Development of the Technology for Large Castings

The German Federal Ministry of Research and Technology became a third negotiating part, which substantially reduced the financial risks that the partners had to accept. 

The preparations for the first casting period were completed in February 1969 when the melting of the first batch started. A series of castings followed which was so successful that this development period ended in December 1969 with the conclusion that 2.5 m blanks can be produced with all properties lying within the required specifications. 

Numerous data, accumulated within a short period of time, were not only used to control the production parameters but they were at the same time analyzed for potential improvements. Some of these improvements were realized immediately during the melting period while others became evident only later when more and more results became available. Among the new data 

The mean value of 0.015 x 10 was considered to be excellent it is almost a factor of 30 smaller than the value for fused silica, the material with the best GTE value so far. At least of similar importance was the fact that the mean value was better by a factor of 10 than the specification of 0.15 x 10 K . If one succeeded in reproducing this value for the 3.6 m blank, the great expectations concerning the performance of the new telescope could even be surpassed. When all the data mentioned before had been collected it became obvious that the new glass ceramic Zerodur was well suited to shift the resolution of state-of-the-art telescopes far beyond the limitations of the time. Only one question remained would it be possible to scale the production process from 2.5 m blanks to 4 m blanks without sacrificing one or more of the excellent properties  

The observed inhomogeneities were analyzed and their reasons discussed. Ideas for improvements were proposed, tested, and finally implemented, resulting in a new tank, which was ready for production at the end of 1973. A three-run series of tests was performed and when the castings could be inspected it became clear that it had been successful. From what was known at this point in time it was concluded that the homogeneity of the glass blanks was so good that the specifications would be met after ceramization. Later this turned out to be true. 

Numerous data, accumulated within a short period of time, were not only used to control the production parameters but they were at the same 

---