Ellen and I are assigned
the task of building a telescope to measure the size
of a crater on the Moon. Unlike other Rough Science
series we have been allowed a step-up in technology
and they have given us polished mirrors and lenses with
which to make the scope. This is great as without them
there simply would not be time to make a decent scope
and make observations.
Kathy and Iain go off to Meteor Crater in Arizona while
Mike is starting to do experiments on projectiles hitting
the ground from various heights - their challenge is
to work out how big the meteorite that made Meteor Crater
would have been.
The first thing Ellen and I have to do is work out
the properties of the mirror. It is the mirror that
collects the light from the distant object and it is
the mirror that, along with the eyepieces, provides
The mirror collects light from the distant object and
focus it to a point some distance in front of the mirror
(this is called the focal length). This is the point
where you want the eye to be but unfortunately if you
look there, your head obscures the very mirror and light
coming in. So what you do is put a small slanting mirror
(called the diagonal) a little closer to the mirror.
This reflects the light from the mirror allowing the
focus to take place at right angles to where it would
have been without the mirror, and in a place where you
can place your head without blocking any of the light.
In order to make up the telescope we need to hold the
mirrors in a frame and the size of this telescope frame
is dependant on the focal length of the mirror. So our
first job is to take the mirror and work out its focal
We did this by taking the mirror outside and arranging
it so that it ‘looked’ straight at the Sun.
Then we took a long piece of wood and moved it between
the front of the mirror until the reflected light shone
on the wood. As we moved it further away the reflected
light became a bright spot because all the suns energy
collected by the mirror was being concentrated. At one
place the spot was so small and powerful it burned a
hole in the wood - the distance from the front of the
mirror to the wood is the focal length of the mirror.
We had a lot of fun playing around with the mirror
burning pieces of wood. We had to be very careful when
moving the mirror so that no reflected light shone on
our faces. This basic experiment got us a focal length
of about 1 metre for the mirror. This is an important
piece of information because, since the diagonal mirror
bends the light away, it means that our telescope will
be less than 1m long – we can start making up
Ellen and I start to make up the woodwork for the telescope,
she made up a ‘cell’ - a wooden base with
three bolts that holds the mirror in place on the telescope
frame and will allow us to position it perfectly in
alignment later on.
I make up a simple wooden frame to hold the mirror
cell and the various other parts of the scope including
the eye piece holder, the diagonal and the fitting /
mounting brackets to hold the thing in place. The telescope
mount will have to wait.
We both get on very well, Ellen is so excited about
both making the telescope and using it for observations,
especially as Mars is so close at the moment (it wont
be this close for many, many years).
At the end of the day we were able to test the scope
out with it lying on the floor propped up with a few
pieces of wood and use it to look out onto the scrubby
hills of the desert floor. It's beginning to work!