basic kit

A "Build Your Own Telescope" course, held at the Sydney Observatory, attracted my attention in February 2002.
The course, to build your own 200mm reflector, was run over an eight week period but, believe me, it took much longer than that to complete!
Seen here are the main components of the telescope mount, which will eventually become a Dobsonian mount for the 1.5m long telescope.

basic kit (with tube)

This picture shows the previous components alongside the telescope tube itself.

mirror kit

The mirror took the most amount of time to prepare. Seen here are parts of the mirror kit with the mirror half way through its preparation.
The mirror blank (bottom right) is ground against a similar blank (known as the tool) with ever finer grades of carborundum grit (bottom left).
The grinding process generates a concave spherical surface on the mirror blank and convex on the tool. With the carborundum grits this leaves a translucent surface.
Once the mirror blank is deep enough (see Ronchi tester / pattern) the polishing process can begin. Here a wood resin (top right) and caster oil mix is poured over the tool (top left) and this is used with cerium oxide to polish the mirror surface back to the original transparent state.

ronchi tester

During the latter grinding and whole polishing process, frequent use is made of the Ronchi (pronounced "Wron-key") tester.
The tester shines light through a slit. The mirror, which is placed at a distance of its 'radius of curvature' from the light source, reflects the light back through a diffraction grating.
The pattern in the diffraction grating shows how spherical the polishing process has made the mirror surface. It also shows any defects clearly.

ronchi pattern

The initial aim is to get a perfectly spherical surface, in which case you're looking for a diffraction pattern of vertical lines.
Thereafter it's a small matter of a little more polishing to obtain a parabolic surface, where the diffraction pattern is shown in this picture. (The picture comes from "Ronchi for Windows")
This test allows a parabola to be formed which is accurate to with 1/8th the wavelength of light (approx 70-80nm).
Once it has been completely polished out, tested and found to be a good parabola, the mirror is then ready for silvering which, fortunately, wasn't a DIY job.

assembled kit

While the mirror is being silvered it's a good time to start assembly of the telescope mount. The three main components are the main stand, rocker box and telescope tube.

primary mirror

This picture shows the final silvered mirror mounted on the mirror holder. The legs are used at attach the holder to the main telescope tube.
There are two blocks of wood, separated by rubber grommets, which get adjusted during the collimation process. The black dot seen in the centre of the mirror is also used during this process.

final assembly

First light, 25th August 2002, 21:10 EST; The Moon.
(Actually first light was very possibly a satellite zooming across the skies)
Not a night usually selected for star gazing, with a full moon bright in the sky, but its a nice big target for the very first real view through the telescope. Since then I've seen Venus, the Jewel Box and the Small Magallenic Cloud.

... and finally

Having been through all that, I really find that I'm much more of an armchair astronomer than a practical one, so now have the scope up for sale to anyone interested. Sydney mobile: 0413 668535