Acquarium Glass

I have acquired the glass for the 76-cm and 51-cm telescope mirrors quite by accident. One day, two of my work colleagues, Johan Fourie, Stephen Wright and I visited the Tygerberg Valley Centre. Johan wanted to buy something at one of the stores. The store also sold telescopes and while we were waiting for Johan I had a look at the telescopes. One of the people in the store noticed that I was looking intently at the telescopes and came over to talk to us. He introduced himself as Bill Hollenbach and when he learned that I was making my own telescope mirrors he mentioned that he had a large piece of glass at home that I could get if I wanted it. At that point he was just on the point of moving to the Aloe Ridge Game Farm to setup a astronomical observatory there. The idea was that people could also have a look at the stars besides the game. After a cup of coffee with Bill we set off to his house to have a look at the glass and if we could pick it up. But the glass was far too large to load into a car. Bill gave me a 49.5 cm piece of glass which he had used to grind one of his mirrors. A few days later another work colleague of mine, Callie van der Merwe, and I drove to Bill's house with Callie's bakkie and picked up the glass.

The glass came from the old Seepoint Acquarium when it was torn down. The old acquarium was built in 1948 under the auspices of the University of Cape Town in honour of the famous LBJ Smith of coelacanth fame. The glass was made at a famous glass works in England and brought by ship into the Victoria Bay of Cape Town harbour.[1] On the right is a photo of the remaining piece. The glass is 37 mm thick.

Cutting of the 76-cm and 51-cm mirrors from the glass

Since my intention was to make the best possible use of the glass I decided to cut the biggest possible mirror from it. But first I had to determine how big the biggest mirror could be. In order to determine that I measured the glass and made a CAD drawing.

The drawing on the left shows the part from which the 76-cm and 51-cm mirrors was cut. The drawing on the right shows the remaining piece of glass from which a 31-cm mirror can be cut.

The 76-cm and 51-cm mirrors was cut by building a temporary cutting machine. A big biscuit cutter was made by cutting a 762 mm disk with a router from block wood. Four zinc plate strips were then screwed to it. The glass was placed under the machine and a dam with window putty was build around the area. The dam is necessary to keep the water and silicon carbide powder inside. The cutting took a few hours and the zinc plates had to be replaced once since they are worned away.

The 31-cm mirror still have to be cut but the Grinding and Polishing machine will be used for that.

References

According to Bill Hollenbach.

Die Burger. Kaapstad het 'n behoefte aan see-akwarium, 7 April 1989, bl. 15.

Die Burger. Bal aan die rol gesit vir opknap van Seepunt-akwarium. 5 Maart 1994, bl. 2.