Radio Telescopes
The radio band is too wide (five
decades in wavelength) to be covered
effectively by a single telescope design. The surface
brightnesses and angular sizes of radio sources span an even wider
range,
so a combination of single telescopes and aperture-synthesis
interferometers are needed to detect and resolve them. It is
not practical to build a single radio telescope that is even close to
optimum for all
of radio astronomy.
The ideal radio telescope should have a large collecting area to detect
faint sources. The
effective collecting
area $A_{\rm e}(\theta, \phi)$ of any antenna averaged over all
directions $(\theta, \phi)$ is
$$\langle A_{\rm e} \rangle=
{\lambda^2 \over 4
\pi}~.$$
Large peak collecting areas imply extremely directive antennas. Only at long wavelengths ($\lambda
> 1$ m) is it feasible to
construct reasonably sensitive antennas from reasonable numbers of
small, nearly
isotropic elements such as dipoles.

The
20.5 MHz Bruce Array used by Karl
Jansky.
Image
credit
Jansky's $\lambda \approx 15$ m
"wire" antenna is an array of phased dipoles. It produces a wide
fan beam on the horizon but has a large collecting area
because $\lambda^2$ is so large. Directive aperture antennas are
needed for sensitivity at
higher frequencies.
The simplest aperture antenna is a waveguide horn.
Radiation
incident on the opening is guided into a tapered waveguide. At
the narrow end of the waveguide is a probe that converts the
electromagnetic wave into an electrical current or measures currents in
the waveguide walls.
Horn antennas pick up very little ground radiation because, unlike most
paraboloidal dishes, their apertures are not partially blocked by
external feeds
and feed-support structures, which scatter ground radiation into the
receiver. This freedom from ground pickup allowed Penzias
&
Wilson (1965, ApJ, 142, 419) to show that the zenith antenna
temperature of the Bell Labs horn was 3.5 K higher at $\nu \approx 4$
GHz than expected—the first detection of the cosmic microwave
background radiation.

The horn
antenna at Bell Labs, Holmdel, NJ that Penzias and Wilson used to
discover the 3 K cosmic
microwave background radiation in 1965.
Image
credit
Because the aperture of a waveguide horn is not blocked by any
feed-support structure, it is also easier to calculate the gain of a
horn
antenna from first principles than to calculate the gain of a partially
blocked aperture. Thus small horn
antennas
have been used by radio astronomers to measure the absolute
flux densities of very strong sources such as Cas A.
Radio
astronomers observing with large dishes typically do not measure the
absolute flux densities of sources, only their relative flux densities by
comparison with calibration sources whose absolute flux densities are
known in advance. The process of measuring the absolute flux
densities of Cas A and comparing them with the flux densities of weaker
point sources suitable for calibrating observations made with large
radio telescopes was described in detail by Baars et al. (1977,
A&A, 61, 99).
Small waveguide horns are frequently used as feed antennas for
paraboloidal radio
telescopes.
Most radio telescopes use circular paraboloidal reflectors to obtain
large collecting areas and high angular resolution over a wide
frequency range.
Because the feed is on the reflector axis, the feed and legs supporting
it partially block the path of radiation falling onto the
reflector. This aperture
blockage has a number of undesirable consequencies:
- The effective collecting area is
reduced because some of the incoming radiation is blocked.
- The beam pattern is degraded by
increased sidelobe levels.
- Radiation from the ground
that is scattered off the feed and its support structure increases the
system noise.

The first
paraboloidal radio antenna,
built in 1937 by Grote Reber. Notice how the feed housing and
feed-support legs cast shadows on the reflector. Image
credit
Radio telescopes are so large that
paraboloids with high $f/D$ ratios are impractical; typically $f/D
\approx 0.4$. Thus radio "dishes" are relatively deep, as shown
in the photo below. Another consequence of a low $f/D$ ratio is a
tiny field of view at the prime focus. The instantaneous imaging
capability of a large single dish is severely limited by the small
number of feeds can fit into the tiny focal
ellipsoid, the ellipsoidal region surrounding the focal point in
which a simple feed yields a beam with minimal gain loss and low
sidelobes.

The 250 foot
Lovell Telescope
in Jodrell Bank, England was the first truly large
steerable dish, completed in 1957 and famous for detecting Sputnik.
The length of the central tower that supports the feed is only
about 40% of the reflector diameter. Image credit
Nearly all radio telescopes have
alt-az
mounts consisting of a horizontal azimuth track on which the
telescope turns in azimuth
(the angle measured clockwise from north in the horizontal plane) and a
horizontal elevation axle about which the telescope tips in elevation
(the angle above the horizon) or zenith
angle (the angle below the
vertical). The 140-foot telescope in Green
Bank is unique among large radio telescopes in having a polar
mount. The advantage of a polar
mount is tracking simplicity—the declination axis is fixed and the
hour-angle axis turns at a constant rate while tracking a distant
celestial source. In contrast, both the altitude and the azimuth
of a celestial source change nonlinearly with time. When the
140-foot telescope was being
designed, the ability of computers to perform the real-time
calculations needed
for an alt-az telescope to track a source accurately was in
doubt. The disadvantage of a polar mount is mechanical—the sloped
hour angle yoke and polar axis with its huge support bearing are very
difficult to build and
support.

The 140 foot
telescope in Green Bank, WV is the largest telescope with
a polar mount. Image
credit
The Parkes 210-foot (renamed 64 m)
telescope in Australia was built about the same time as the 140-foot
telescope, but its alt-az mount and centrally concentrated reflector
backup structure pointed
the way to the design of modern radio telescopes.
The Parkes
telescope was built about the same time as
the Green Bank 140 foot telescope,
but it has an excellent alt-az design and is still in active use with
multibeam receivers making HI and pulsar surveys of the southern sky. Image
credit
While the 140-foot telescope in Green
Bank
was being constructed, a very simple and inexpensive 300-foot telescope
was built nearby. It was transit
telescope that could
move in elevation
along the north-south line but not in azimuth. It could observe
sources over a wide range of declinations, but only when they were near
the meridian. The prime-focus feeds could be moved slightly in
the east-west direction to track a source for a few minutes while it
remained within the focal ellipsoid.
The surface of the 300-foot
telescope was an open mesh with square holes about 6 mm on a side to
reduce both weight and wind loading. So long as the openings are
much smaller than a wavelength, a mesh reflector appears nearly solid
to
radio waves and only a small amount of ground radiation leaks through
the reflector to be picked up by the feed.

The 300-foot
transit telescope in Green Bank, WV was built as a stopgap during the
delayed construction of the 140-foot telescope. Originally
expected to have a scientific useful lifetime of only five years, it
actually
lasted 26 years. Image
credit

At 9:43 p.m.
EST on Tuesday the 15th of November 1988, the 300-foot
telescope in Green Bank collapsed. The collapse was due to the sudden
failure of a key structural element—a large gusset plate in the box
girder assembly that formed the main support for the antenna.
This is a photograph of the 300-foot telescope taken on November 16,
1988 after the collapse.
The loss of the 300-foot telescope resulted in the Green Bank Telescope
Project.
Image
credit

The real reason
for the collapse of the 300-foot telescope!
The 1000 foot
(305 m) fixed spherical dish near Arecibo, PR.
Image
credit
The Arecibo
radio telescope was
originally designed for
incoherent backscatter ionospheric radar at 430 MHz, but such a large
dish is not
needed for that purpose because because the backscatter is coherent
when the number $n$ of electrons
in a volume $< \lambda/2$ on a side is more than one:
$$P = {2 q^2
\dot{v}^2 \over 3 c^3}$$ becomes $${2 (n e)^2 \dot{v}^2 \over 3 c^3} =
n^2 \times {2 e^2 \dot{v}^2 \over 3 c^3} {\rm ,~not~~} n \times {2 e^2
\dot{v}^2 \over 3 c^3}$$ Its huge collecting area and
forward gain at frequencies up to about 10 GHz are now used by
astronomers for
radar (planets, moons, asteroids), pulsars, HI 21 cm line observations
of galaxies, and other observations that benefit from Arecibo's
unparalleled sensitivity.
The spherical reflector can be very large because it is does not
move. Because a sphere is symmetric about any axis passing
through its
center, the Arecibo beam can be steered by moving the feed
instead. The curved feed arm visible in the photo above is 300
feet long and rotates in azimuth under the triangular support
structure. The feeds are mounted under two carriage houses that
move along tracks on the bottom of the feed arm. This permits elevation
tracking at zenith angles up to 20 degrees. The feed illumination
tends to spill over the edge of the fixed reflector at high zenith
angles, so a large ground screen
surrounds the spherical reflector to
reflect the spillover onto the cold sky instead of the warm and noisy
ground.

The fixed spherical reflector is
suspended over a huge limestone
sinkhole near Arecibo, PR. Holes in the reflector pass sunlight
so erosion-controlling plants can grow underneath. Photo by J.
Condon.
A paraboloid focuses a distant point
source onto a point, so a simple point feed can be used. A
spherical reflector focuses a distant point source onto a line segment,
so a line feed is needed to illuminate the entire aperture efficiently
at prime focus. Slotted-waveguide line feeds are inherently
narrow band, and ohmic losses increase the system temperature
significantly at short wavelengths. The large "golf ball" under
the feed arm at Arecibo house an enormous Gregorian subreflector and a
tertiary reflector that allow low-noise wide-band point feeds to
illuminate an ellipse about 200 m by 225 m.

This
96-foot-long slotted waveguide
can illuminate the entire 1000 foot
reflector at 430 MHz. The ground screen surrounding the
reflector reduces noise pickup from ground radiation. Photo by J.
Condon.
Gravitational deformations limit
the short-wavelength performance of tilting reflectors. The
deformations can be controlled by clever design of the backup structure
so that the deforming surface remains nearly paraboloidal at all
elevations. Such homologous deformations
cause the focal point to shift slightly in elevation, but this can be
corrected by moving the feed slightly to track the focal shift.
The first large homology
telescope deliberately designed to deform in this
way is the 100 m telescope of the Max Planck Institut für
Radioastronomie (MPIfR) near Effelsberg, Germany. Despite its
huge size, the surface is accurate enough to work at wavelengths as
short as $\lambda = 7$ mm.
The photo below clearly shows the
Cassegrain optical system of the 100
m telescope. Radiation reflected from the main dish is reflected
from the convex Cassegrain
subreflector just below the focal point down
to receivers at the vertex of the paraboloid. Some advantages of
a subreflector system over prime-focus system are:
- The magnifying subreflector can multiply the effective $f/D$
ratio; values of $f/D \sim 2$ are typical. This greatly increases
the size of the focal ellipsoid.
- The subreflector can be used to tailor the illumination taper to
optimize the tradeoff between high aperture efficiency and low
sidelobes.
- Receivers are located at the vertex, not the focal point, where
they are easier to access.
- The large focal ellipsoid can contain a number of receivers and
feeds, not just one at a time, making it much faster and easier
to change from one frequeny range to another.
- Feed spillover radiation is directed toward the cold sky instead
of the warm ground, lowering overall system temperatures.
- The subreflector can by nutated
rapidly to switch the beam between two adjacent positions on
the sky. Such differential observations can be used to remove
receiver baseline drift and large-scale fluctuations in atmospheric
noise.
Some disadvantages of a
subreflector
system are:
- Very large feeds are required to
produce the narrow beams needed to illuminate the subreflector that
subtends only a small angle as viewed from the vertex.
- Standing waves in the leaky
cavity formed by the reflector and
subreflector cause sinusoidal ripples in the observed spectra of strong
radio sources.

The 100 m
telescope near Effelsberg, Germany. The first homologous
telescope, it works to $\lambda
\sim 7$ mm. Note the large Cassegrain (convex shape, below the
focal point)
subreflector.
Image
credit
The 100 m Robert C. Byrd Green
Bank Telescope (GBT)
is the successor to the 300 foot telescope, and it
incorporates a number of new design features to optimize its
performance.
The actual reflector is a 110 m
$\times$ 100 m off-axis section of an imaginary symmetric paraboloid
208 m in diameter. Projected onto a plane normal to the beam, it
is a 100 m diameter circle. Because the projected edge of the
actual reflector is 4 m away from the axis of the 208 m paraboloid, the
focal point does not block the aperture. The GBT enjoys the
same clear-aperture benefits of waveguide horns—a very clean beam and
low spillover noise—but is much larger than any practical horn
antenna. The clean beam is especially valuable for suppressing
radio-frequency interference (RFI) and stray radiation from very
extended sources, such as HI emission from the Galaxy.

The GBT reflector is an off-axis
section of a larger symmetric parent paraboloid.
The large feed-support arm is over
60 m long, the focal length of the
208 m paraboloid. The
feed-support arm has a much
larger cross section than the feed-support structures of symmetrical
telescopes, which must be kept as thin as possible to minimize
blockage. This large
arm is very strong and can support large subreflectors, feeds, and
equipment rooms. At the top of the arm and above the focal point
is the concave Gregorian
subreflector. This subreflector
illuminates feeds in the roof of a large receiver cabin attached to the
feed arm a short distance below. Since these feeds are relatively
close to the subreflector, even a moderately small subreflector
subtends a large angle as viewed from the feeds, which can then be
moderately small themselves. All of the receivers and feeds
needed to cover the frequency range $1 < \nu{\rm (GHz)} < 100$
can fit into the receiver cabin simultaneously and are available for
use on short notice.

Aerial view of
the 100 m
Robert C. Byrd Green Bank Telescope (GBT).
Image
credit
Jim's Photo Tour of the GBT

This side view
shows the major components of the GBT: the azimuth ring on which the
telescope rotates in azimuth, the pyramidal alidade that supports the
tipping structure, the elevation bull gear and axle, the reflector
backup structure, and the offset feed arm with the receiver cabin and
feeds at the top.

Each corner of
the alidade is supported by a two layers of whiffletrees
(pivoted horizontal arms) that divide the corner weight evenly among
the four wheels. The GBT is the largest moving structure on
land. The total moving weight is 16 million pounds, so each wheel
must support about one million pounds.

This closeup
shows one wheel and a short section of the azimuth track. The
track is overstressed and will be replaced during the summer of 2007.

The pintle
bearing under the center of the alidade supports the GBT against
horizontal loads. This concrete supporting the azimuth track and
pintle bearing extended to bedrock about 16 feet below ground.

This stairway
and an elevator extend up one side of the alidade from the bottom, past
the elevation drive, to the level of the elevation axle and the
horizontal feed arm.

This walkway
goes to the bottom of the elevation bull gear, where it is driven by
pairs of motors.

These motors
drive the telescope in elevation via the bull gear. The wide structure
just overhead carries the concrete counterweights needed to balance the
tipping structure.

The bearing at
one end of the elevation axle.

This side view
shows additional counterweight plates attached to the side of the
original counterweight structure.

This walkway
extends from the elevation axle to the elevator at the base of the
vertical feed arm. Two workmen are just visible on the
walkway. The complex backup structure for the reflector consists
of many small beams in order to deform homologously; that is, it keeps
a paraboloidal shape under gravitational loading as the telescope is
tipped in elevation. This requires that the backup structure be
equally soft and not contain any hard points supported by the strongest
structural members.
The main reflector is supported by a
backup structure that deforms homologously to ensure good efficiency at
wavelengths as short as $\lambda = 2$ cm. The reflecting surface
consists of approximately two thousand panels, each about two meters on
a side. The corners of individual panels are mounted on
computer-controlled actuators that can move the panels up or down as
needed to make active corrections to the overall shape of the
surface. Photogrammetry was used to measure the surface at the
"rigging elevation," and the panels were initially adjusted to correct
the measured surface at that elevation. The gravitational
deformations at other elevation angles predicted by the finite-element
computer model of the GBT are continuously removed by the actuators as
the telescope moves. As a result, the GBT has a high surface
efficiency at wavelengths as short as $\lambda = 6$ mm. Efforts
are underway to make the GBT a $\lambda = 3$ mm telescope.

The GBT has an
"active" surface of panels whose corner heights are continuously
adjustable by motor-driven
screws at their corners.
Image
credit

The concave Gregorian subreflector
just above the reflector focal point
images sources onto conical horn feeds extending through the top of the
receiver cabin. The prime-focus feed arm is shown stowed out of the way
of the subreflector. None of these offset structures block the
main
aperture. Image
credit

The Gregorian
feed horns are mounted on a rotating turret in a hole at
the top of the receiver cabin so that the desired feed can be placed at
the secondary focus of the Gregorian subreflector. The largest is
the L-band (approximately 1–2 GHz) feed horn.

Inside the
receiver cabin, the large L-band feed and receiver extend almost to the
floor.

This photograph shows the prime-focus
boom and feed extended in front of the Gregorian subreflector.

The GBT is
about 480 feet tall. Antennas always seem taller when you look
down from the top. The small white rectangle in the upper right
of this photo is the top of a large tour bus.
The pioneering millimeter-wave
telescope was the 36-foot telescope on
Kitt Peak, since upgraded and enlarged slightly to become the 12 m
telescope. Millimeter-wave telescopes must be located on high dry
sites to minimize atmospheric emission and absorption.

The 12 m
telescope at Kitt Peak, AZ
pioneered millimeter-wave astronomy
and discovered many new interstellar molecules.
Image
credit
SCUBA (the
Submillimetere
Common-User Bolometer Array) is the bolometer array "camera"
for $\lambda = 850 \mu$m and $\lambda = 450 \mu$m (Holland et al. 1999,
MNRAS, 303, 659) used on the 15 m JCMT (James Clerk Maxwell
Telescope) to make sensitive continuum images and detect dust emission
from ultraluminous starburst galaxies at cosmological distances.

The 15 m James
Clerk Maxwell Telescope (JCMT) on Mauna Kea, HI.
Image
credit