Instrument Making Section

This Section is one of the oldest in the Society, and could also be known as the 'Telescope Making Section'. Its prime purpose is to provide a wealth of knowledge and experience to any member who wishes to construct an instrument which is the equal of any commercial telescope of similar aperture.

This Section meets on the third Saturday afternoon of each month from 2 to 5 p.m. at the Society's Clubrooms.

The general structure of the meeting falls into several categories such as:

Membership comprises a nucleus of dedicated telescope makers and there is always a passing parade of those people who have successfully completed making an instrument, to the gratification of the Director and the Section.

As an enthusiastic astronomer, whether new to astronomy or a 'seasoned armchair' astronomer, the question of needing your own telescope for casual or serious observations arises. If the need is there, then you are urged to read on and become, with little effort on your part, a 'qualified' amateur telescope maker.

Whilst the construction of a telescope for astronomical observation may seem to be ambitious, any member can be comforted by the fact that a number of successful telescopes have been constructed in this Section over the years. A testament to the fact that the Section is still operational to this day is that instruments are still being successfully produced to the pride and joy of their owners.

Should you have the desire, time and energy to join our Section, the Director and experienced members have the enthusiasm to assist you in the production of your very own telescope, one you will be proud of.

So, let's start at the beginning. Telescopes fall into two basic types, the refracting telescope where the light passes through the objective lens at the front of the telescope and is refracted to a focus at the end of the instrument's tube. A well-known example is a common pair of binoculars. With the reflecting telescope, the light passes down the tube to a mirror and due to the shape of the mirror the light is reflected to a focus at the top of the instrument.

The refracting telescope requires at least two matching lenses to be fabricated and is of a larger 'f' ratio, typically f15 and could not be considered to be a portable instrument if the aperture is greater than 150mm. The 'f' ratio as designated above being the instrument's focal length divided by the diameter of the lens at the front of the instrument.

The refractor is considered to be a more advanced instrument as there are four surfaces to be ground and polished. On the other hand, a 'Newtonian' telescope, which is a reflecting telescope and to which the following comments are directed, contains only one surface to be ground and polished to complete an instrument which will satisfy your immediate curiosity until 'aperture fever' sets in (The need to build ever larger telescopes to gain brightness and resolution).

The typical instrument recommended to all beginners is a 205mm Newtonian telescope and as a good compromise a 'f' ratio of f7 has been chosen. In this case, the 'f' ratio is the mirror's diameter divided by its focal length.

The steps to be taken are quite simple: Firstly, the generation of a shallow concave curve in the mirror blank. In the case of the recommended scope above, less than 2mm. This is achieved by the grinding of the blank on top of a second glass blank, the tool, with a coarse grade of carborundum. Carborundum is a trade name for silicon carbide, an extremely hard material crushed into a range of graduated powders. The rough ground surface is then fined to a point where the final polishing can take place by using these graduated powders, each powder producing a finer surface. The final step is of course the polishing stage. This is now performed on a surface created by pitch and is called the lap. Unlike the glass tool, pitch is chosen as the polishing medium as it yields under pressure and conforms precisely to the shape of the mirror surface. Once polished, the mirror can be tested for the first time and quite astonishing accuracy can be achieved using simplified testing apparatus in conjunction with the polishing action.

It should be noted that advice, help and guidance will always be at hand, and a stock of all the necessary materials is available for sale. Materials are sold 'at cost' to Society members.

From the basic steps above it becomes obvious that time and perseverance are the basic resources to producing a mirror.

The next step is to space the optical components at the correct distances and this is the role of the tube assembly. Finally, a steady rest for holding the tube and pointing the tube assembly at the object to be observed, this is the mount and the design should be chosen to suit your particular needs.

While there is now a wealth of technical papers available including an ever-increasing proliferation of computer programs dealing with the design and evaluation of designs, the basics are to be learned from some of the older publications considered to be essential reading and whilst the list is by no means exhaustive, amongst these are:

These publications will provide an excellent understanding of amateur telescopes and are available from the Society's library. Additional publications on the subject are also available and includes the magazines, such as 'Sky and Telescope' which devotes a section to this subject.

The above publications could be considered as the 'Old Testament' for amateur telescope makers, however, some of the newer publications considered worthwhile additional reading are:

Advanced Telescope Making Techniques - Vol. One and Two A Mackintosh ed. Willmann-Bell inc. Richmond Virg. USA

Telescope Optics Evaluation and Design -Harrie G J Rutten & Martin A N van Venrooji Willmann-Bell Inc. Richmond Virg. USA (Aided by a computer program, analyses in greater depth - the intricacies of various telescope designs)

 

Ken Beard