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Astronomy

150mm Newtonian Telescope

This is an account of how my 150mm Newtonian telescope was built.

It started out as two glass blanks (mirror and tool) that my dad had bought from a company in the USA, many years ago, but he never completed grinding it. I grew up knowing about the existence of the '6-inch telescope mirror', because my dad would occasionally show it to us kids. I even vaguely remember watching him grinding the miror while slowly shuffling around a 44-gallon oil drum, but I never ever dreamed that one day, I would be the one who would finish building the telescope.

Fast-forward about 50 years to August 2009 when I decided to resurrect my dad's abandoned project and started working on the mirror. I joined a group of telescope nuts (members of the Astronomical Society of SA) who meet at the Design & Technology centre of Parktown Boys' High School in Johannesburg to work on DIY telescopes on Saturday afternoons.

With the friendly and expert guidance of mentor Johan Smit, my project quickly started taking shape. Working most Saturday afternoons, the partly-ground mirror was re-ground with 500-grade grit to get rid of a few scratches that had accumulated while in storage. Progressing through the various grades to 1000-grit and then finally polishing with Cerium Oxide, the mirror was ready for 'figuring' into a paraboloid, the optical shape required for a Newtonian telescope.

After using Johan Smit's Foucault tester for a while, I built a very simple Rochi tester so that mirror figuring and testing could be done at home. When the figuring was completed, the final Foucault test in Parktown was performed by the fundi's who all agreed that the mirror was ready for the final step; receiving a thin layer of aluminium on the polished surface.

Next, a Dobsonian mount was designed and built, using ideas found on the Internet. The mirror was then mounted in a 200mm diameter cardboard tube that was sourced at the telescope class. Finally, the secondary optical, in this case a prism from old binoculars, was mounted in a curved spider made of 1mm steel plate from the case of an old PC CD-ROM drive. The whole process took about four months from start to finish.

'First light' was on 3 January 2010. What a feeling it is to view the moon, planets, stars clusters and even some galaxies through a precision instrument that was made with one's own hands!

Painting and final finishing continued during the next 4 months and the completed telescope was displayed at Scope-X on 17 April 2010.

Specifications

Primary Mirror: 6" float glass (150mm)
Focal Ratio: f7.7
Focal length: 1150mm
Secondary: 25mm Prism (from old binoculars)
Focuser: Home-made 1.25" Helical Focuser
Tube: 200mm Cardboard tube covered in white Solartex