ASTR 1230 (O'Connell) Supplementary Lecture Notes
7.1 MODERN OBSERVATIONAL ASTRONOMY
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
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
Thus, progress has coupled new technology and visionary
astronomical pioneers with the generosity of wealthy private donors or
the financial strength of governments.
100-in reflector on Mt. Wilson. Click for
B. AMERICAN OBSERVATORIES 1880-1950
Initially, telescopes were built adjacent to universities, usually in
or near cities. However, the growth of artificial
drove telescope construction to ever more remote
sites, first in the southwestern US (e.g. California, Texas, Arizona),
and ultimately to high mountains like
in Hawaii or the northern deserts of Chile.
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
The Yerkes Observatory 40-in refractor (Univ. of Chicago, 1897).
The largest refractor ever built (see picture above right). It was
not practical to build larger refractors
(see Lecture 2 for details).
The Mount Wilson Observatory 100-in reflector (1917), the most
important telescope of the first half of the 20th century (see photo
at beginning of this section). Hubble proved the existence of other
galaxies and discovered the expanding universe with the 100-in.
The Palomar Observatory 200-in (5-m) reflector (1948), the largest
working telescope until 1992. The 20-year process of planning &
building Palomar is described in a photo-history here.
At right is a photo of the 200-in dedication in 1948.
Here is a diagram of the
telescope's unusual design. Work with the 200-in has concentrated on
the structure & evolution of stars and galaxies, quasars (discovered
with the 200-in in 1963), formation of galaxies out of intergalactic
gas, measuring the expansion of the universe, and supermassive black
holes in galaxy nuclei.
C. AMERICAN ASTRONOMY 1950-2000
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
per unit optical collecting area. These included:
- Shorter focal length optics (requiring smaller domes)
- Lightweight structural materials
- Lightweight monolithic mirrors (thinner designs or honeycombed)
- Multiple-mirror designs, in which a large reflecting surface is
tiled with smaller (usually hexagonal) mirrors; this allows
construction of arbitrarily large telescopes
- Alt-azimuth mounts (less costly)
- High performance computer control for
active figure correction of thin mirrors and directional
control of alt-az mounts
- Thorough and rapid ventilation of domes to keep nighttime
temperatures uniform and therefore reduce "seeing"
Another key innovation was the development of
, 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
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
(see picture above right). These have an
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
- The National Science Foundation
(NSF), which provides general research grants in astronomy
and directly supports the operations of, among others:
- The National Optical Astronomy Observatories.
In the 1970's, NOAO developed a
160-in (4-m) telescope design based on the 200-in, and this has
been reproduced, more or less closely, in multiple versions around the
world. At right is the 4-m dome at Kitt Peak National
- The National Radio Astronomy
Observatory, which operates radio telescopes at a number of
sites in the US and
millimeter-wave array in Chile. The headquarters of NRAO is in
- The Gemini
Observatories, two 8-m class telescopes operated by an
- The Large Synoptic Survey
Telescope, which features
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 and achieves a wide field of view.
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. LSST is now under construction in Chile.
National Aeronautics and Space Administration (NASA), which
designs and deploys a wide variety of satellite observatories and deep
space probes. Because of their cost (see below), NASA contributes
over 10 times as much funding per year to astronomy as does NSF.
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
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
, 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
The European Southern Observatory Very
D. EM SPECTRUM COVERAGE
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
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
Because of absorption by the Earth's atmosphere,
observations of most cosmic EM radiation other than optical and radio
require a telescope in space
You can find compilations of information on telescopes at the following
E. COMPETITION FOR TELESCOPE TIME
Access to powerful telescopes is provided through a competitive
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.
Telescope time allocation committees are one example
of peer review, which is the basic method by which
standards are preserved in all sciences. Scientists seeking access to
privileges or special resources (telescopes, supercomputers,
acclerators, space missions, laboratory space, grant money,
publication space, etc.) must regularly submit their ideas to the
scrutiny of experts. Typically, scientists spend 1-2 weeks each year
reviewing and reporting on the work of other scientists.
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
F. THE LARGE BINOCULAR TELESCOPE
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.
- The LBT consists of two
8.4-m diameter mirrors on a single mount (corresponding to the light
collecting area of 3,500 8-in telescopes).
- It can operate as
two separate telescopes (pointing at the same object), or it can
combine the beams of the two mirrors to act as an
interferometer yielding the effective optical resolution (see
Lecture 2) of a 23-m diameter
here for pictures of the LBT being assembled. The official
dedication was held in October 2004.
Astro-2 UV observatory in Shuttle payload bay.
G. SPACE ASTRONOMY
1. Why telescopes in space?
- Freedom from
atmospheric absorption (see Lecture
2) permits observations in the UV, X-Ray, Gamma Ray, Infrared, and
Sub-millimeter regions that are blocked for ground-based
- Freedom from the bright atmospheric night sky. The sky on
the ground is 3-10x brighter in
the visible, 60-1000x brighter in the near-IR.
- Freedom from atmospheric turbulence (seeing), allows
2. Why do space telescopes cost up to several 100x as much as
equivalent sized ground-based facilities?
- Must be self-contained: power, pointing, computer control,
heating/cooling, communications with Earth
- Must be highly reliable (failure probability---for any
reason---only 2-5% per year during early deployment); no repair possible
(except HST); must survive launch; must survive harsh environment
(e.g. radiation, vacuum, 300o temp differential
- Complexity: high tech equipment; complicated operations.
- Expensive transportation (at right)
- ====> Labor intensive!
- Hubble Space
- First proposed by Lyman Spitzer in 1946 but not launched until
1990. Long lifetime (to 2020 or beyond). Orbits at 300 mi altitude
(see picture at top of page).
- Small (94-in) mirror, but very high precision. Can be serviced
by Space Shuttle crews (only such scientific satellite). Carries up
to 6 powerful instruments (imagers, spectrographs). Highest
resolution optical-band images ever (0.04 arcsec). Deepest images
(4 billion times fainter than naked eye limit) of universe in
UV, optical, near-IR.
- The final servicing mission for HST was completed in May 2009,
with the delivery of two new instruments, the repair of two others,
and installation of a number of housekeeping upgrades. You can find
full coverage of the servicing mission
- Chandra X-Ray Observatory
- Long-lifetime satellite observatory for observing cosmic X-ray sources.
X-Rays have EM wavelengths of a few Angstroms, about 2000 times smaller than
- Uses grazing
incidence optics with very high precision, nested mirrors (46-in diameter).
Gives resolution of about 1 arc-sec (highest ever for X-rays).
- X-rays are only emitted by very high-temperature gas (10
to 100 million degrees), so Chandra probes the "high energy" universe.
Chandra X-Ray image of hot interstellar gas and accreting
in the galaxy NGC 4697. (C. Sarazin,
- ASTRO missions
- First proposed 1978 for multiple short-duration missions
on Space Shuttle. First launched in 1990.
- UV observatory with 3 telescopes, 2 missions 1990, 1995, up to
15 days in orbit. (Picture at beginning of this section).
- Click here for pictures of UIT and the Astro missions.
Web links: links are embedded in the text above.
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