The last day and most of the crew are having a little
lie in to catch up on some sleep (the guys have been
away at the massive meteor crater in Arizona and have
been up all hours). Ellen and I get in early because
we still have loads to do.
We spend hours trying to make sure that the holder
for the eye pieces is good enough. So far we have made
up something that works but it’s a little fiddly
to use and not very smooth to get a good focus. It proves
to be difficult to make something from the various pipe
off-cuts and bits and piece lying around, but eventually
we make up something that works and, most importantly,
is reliable and gives reproducible results.
Ellen does some finishing touches to the diagonal support
which is now perfect. She has to go off and help the
other Roughies and I am left to crack on with the telescope
mount that will hold and guide the scope.
The most basic mount would be to fix the scope to a
bench or other solid object - however, this is fine
for looking at terrestrial objects but no good for looking
at things in space. The reason is that the Earth rotates
once in a day and this movement causes everything seen
outside of the Earth to move as well. It is especially
a problem with a powerful telescope as it magnifies
the effect. The Moon seen through a telescope of 100
times magnification will therefore appear to cross the
field of view in just a few minutes.
We can’t stop the Earth rotating so we have to
live with the motion but the way to overcome the problem
is to make an equatorial mount for the scope. Like all
mounts this is a telescope mount that can move both
across the sky and up and down. One axis is fixed in
alignment with the axis of rotation of the Earth and
when set up correctly all one has to do is adjust it
once for both axes, and then afterwards only one axis
has to be moved to track the object. This makes for
greater simplification when tracking astronomical objects
in the night sky. As it points along the axis of the
Earth the main body is broadside to this which is aligned
to the Earth's equator – hence the name, equatorial
Ellen goes off site to collect resin for the candles
she is going to make for the end of the programme while
I continue with the telescope mount. I start on my design.
It is a large frame in which the telescope sits and
can move up and down freely, but can be locked when
required. This is mounted on bearings that allow the
whole thing to rotate so that it can track the astronomical
It takes me a lot longer than I thought to make and
it’s baking hot. Each part has to be carefully
calculated and cut so that it will make the mount have
the correct angle to point to the Earth's axis. Also
I add further support because I find the basic thing
is a little wobbly. Wobble is not something we can have
when we look at the heavens at 100 times magnification
– otherwise everything will be seen with 100 times
Everything is ready now but I don’t want to fix
the telescope onto the mount until Ellen gets back so
that we can share in the excitement! So I have a well
earned break and sit and watch the desert of a while.
Ellen comes back with the crew and we assemble the scope
and have a look around. It's early evening and the Moon
is out. In the deepening blue sky we can see that the
Moon is about a quarter around its monthly cycle making
it a D shape - it’s at first quarter. We can see
it clearly in the evening sky.
Then we have a shock - we still haven’t made
a cross hair so that we can make the measurements. We
can look as much as we like but we can’t actually
make a measurement without it! Ellen and I talk for
ages and with the crew to try and work out how to make
such a thing. It’s not as easy as you might think.
What we need is a very thin hair to be put on the eye
piece so that we can measure the movement across the
field of view. If you put a hair across the eye piece
you either see the hair or the image but you can’t
focus on both at once! If you put it behind the eye
piece it's better but still a problem. In the end (after
taking the complex eye pieces to bits and reassembling
it several times) we realised that the hair needed to
go at exactly the focal point between the main mirror
and the eyepiece. This place was found to be in the
eye piece holder and after mucking around we could get
a clear sharp hair and a clear sharp image at the same
Measuring the Moon
We set up the scope in a spot where there was not much
wind and also that had a very good view in all directions
.. and waited for the Sun to go down .. Ellen started
to align the telescope onto the Moon and shouted out
when the start of the Moon's disc just crossed the hair.
Then a few moments later she called out when the end
of the disc had finally passed through. I sat with the
cameraman’s watch and measured the time it took.
We did this nine times to get a good average.
We actually measure how long it took for the half Moon
to cross the hair and then doubled the results. We got:
82, 83, 82, 80, 80, 80, 80, 80, 81 seconds giving an
average of 80.9 seconds
which when doubled gives 161.8 seconds
Measuring the Crater
Which crater to chose? We decided on Archimedes as it
was a relatively large crater, was well lit up and was
seen almost full on. Looking through the scope there
is a large crater near to the bottom (the North, as
the telescope inverts) this is Plato. Archimedes is
above this about a 1/3 of the way up near to the centre
line. It has two small craters to its side. Ellen now
made the same measurements on the crater but obviously
because the crater is so small it passes much faster!
In fact each of the crew and Roughies took a turn to
measure 5 readings for the crater to get some good statistics!
We took it in turn, a drink in hand, it was a great
Rough Science moment shared with everyone.
We got 21 readings:
3.75, 2.91, 3.77, 3.24, 3.30, 3.30, 3.20, 3.16, 3.23,
3.64, 3.34, 3.26, 3.89, 3.89, 3.55, 3.58, 3.03, 3.57,
2.95, 3.36 and 3.47 seconds for the full crater.
Giving an average of 3.34 plus or minus 0.5 seconds
Now we know the size of the Moon (3500km) and we know
this takes 161.8 seconds to pass through so if we know
that the crater takes 3.34 seconds we get
Archimedies = 3500 x 3.34 / 161.8 = 72 km plus or minus
11 km diameter
Which is within error of the true value taken by photographic
means during the space ship measurements made on the
Moon prior to the Moon landings.
Get to bed about 12:30 and then was up at 8:30 slightly
paranoid so I go through the calculation again –
all OK – don’t awake again till 12:30!