Astronomical instruments

Astronomical instruments can be divided into two
major categories. The first category might include
all of the instruments which are used in the actual
process of observing celestial objects. Some of
these, like the meridian transit, are designed for
specific tasks such as the precise determination of
an observer's position on the earth or a star's
position in the sky; other observational
instruments are principally collectors of the
radiation emitted by stars, planets, nebulas, and
galaxies. These latter, which are generally referred
to as telescopes, enable objects invisible to the
naked eye to be seen, photographed, or otherwise
detected.

 In the second category may be grouped the auxiliary
instruments which are used to standardize, record, or
analyze the data obtained by the observational
equipment. Devices to provide an accurate standard of
time, to determine the brightnesses of stars, to record
their spectra, or to measure the positions of stars on
photographic plates, are examples of instruments
belonging to this second category.

 It should be mentioned at the outset that the radiation
gathered from a celestial object by a conventional
astronomical telescope lies in the visible and near visible
region of the electromagnetic spectrum. Over the past few
decades, however, an entirely different type of astronomical
telescope has come into wide use. These instruments, known
as radio telescopes, have been developed as the result of the
discovery in 1928-1932 by Karl G. Jansky of the Bell Telephone
Laboratories that the center of our own galaxy is a powerful
emitter of electromagnetic radiation in the radio wavelength
region. Since Jansky's initial discovery, many other celestial
"radio sources" have been found. The operating
principles and the evolution of radio telescopes, as well as the
significance and importance of the new field of radio
astronomy which they have fostered, are treated under Radio
Astronomy and Radar Astronomy. Also described elsewhere
are certain other electronic devices, such as "image
intensifiers," which already belong, or soon will belong, to the
growing list of techniques employed in modern astronomy.

Optical Telescopes
Optical Telescopes -- The purpose of a telescope is to collect light
and then to have the image magnified. The larger the telescope's
main light-collecting element, whether lens or mirror, the more
light is collected. It is the total amount of light collected that
ultimately determines the level of detail. All optical telescopes fall
into one of three classes (see Figure 01). In the refracting
telescope, light is collected by a 2-element objective lens and
brought to a focal plane. By contrast the reflecting telescope uses
a concave mirror for this purpose. The mirror-lens, or
catadioptric, telescope employes a combination of both mirrors
and lenses, resulting in a shorter, more portable optical tube
assembly. All telescopes use an eyepiece (located behind the focal
plane) to magnify the image formed by the primary optical
system. Other instruments can be placed in the focal plane for
various purposes, e.g., a photo-electric cell to measure the
luminosity, the slit of a spectrograph to analyse the light, or a
thermo-couple to measure temperature. The advantage of
reflecting telescope is that it has no chromatic aberration.
Moreover, mirrors can be manufactured to much larger
dimensions

than lenses. Large lenses sage in the middle and distort the received
image. Reflectors can also be made from a great variety of materials,
because all that matters is the reflecting surface, whereas lenses have to
be made from special types of glass. Figure 02a is the aerial view of
Mauna Kea in Hawaii. It shows the domes that house many of the world's
largest telescopes.

 Figure 01 Telescope, Types
 Figure 02a Mauna Kea

 The magnification of a telescope is given by the formula:

 M = OF / f, -------------------- (1)
where OF is the focal length of the objective, and f that of
the eyepiece.
The resolution (limit) of a telescope is given by the
formula:
R (in sec of arc) = 2.3 x 105 x ( /A), ------------------- (2)
where is the wavelength and A is the diameter of the
aperture. For example, if A = 100 cm and = 4000x10-8
(yellow light) then R = 0.092".

 Interferometer is used to measure the size of
astronomical objects. By careful analysis of
the resulting interference pattern, the
position of a point source, or fine detail in
an extended one, can be resolved. The same
formula for the resolution above is
applicable except the aperture A is replaced
by the baseline between the two receivers.
This technique has been used extensively
with radio telescopes; it is now also applied
to the optical telescopes as well.
The table below lists some of the optical
telescopes in the world (a complete listing
can be found in reference 1):

Observatory Location Aperture (m) Characteristics
Keck Mauna Kea, Hawaii 10.0 36 segment mirror
Keck II Mauna Kea, Hawaii 10.0 Interferometry optical

Hobby-Eberly Mt. Fowlkes, Texas 9.2 inexpensive, spectroscopy only

Observational performance
Subaru Mauna Kea, Hawaii 8.3
optimized

4 units combined as an
VLT (Very Large Telescope) Cerro Paranal, Chile 8.2
interferometer

Gemini North Mauna Kea, Hawaii 8.1 Twin of Gemini South

All sky coverage with Gemini
Gemini South Cerro Pachon, Chile 8.1
North

Next Generation Space Telescope Halo orbit 7-9 Scheduled for launch in 2007

Previous generation (1950-1990)
Hale Mt. Palomar, Ca., 5.0
limit

New Technology Cerro La Silla, Chile 3.5 Adaptive opticsa

Discovery of cosmic expansion
Hooker Mt. Wilson, Ca 2.5
(1917)

Observations outside the
Hubble Space Telescope Low Earth orbit 2.4
atmosphere

Solar Tower Kitt Peak, Arizona 1.8 Study of the Sun

Yerkes Williams Bay, Wisconsin, 1.0 World's largest refractor (1897)

Table 01 Optical Telescopes

Figure 02b Large Telescopes of the World

 By 2010 there are about 50 telescopes on Earth with at
least 2.5 meters in aperture diameter. Figure 02b below
shows the locations for these giant telescopes (from
"Astronomy", Vol. 38, Issue 11, November 2010).

Radio Telescopes · Radio Telescopes -- It is an instrument for
collecting radio waves from celestial objects.
The radiation is reflected from a parabolic
dish to an aerial (dipole), situated at the
focus, from which the signals are led to a
radio receiver. Because the wavelength of
radio waves is very large (from 0.3 mm to 30
cm), a radio telescope with an aperture
comparable to the optical telescope would
have a very poor resolution according to Eq.
(2). A considerable increase in resolution
can be obtained by using an interferometer
- an array of identical antennae spaced at
regular intervals as shown in Figure 03,
which shows that the elevation angle of a
celestial object (from the horizon) can be
calculated from the time difference and the
distance between the receivers (the
baseline). The angular separation
(resolution) is obtained from the difference
of the elevation angles corresponding to the
resolution of the time difference of T1 and
T2.

Interferometer
 The table below lists some of the radio telescopes in the world:

Observatory Location Resolution (arcsec) Characteristics

Very Long Baseline
VLBI Intercontinental > 0.001
Interferometer

Largest (dish) synthesis
VLA Socorro, NM > 0.04
array

Arecibo Puerto Rico > 0.2 Largest fixed dish

Effelsberg Effelsberg, Germany > 0.6 Largest single dish

Largest in southern
Parkes NSW, > 0.9
hemisphere

Table 02 Radio Telescopes

Infrared Telescopes
 Infrared Telescopes -- Infrared radiation
(wavelength between 1 and 1000 mm) from space
is mostly absorbed by the atmosphere (see Figure
04): so the largest infrared telescopes are built on
the tops of high mountains, installed on special
high flying aircraft or balloons, or better yet on
satellites orbiting the earth. However, atmospheric
absorption is not the only obstacle to analyse this
type of radiation on earth: the main problem,
which also occurs in space, is to distinguish the
signal collected from the "background noise", i.e.,
from the enormous infrared emissions of the Earth
or

 of the instruments themselves, since object
which is not at absolute zero, emits infrared
radiation. So everything around the
instruments (including the telescope)
produces "backround noise". Therefore,
special photographic film is used to
produce a "thermograph" of a
 celestial body, and the instruments must be
cooled continuously by immersion in liquid
nitrogen or helium (Figure 05).

Figure 04 Atomspheric Absorption
Figure 05 Infrared Telescope
 The table below lists some of the infrared telescopes in the world:

Observatory Location Aperture (m) Date
UKIRT Mauna Kea, Hawaii 3.8 Since 1978
To be launched in
FIRST Orbiting 3.0
2007
NASA IRTF Mauna Kea, Hawaii 3.0 Since 1979

Started operation in
SOFIA Airborne 2.5
February, 2006
Launched in August,
SIRTF Heliocentric orbit 0.85
2003
Launched in
ISO Geocentric orbit 0.6
November, 1995
Operated for ten
IRAS Geocentric orbit 0.6
months in 1983
Table 03 Infrared Telescopes

Acronym: UKIRT - United Kingdom Infrared
Telescope.
FIRST - Far Infrared Space Telescope.
IRTF - Infrared Telescope Facility.
SOFIA - Stratosphere Observatory for
Infrared Astronomy.
SIRTF - Space Infrared Telescope Facility;
renamed to Spitzer Space Telescope in honor of the
late
astrophysicist Lyman Spitzer Jr.,
who first conceived of a large telescope in orbit.
ISO - Infrared Space Observatory.
IRAS - Infrared Astronomical Satellite.

High Frequency Observations
 Observations of High
Frequency Radiation --
As shown in Figure 04,
radiation with
wavelength shorter than
310 nm are absorbed in
the Earth's
stratosphere, high
above any terrestrial
observatory. These
short wavelength
radiations can be
studied only by
instruments carried on
very-high--altitude
balloons, rockets,
satellites, and

Figure 06 Grazing Telescope
Figure 07 Scintillator
 Although attempts to study the sun's UV spectrum
from balloons were made during the 1920s, it was not
until 1946 that rocket-borne instruments made this
possible. Only limited additional progress was made
until 1962, when the first Orbiting Solar Observatory
(OSO) satellite was launched by the National
Aeronautics and Space Administration (NASA). It
returned thousands of UV spectra, including the first
exteme-ultraviolet (wavelengths below 91 nm)
observations of the solar corona. The International
Ultraviolet Explorer (IUE) was in orbit between 1978
and 1996.

Its large telescope (0.45 m aperture) made possible
the first UV observations of objects beyond the
Milky Way. The Extreme Ultraviolet Explorer
(EUVE, 1990-1999) was the first orbiting
observatory to focus on that part of the spectrum.
Ultraviolet spectroscopy has been particularly
valuable because many of the most abundant
atoms and ions in the universe have their strongest
lines in that region of the spectrum. Right now
there are no dedicated ultraviolet observatories in
orbit. The Hubble Space Telescope can perform a
great deal of observing at ultraviolet wavelengths,
but it has a very fairly small field of view.

 X-rays are typically emitted by gaseous bodies with
temperatures ranging from 106 to 108 oK. Conventional
telescopes cannot be used at x-ray (or EUV) wavelengths
because mirrors abosrb x-rays rather than reflect them,
unless the x-rays graze the surface at a very shallow
angle. Satellites such as ROSAT (in orbit between 1990-
1999), and the Chandra X-ray observatory (since July,
1999) utilize "grazing incidence" telescopes (GRITs),
which bring x-rays to a focus by reflecting them at
shallow angles from the surfaces of nested sets of
tapering, tubelike reflectors as shown in Figure 06. X-
ray observations have discovered a variety of objects,
ranging from hot patches and cool "holes" in the Sun's
outer atmosphere to swirling discs of hot gas
surrounding collapsed stars and black holes to hot
clouds of gas in intergalactic space.

Gamma rays are the most energetic form of
electromagnetic radiation. Because their wavelengths
are far smaller than the sizes of the atoms in a mirror,
gamma rays cannot be focued by reflection, and the
early gamma-ray satellites were unable to form images
of sources or even determine their positions with
confidence. Modern gamma-ray imaging systems and
spectrometers, such as those carried on board the
Compton Gamma Ray Observatory (CGRO, 1991-2000),
make use of scintillators (see Figure 07), which are
devices that convert gamma rays into visible photons
that are more easily detected and analyzed. Among
known gamma-ray sources are the Milky Way, some
pulsars, and some

 quasars. Most puzzling of all are the 2600 gamma-ray
bursts detected by CGRO. The next generation
gamma-ray observatory is GLAST (Gamma-ray Large
Area Space Telescope) scheduled to be launched in
2005. It is designed for making observations of
celestial gamma-ray sources in the energy band
extending from 20 MeV to more than 300 GeV. Figure
08a is a gamma-ray sky animation - constructed from
simulating the first 55 days of GLAST observations of
cosmic gamma-ray sources. It shows the plane of our
Milky Way Galaxy as a broad U-shape, with the center
of the galaxy toward the right. Besides the diffuse
Milky Way glow, the simulated objects include flaring
active galaxies, pulsars, gamma-ray bursts, the flaring
Sun, and the gamma-ray Moon. The GLAST was

Figure 08a Gamma-ray
Sky

finally launched on June 11, 2008 many years behind schedule. It will study
gamma-rays from extreme environments in our own Milky Way galaxy, as
well as supermassive black holes at the centers of distant active galaxies,
and the sources of powerful gamma-ray bursts.

Figure 08b is another GLAST
gamma-ray sky map taken in
the period from August 4 to
October 30, 2008. The map
highlights the "top ten" list
of five sources within, and
beyond the Milky Way.
Within our galaxy: the Sun
traces a faint arc across the
sky during the observation
dates, LSI +61 303 is an X-ray
binary star, PSR J1836+5925
is a type of pulsar that is only
seen to pulse at gamma-ray
energies, and 47 Tuc is a
globular star cluster.
Figure 08b Gamma-ray Sky

A fifth galactic source (unidentified), just
above the center of the galactic plane, is a
variable source and has no clear counterpart
at other wavelengths. Beyond our galaxy:
NGC 1275 is a large galaxy at the heart of the
Perseus galaxy cluster, while 3C 454.3, PKS
1502+106, and PKS 0727-115 are active galaxies
billions of light-years away. Another
unidentified source, seen below the galactic
plane, is likely beyond the boundaries of the
Milky Way. Its nature remains a mystery.

Table 04 lists the three types of gamma-ray sources in the Milky Way (MW). It is
thought that they may be associated with the unknown dark matter particle
(dmp). The mp denotes the mass of proton (nucleon).
Gamma-ray
Instrument Process Source Distribution
Energy

Annihilation of ~ 0.003 mp light Around the
INTEGRAL 511 kev
e-e+ dmp center of MW

Annihilation of ~ 60 mp Faint galactic
EGRET ~ 1 Gev
dmp neutralino background

Annihilation of ~ 20000 mp heavy Point source at
HESS ~ 100 Gev
dmp dmp MW center

Table 04 Gamma-ray Sources in Milky Way

 Acronym: INTEGRAL - INTErnational Gamma-Ray
Astrophysics Laboratory (a space instrument
combining fine spectroscopy and imaging
of gamma-ray emissions in the energy range of 15
keV to 10 MeV ).
EGRET - Energetic Gamma Ray
Experiment Telescope (space telescope for detecting
30 MeV - 30 GeV gamma-rays).
HESS - High Energy Stereoscopic System
(a system of ground based Cherenkov Telescopes for
the investigation of cosmic gamma rays in
the 100 GeV energy range).

Meanwhile the Swift satellite (Figure 09) was launched
into a low-Earth orbit on November 20, 2004. Its
mission is to solve the mystery of Gamma-ray bursts
(GRBs). They are the most powerful explosions in the
Universe since the Big Bang. They occur approximately
once per day and are brief, but intense, flashes of
gamma radiation. They come from all different
directions of the sky and last from a few milliseconds
to a few hundred seconds. So far scientists do not
know what causes them. Do they signal the birth of a
black hole in a massive stellar explosion? Are they the
product of

Figure 09 Swift
 the collision of two neutron stars? Or is it some other exotic
phenomenon that causes these bursts? Swift is designed to look for
faint bursts coming from the edge of the universe. On September
2005, astronomers announce that they have detected a cosmic
explosion (GRB) at the very edge of the visible universe. The
explosion occurred soon after the first stars and galaxies formed,
perhaps 500 million to 1 billion years after the Big Bang. It was
probably caused by the death of a massive star. It is believed that this
observation opens the door to the use of GRBs as unique and
powerful probes of the early universe.

 The key to unravel the nature of GRB comes in part from
the discovery that they are narrowly focused beams. This
realization allowed astronomers to estimate energies for
individual bursts and hypothesize the number of total
bursts occurring over a given time interval. The usual
assumption of spherical emission would over estimate the
released energy and under estimate the number of GRB. By
2007, astronomers can link the most common type of GRB,
those lasting 20 seconds or longer, with the collapse of
massive stars about 30 or more times larger than the Sun.
While the short GRB (lasting just a few milliseconds) came
from a neutron star crashing into a black hole or another
neutron star.

 Footnotes:
 aIn adaptive optics, light from the primary mirror is
directed onto a smaller flexible mirror behind which a
large number of actuators are located. The actuators
distort the shape of the mirror to cancel out distortions
in the incoming wavefronts of light that have been
caused by the atmosphere. Wavefront distortions are
sensed by monitoring a suitable bright star, or an
artificial "star" generatged by shining a powerful laser
beam into the upper atmosphere. All these operations
are computerized. Computers have so revolutionized
astronomy, in fact, that researchers rarely look
through the telescope or work in the dome during
observations. Instead, they operate the instrument in
comfort from a control room.


Astronomical instruments

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