Re-calibrate; Why do we have to re-calibrate our telescope every time we observe?
Why do we have to re-calibrate our telescope every time we observe?
As a matter of fact, we actually have to
re-calibrate the telescope several times an hour. Why isn't one time
good enough? There are two reasons. The first is that some of the
effects we are trying to account for depend on the position of the
telescope. Since the telescope is moving all the time as it tracks a
source across the sky, these effects are constantly changing. The second
reason we need to re-calibrate is that certain effects are changing
over time. Even if the telescope were not moving, we would have to
account for these changes.
What are some of the things that change with the position of the
telescope? One thing that changes is the shape of our telescope.
Surprising, but true! The DSS-12 telescope weighs around 2 million
pounds, and the big dish sags and stretches in different ways as it
points to different parts of the sky. The change in shape alters how
effectively the dish reflects energy into our receivers. Another thing
that changes is the amount of energy we pick up from the ground. When we
try to observe an object low in the sky, you can imagine that more
energy from the surrounding ground or mountains makes it into our
receivers than when we are observing something straight up.
What are some of the things that change over time, even if the
telescope is not moving? The main thing that changes is the performance
of our electronics, particularly the amplifiers used to boost the faint
whisper of energy we get from space. (If the amplifiers are a problem,
why do we use them? The reason is that without them, the signals are too
weak to be detected!) Other things that change are the wind here on
Earth (which can influence the shape of the antenna and our ability to
point it accurately), and the amount of energy emitted by the Earth's
atmosphere. (Think about a cloud moving across the sky. When it passes
in front of our telescope, we might get more signal from the cloud, and
less from the object in space.)
Because of all these things that change, we must constantly
re-calibrate our instrument if we wish to get an accurate measure of the
amount of energy coming from space.