The first try

<Prologue

Having spend the whole of 2001 and 2002 drawing and calculating, construction on the first version of the 17.5 inch Newton finally started at end of 2003. The idea was to build a Dobson, but an ultralight one with strings instead of trusses.

Mirrorbox

The telescope had a 9 point mirror mount calculated with PLOP. The aluminium triangles were mounted on small composite bearings (IGUS) which in turn were fixed to a plywood plate. The plywood was reinforced with carbon as was most of the mirrorbox. This stiffened up the box substantially and made it possible to use mostly 9mm multiplex. Rollerskate bearings provided the lateral support of the mirror. a semi-finished project The top of the box was reinforced with a double layer of carbon to resist bending due to the forces exerted by the strings.

Strings

The strings in this first version consisted of 20 single threads per string (6 strings total). They were connected to the mirrorbox and the secondary ring with small carabiners. I needed to keep the weight of the secondary ring low, which was difficult considering that the secondary mirror weighted 1kg and the focuser, eyepiece and finder added another 2 kg. The rest of the weight needed to be less than 1 kg. In the end the ring weighed a little under 4 kg. The secondary vanes where made out of 2 mm balsa wood covered with 4 layers of carbon. The ends where clamped in a ring of polyurethane foam sandwiched between two rings of 2 mm balsa.

Secondary ring

The ring was then wrapped with carbon band. This created a particularly light and rigid structure. Most telescopes constructed with a single ring have a focuser that is mounted below the ring. Following the example of Greg Babcock 24 inch Newton I chose to mount the focuser on top of the ring. The pyramid construction of the vanes is very stiff and has the added advantage of being able to use shorter tubes and strings. This also allows for the secondary ring to rest on the mirrorbox during transport. The secondary mirror was glued to the vanes.

Dobsonmount

The side bearings where made of 30 mm plywood, ebony star was used as a bearingsurface. The azimuth was constructed as a so-called flex rocker. Not 3 but 4 points are in contact with the ground ring. The telescope weighed about 27 kg and easily transported in a Volkswagen Golf with room for one extra passenger. ┬áThe final result got first light in the summer of 2004 under a beautiful sky in the southern French village of St Michel l’Observatoire.

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