Calibration: What is calibration?
When we use our radio telescope, we are
generally trying to determine how much radio energy an object out in
space is emitting. (The amount it emits helps us learn what it is made
of, its temperature, and other things.) When we point our telescope at
the object, sensitive electronic devices report how much power they are
receiving. The amount of power received in our instrument, however, is
not the same as the power emitted by the object in space! When we
calibrate our instrument, we are learning how to translate the power
measured in our detectors into the amount of energy emitted by the
target.
There are many things that reduce the amount of power we detect - for
example, the metal dish and mirrors that reflect energy into our
receivers are not perfect, and some of the energy is scattered away or
absorbed. Can you think of other things that cause our telescope to not
receive exactly as much energy as the object in space emits? Those of
you who have done the "Bucket of Noise" activity might remember that not
all of the fuzz-balls "emitted" actually hit the telescope. And those
of you who have studied the inverse square law might be able to describe
how the farther away something is, the less energy we receive.
In addition to things that reduce the amount of power we detect,
there are even some things that put extra energy into our detectors,
causing us to see more than is coming from the object in space. For
example, you may have learned that everything emits radio waves through a
process called thermal radiation, also referred to as blackbody
radiation. This means that the Earth's atmosphere, as well as all the
pieces of our telescope and electronics, are emitting energy which our
detectors see. Can you think of other things that might emit energy
which our telescope picks up?
When we calibrate the telescope, we are determining the effect of all
these other factors; some of which have increased the power we receive,
some which have decreased it. We then know how to convert the readings
from our power meters into a measurement of just the energy coming from
the object in space. That is the process of calibration.