When Peter and I had been going out with each other for a year or so I remember asking him what his greatest ambition in life was. It was one of those typical female questions aimed really at checking relationship compatibility, and what I was not expecting was the answer: "To build a one metre telescope"

In the following years his enthusiasm for this project has not diminished. We now have three of four books on our bookshelves on building large telescopes. But the interest has remained pretty theoretical. Living in a flat we don’t have the space to build a telescope and even if we did we are very inexperienced at practical skills and would need to start with something less ambitious!

And then December 2000’s Sky and Telescope magazine came to the rescue. There was an article by Barry Leger on how he turned his traditional Dobsonian into a light weight model. We immediately realised the implications. Peter had a 8 ¾" Dobsonian made by Dark Star (RIP) which had been sitting idly at his parents’ house since he went to university. I had objected to having it in the flat because it would have taken up too much space and we’d never get it to the park, but now we realised we could convert it. This was excellent! We would be doing something towards building a telescope, learning skills that we could adapt to bigger and better ‘scopes in the future, we would have a ‘scope that would be small enough to fit in the flat and to trolley into Richmond Park on a dark evening, and yet would be a size worth observing through when we got it together. So our peaceful Christmas holiday with his parents in North London became a frantic building project in a very cold garden shed.

The truss-tube design replaces the bottom rocker with something smaller and more basic, but the main improvement is in replacing the solid tube with 8 poles. In the article this was the end of the story, but when you live in light-polluted London rather than dark New Brunswick you need more shielding than this. We found some black rip-stop nylon from a kite making company on the internet which we simply wrap round the rods.

We had emailed his Dad a shopping list before we came up, so we had a huge sheet of ¾" hardwood ply and some pine ¾" dowels to make the main structure. In addition we had ordered some Formica and PTFE from Beacon Hill Telescopes and a red dot finder (his existing Telrad being too heavy).

The first aim was to make the base of the telescope. We cut the rectangles and circles out of the ply, blunting a number of jigsaw blades in the process, then tidied the edges up as much as possible. The diagram in the magazine was not exactly clear about the angle of the scope tube to the horizontal, but we guessed 45º, and that seemed to make sense. We also suffered from a good deal of NASA trouble, mixing metric and imperial units (particularly bad when we both work for NPL). This, added to the fact that are woodworking skills were, to be polite, in need of some practice, meant that it is perhaps not the most attractive mirror base, but it is rigid and seems to work.

The "mirror rocker" consists of two large semi-circles which rock on supports attached to the inside walls of the rocker base. There are struts between the semi-circles to hold the mirror and the poles. Since we were doing this over the Christmas week and we couldn’t bear to be in the shed for too many hours when snow was on the garden lawn, we didn’t get a whole lot done. We had decided to modify the plans in the magazine so that the secondary holder was more rigid and protected the secondary when placed on the ground. We made it from two octagonal rings of ply held rigidly by four thinner ply walls on alternate inner sides of the octagon. We mounted the focusser on one of these boards. When we decided that the structure was too heavy, my father-in-law just cut some spare ply from round the edges of the rings. It was amusing seeing him approach it practically, whereas Peter knew all the theory and used mathematics.

We had decided to use the existing focusser, mirror and cell and secondary and spider. The secondary spider attaches to the underside of the top of the octagon so that the bottom octagon can protect it when the structure is placed on the ground. The focusser is attached to one of the wooden uprights.

So when we left on New Year’s Eve we had built the rocker base, mirror rocker and secondary cage. At the end of January we went back to finish off.

While we were away Peter’s father had painted the inside of the mirror rocker and secondary cage with a very black matt paint to help prevent stray light. He’d painted the outside a brown colour with some spare paint he had lying around. Our first job was to attach the PTFE pads to the rocker base and Formica strips to the mirror rocker sides. These materials have a very low amount of friction so they slide very easily over each other making telescope pointing easy. We then mounted the primary mirror, the secondary mirror and the focusser. Now we only had to cut the rods to the right length and bolt them in place. After careful calculation we came to the conclusion that we weren’t too good at working out 3D trig, so we drilled holes in the bottom of the rods and screwed them in. Holding the secondary cage at the right height (as determined from the old scope) this allowed us to cut the rods (slightly too long to allow for mistakes!)

We temporarily held the secondary cage in place with rubber bands. WE wedged the mirror rocker horizontal (using a spirit level) and made sure that the secondary cage was also horizontal and using a thread-and-BlueTak plumb-bob, that they were directly above one another. The rods were then drilled and joined to make the completed telescope. As the rods are different lengths, they were colour coded so that they always go together the same way round. They are attached to the mirror rocker and the secondary cage with wing-nuts, so no tools are needed in the field, though care is required to prevent dropping them on the mirror.

As the telescope was now made we had to collimate it. This involves careful alignment of the mirrors to optimise the image formed. You can buy expensive collimation tools, but we went for the home brewed option. The instructions were found on the web but we had trouble finding some tubing of the right size to fit the eyepiece tube snugly. We had a flash of inspiration and used some old vacuum cleaner tubing (an exact fit when the tapered part was lopped off). Following the instructions we worked out the ideal length and then after finding this was too long, we lopped a bit more off. We made a peep hole for one end by using a film canister lid. The crosshairs were made from the other end of the film canister with some thread stretched across it. The vacuum tube with peephole goes into the focusser, with the crosshairs on the inside of the focusser. First we made sure that the secondary was in the centre of the focusser cage and was facing the eyepiece, then the secondary was tilted to point at the primary mirror and finally the primary mirror is tilted so that the reflection of the peephole, and secondary are all centred.

By now it was Sunday evening (time to go home) and already dark, so we took it outside for a quick look at the moon. I was given the honour of first look. "Peter I can’t focus!". Unfortunately we had forgotten to alter the length of the old tube to take into consideration the focusser being further from the secondary than before. There was nothing for it, we had to apply some ‘scientific’ botching and lopped the "odd 3.5 cm" (metric again!) off all the poles. When we reconnected and recollimated, things were somewhat different. The view was incredible; we quickly saw a crescent Venus, the rings of Saturn and clear banding on Jupiter.

Later we went back to put on counterweights, to make moving it easier, the shroud, to increase the contrast of deap-sky objects and the red dot finder, to make finding things possible.

An unwieldy telescope has been transformed into a fully portable one. With the rods removed it is easy to store and transport and it is easy to put together on site. There has been an additional surprising benefit—previously Peter had never dared to recollimate or change the original scope, but now that he had he saw the moon craters Messier A+B for the first time.

If you want to know more or have any comments, get in touch through the internet or at any Ewell AS meeting.

Emma Woolliams