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 magnification.
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 put 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 length.
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 the frame.
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 won't 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!
Today Ellen and I spend a lot of time trying to make sure the diagonal mirror (the little mirror that bends the focused light into the eyepiece) is correctly supported. This was harder than we thought and it took a good half of the day to make something that would hold it steady. In the end Ellen made up a cross support out of thin metal. This metal was taken from an old Californian number plate – very appropriate, we thought!
After putting the diagonal in place, we adjusted the mirror cell setting bolts so that it was all central and looked through the scope. Without an eye piece what we could see was an image of our own eye reflected back – it looks as if your eye is trapped, caged in the frame of the round mirror. By adjusting the three bolts at the back of the mirror we could make sure that this image of the eye was right in the middle. This way we could set up the scope.
We needed to make sure that the eye pieces could be held in the correct place and were able to be adjusted back and forth to get the best focus. As we didn’t know exactly where the place of best focus would be, we had to play around with the scope looking at a lot of different things in the desert – good fun.
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 mount.
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 objects.
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 the wobble!
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 place!!
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!