Astronomy
began as
people
viewed the
night sky
with their
naked
eyes. When
the
optical
telescope
was
invented
in the
17th
century,
optical
astronomy
made great
advances.
Early
errors,
such as
the
assumption
that the
universe
rotated
around the
earth,
were
discarded.
The use of
photographic
methods,
introduced
in the
last
century,
further
advanced
optical
astronomy.
Radio
astronomy
began by
accident.
In 1931,
Karl
Jansky was
looking
for the
source of
radio
interference
at Bell
Labs. He
discovered
a source
of noise
that moved
across the
sky so
that it
took not
quite 24
hours to
re-appear
at the
same
place. The
source
moved with
the stars,
which lag
the sun's
apparent
movement
across the
sky by
about 4
minutes
per day.
Although
it was not
known
then, the
source of
the radio
waves was
the center
of our
galaxy,
where a
large
black hole
resides.
Radio
waves are
emitted
because
the black
hole
ingests
hydrogen
gas, along
with
anything
else that
comes near
it, and
the
captured
material
is
accelerated
and
ionized on
approach
and in the
presence
of a
magnetic
field
produces
radiation
at radio
frequencies.
Of course,
Jansky did
not
realize he
had
located a
black
hole!
Jansky
was
assigned
to other
projects
when it
was
discovered
that the
interference
could not
be
avoided.
His
discovery
might have
been lost,
however,
Grote
Reber, a
young
amateur
radio
operator
and later
an
electrical
engineer,
learned of
Jansky's
discovery.
Reber, on
his own,
built a
9.5 meter
parabolic
dish
antenna in
his back
yard. His
equipment
was
primitive
and it is
a great
tribute to
him that
he
compiled
the first
map of the
radio sky.
When
astronomers
realized
that the
new radio
telescope
could
observe
radio
waves from
the center
of the
galaxy,
which was
blocked to
their
optical
instruments
by clouds
of dust,
the
building
of the
great
radio
telescopes
began.
What
comes out
of a radio
telescope
is not a
picture,
as we
cannot
picture
radio
waves. A
change in
the
intensity
of radio
waves is
marked by
a change
in the
voltage at
the output
of the
radio
astronomy
receiver.
This was
recorded
on a strip
chart in
the early
days, now
the data,
which is
the time,
the
voltage
output of
the
receiver
and the
point in
the sky at
which the
radio
telescope
is
pointing,
is
organized
by a
computer
and
recorded
on a hard
disk or on
computer
tape. Here
is a
sample
from our
radio
telescope
Observe
this
graph. The
numbers
across the
bottom are
the number
of seconds
since the
observation
file
started.
The graph
starts at
1800
seconds,
which is 5
hours
after the
observation
file
started.
The file
name,
03290000.txt,
appears in
the upper
right
corner of
the graph.
The file
name is
based on
the
starting
month,
day, hour
and minute
in "mmddhhmm"
format.
This file
started on
March 29
at 00:00,
which is
midnight.
Therefore,
the graph
starts at
5 am on
March 29.
At about
5:33 am, a
radio
source
transited
the south
meridian (
moved
across the
beamwidth
of the
dish which
was
pointed
south ).
The
increase
in radio
noise
raised the
total
power in
the 35 MHz
wide
spectrum
the
receiver
"sees"
at one
time. That
resulted
in the
output
voltage of
the
receiver
increasing,
which is
the
vertical
component
of the
graph. The
voltage
went up
from 1.8
volts to
2.4 volts.
How did
we know
that it
was the
black hole
first
discovered
by Jansky?
We had
access to
the
co-ordinates
of
Sagittarius
A (right
ascension
and
declination)
from
several
sources.
One source
of that
information
is the
astronomical
database
of radio,
x-ray and
gamma ray
sources
called the
Vizier
database
. If you
check the
time of
the
transit,
you will
discover
that the
dish was a
pointing a
few
degrees
away from
due south.
If you
looked at
the graph
with care,
you
noticed a
second
graph line
just above
the 0.5
volt grid
line. That
is the
temperature,
where 0 is
0 Deg. F
and 1.00
is 100
Deg. F.
The
temperature
affects
the LNB,
part of
the
receiver
mounted
right on
the dish.
The LNB
has
greater
gain when
it is
cooler.
However,
that day
the
temperature
stayed
around 60
deg for
the period
of the
graph.
Since
March 29,
1989, we
have had a
temperature
sensor
right on
the LNB.
Actually,
the
temperature
shown in
the above
graph is
the
temperature
in a box
on the
post on
which the
dish is
mounted.
If you
graph the
entire
file
03290000.txt
on our graph
a file on
the web
sub-page,
you will
see that
we moved
the
temperature
sensor
that day,
as the
temperature
goes to 0
when we
pulled the
power to
the
sensor.
There are
very
strong
spikes in
the
receiver
output
when we
got in
front of
the dish
on a
stepladder
to attach
the sensor
to the LNB!
Apparently
our bodies
are warmer
than cold
sky!
You may
have seen
pictures
showing
part of
the sky as
it appears
at radio
frequencies.
Those are
a
graphical
combination
of data
from many
observations
at a
particular
frequency.
Here
is one:View
This
information
page is
long
enough,
and we
haven't
begun to
really
explain
radio
astronomy.
NASA also
has a very
good
simple
explaination
of radio
astronomy
on the web
in their Basics
of Radio
Astronomy,
download
each of
the
interested
chapter in
PDF
format.
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