 1.75. The optical system is a Ritchey-Chretien
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 arcsec at
IR wavelengths. (Ground-based seeing becomes worse
at shorter wavelengths, and AO systems do not work well at
wavelengths below 1 µ or over fields larger than ~30 arcsec.)
Principal foci: Naysmith, Cassegrain. Interferometric combination of
beams from the two telescopes was implemented until 2012.
Telescopio Canarias, on La Palma island, is operated by a consortium
led by Spain and Mexico. It has a segmented-primary design similar to
each Keck telescope but with slightly larger segments and a collecting area
of 78 square meters.
- Hobby-Eberly 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 was constructed in South Africa
- The Gemini
Observatories: two telescopes (Mauna Kea, Hawaii & Paranal, Chile)
operated by an international consortium, including the US. 8.1-m,
20-cm thick Corning ULE meniscus mirrors with 120 figure control
actuators. IR-optimized, using silver coatings. Total operations
cost, about $45,000 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,
- The Hubble Space
Spitzer in 1946 but launched in 1990. HST orbits at 300 mi altitude,
where the deleterious effects of the Earth's atmosphere (seeing,
absorption, light emission and scattering) are eliminated. Serviced 5
times by Space Shuttle crews (HST is the only scientific satellite
capable of human servicing). Instrument and spacecraft upgrades were
essential for its long lifetime (27 years to date) and outstanding
HST has a small (2.4-m) but very high precision mirror in a
conventional Ritchey-Chretien Cassegrain configuration. Primary
mirror shape was inaccurate, however, owing to miscalibrated testing
tools, with an edge about 2 µ too low. This produced large
aberration (a 38 mm difference in focal length between the inner
and outer mirror areas). That was correctable, however, with small
additional optical elements in each instrument. The first servicing
mission in 1993 carried correcting optics, and HST achieved its design
Carries up to 6 instruments (imagers, spectrographs, interferometers)
covering the band 1100-22500 Å. Owing to its outstanding
instrumental stability, optical precision, excellent pointing, and the
absence of atmospheric blurring, HST has produced the
resolution UV-optical images ever (0.05 arcsec). It has produced the
deepest images yet made, to m ~ 30 mag (4 billion times fainter
than the naked eye limit).
- Special Survey Telescopes: Sloan
Digital Sky Survey (imaging & spectroscopy),
Infrared Survey (imaging):
see Lecture 15.
The Large Binocular Telescope
V. 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.
- The LBT consists of two
8.4-m diameter mirrors on a single alt-azimuth mount. It carries
6 pairs of focal stations.
- 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
diameter telescope (about 20 mas at 2 µ). The combined
collecting area of the two mirrors is 111 square meters, corresponding
to a single 11.8-meter (460-in) diameter mirror.
- 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
seeing. Multiple laser guide-star adaptive optics system.
here for pictures of LBT construction.
VI. THE NEXT GENERATION
Two very large telescopes based on the segmented-mirror concept
of Keck have been designed:
Thirty Meter Telescope
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
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
Additional References and Web links
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.