Listen to the Music – the making of Lyra
By Bosman Olivier
It all started on 21 February 2013, when I slipped in die gym and broke both my left hip and wrist. At the time I was busy with a 14" f5 mirror, using a concrete tool to try and recover the turned down edge.
When I was eventually released from hospital and rehab, I could start the slow process of recovery. My leg recovered quickly, so I could walk around the drum fairly soon, but handling the heavy concrete tool was a problem – a very frustrating problem since Scopex was only a few months away and I was hoping....
My good friend, Johan Smit, probably saw my frustration and decided to come to the rescue – he offered me a 10" half polished mirror with, I might add, a turned down edge of note! There was no way I was going to finish the mirror in time for Scopex, but Johan had a solution for that problem as well.
He planted the idea of 'n string 'scope as the instrument to use with the 10" in my mind. I did a bit of research and soon became convinced that this could be a viable solution. And, anyway, if it did not work, one could always revert to trusses. Johan is a great guy for trying new ideas!
Around mid April I got hold of some Russian Birch plywood from a colleague of ours. This 15mm plywood was ideally suited to become the mirror box and rocker box of the proposed telescope. We started measuring and cutting the wood to size, drilled the holes that were required and built the mirror and rocker boxes.
The idea was, in order to saveweight and space and to make transport easier, to fit these boxes inside one another. The result was an almost cube of some 450mm. The top ring, containing the spider and secondary and the removable focuser, were to fit inside the mirror box, making the parcel even more compact.
The next step was to construct the top ring. We used 9mm ply for this section, cutting an octagonal ring with a 300 mm central opening. The eight side panels were fixed to the ring.
An unique feature of the top ring is the straight spider vane with a home made diagonal holder that can be moved along the vane to place it in the centre of the central opening.
The mirror cell consists of a solid piece of 20mm plywood that acts as a base. An iron bearing separates the base and the mirror cell proper. The mirror sits in a 'dam' formed by another octagonal ring with a 245mm hole in the middle. The mirror fits snugly in this hole and is supported on a layer of felt. This solution was chosen since the mirror is only 20mm thick and we believe it requires a solid, firm base to rest on.
Collimation of the mirror is effected by means of two threaded rods that can be manipulated from the top of the mirror box. The rods lift and lower the corners of the mirror cell proper and the action is balanced by means of springs to ensure that collimation does not wander during use or transport. No more crawling on the ground trying to collimate the 'scope!!
It was now time for the real test, keeping the top ring and the mirror box apart at the correct position. Basic physics was applied, the principle being the fact that the triangle is nature's strongest structure. Steel cables (1.1mm, used in electric fencing) were attached to the corners of the mirror box and the centre of corresponding sides of the top ring, forming a triangle with a fairly broad base. To test the concept, four broom sticks were used. These were later replaced by aluminium tubes. Threads were cut into the tubes at both ends. At the bottom bolts were screwed in and at the top dome nuts ensure that the tubes remain seated properly.
Most articles on the 'net describe how the cables are tensioned by various means. We took a different approach by ensuring that the cables are of fixed and equal length. Adjustment is effected by turning the aluminium tubes on the bottom bolts, making them longer and thus lifting the top ring and tensioning the cables.
The painting and beautifying was left to me. I spent many hours disassembling, sanding, varnishing and painting, sanding again, varnishing, sanding and varnishing. In the end the matt finish marine varnish turned out very well. I believe it does justice to the time and thought Johan and I spent in the design and construction.
I think most people who look at the concept of the string 'scope, believe it is a flimsy design, that cannot be trusted. This project proves the opposite. This telescope proves that the design is extremely rigid and strong. One might as well have built it using a solid tube. There is absolutely no flexing when the 'scope is pointed at the horizon, and one can even lift the mirror box by the top ring. This is not recommended normal operating procedure, though!!
During the testing phase the cables were stretched to the point of emitting musical sounds when plucked – thus Lyra, the Lyre, was born!
On its first outing to Scopex, with the unfinished mirror in place, Lyra attracted a lot of attention and we proudly and enthusiastically described its features to the many visitors. The impression we got was that many more Lyras will see starlight in the future.
What will possibly change? The cables tend to be very stiff and one battles to fold everything away neatly. Cables that are more pliable will most certainly help keep things tidy.