The
antenna is
the most
obvious
part of a
radio
telescope.
It is
analogous
to the
lens of an
optical
telescope.
The
antenna
gathers
the minute
amount of
radio
frequency
energy
from the
sky and
transforms
it to a
tiny
electrical
current
which,
after much
processing,
we can
then
measure.
Most radio
telescope
antennas
are quite
large due
to the
resolving
power
desired.
Larger
antennas
may better
focus the
energy
from a
smaller
region on
the
celestial
sphere.
This
region of
the sky to
which the
antenna is
most
sensitive
may be
thought of
as the
beam
pattern of
the
antenna.
The
resolving
power
RP=1.22L/D
x 206265
Where
L=
wavelength
in
millimeters
D=
collector
diameter
in
millimeters
206265
is
deducted
from the
parallax
(1pc /1
AU)
The
resolution
power is
expressed
in arc
seconds.
The
resolution
of a radio
telescope
is linked
to the
frequency
by the
herewith
formula.
We
immediately
see that
the dish
diameter
becomes rapidly
huge if we
want a
resolution
similar to
optical
telescopes.
We can
work at a
few GHz
but here
silicon
components
are
useless.
The only
solution
is using
an inter-ferometry
base and
to
integrate
results (VLA,
VLBI,
etc).
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While
the type
of antenna
most often
thought of
in
relation
to radio
astronomy
is the
parabolic
dish
antenna,
many other
types of
antennas
are also
used.
Large
arrays of
dipole
antennas
have been
used to
discover
pulsars
and probe
the noise
storms of
Jupiter.
Long
trough-like
antennas,
the cylindrical
parabolic,
are still
used in
observatories
around the
world.
Arrays of
Yagi
antennas,
horn
antennas,
Mills
crosses,
and many
others
have
contributed
to radio
astronomy.
Virtually
any
antenna
which has
a
reasonably
small beam
pattern
has been
used.
Very
often,
amateur
radio
telescopes
will keep
the
direction
of the
antenna
fixed
along the
north-south
line, or
meridian.
The
antenna is
adjusted
in
elevation
to a given
angle and
the cosmic
radio
source
allowed to
pass
through
the
antenna
beam as
the Earth
rotates.
This is
called a
meridian
drift scan
observation.
As the
radio
source
passes
through
the
antenna
pattern,
an
increase
in energy
is
recorded
as a rise
and then a
decline in
the data
recording
device.
Meridian
drift
scans
offer the
advantage
that
calculation
of the
source
coordinates
becomes a
simple
matter.
The right
ascension
of the
source is
equal to
local
sidereal
time at
which the
source
passes.
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can buy
"Jove
Receiver"
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On-Line
|