From October 1986 to April 1988 Carl Zeiss has fabricated the 3.5m primary mirror of the "New
Technology Telescope" (NY!') for the European Southern Observatory (ESO). This was the first time at
Carl Zeiss that at no stage of manufacturing a skilled optician was necessary for doing manual corrections
on the mirror. In the contrary, all the shaping and fine correction was done by computer controlled
machine work. Computerized interferometry was the tool to deliver the necessary data for closing the
As a rule of thumb, in a sound fabrication process the logarithm of the rms-values of the remaining
figure-errors drops linearly with time. For the LOT (Large Optical Telescope, Iraq) 3.6m-primary mirror
with an f-number of 3.5, which was fabricated at Carl Zeiss in a traditional way and finished in 1984, the
rms-value was brought down by a factor of two in about 81 days. For the ESO-N1T the time for cutting
the rms-value to half was 35 days3.
Next month the fabrication of another 3.5m mirror, the primary for the Telescopio Nationale Galileo
(TNG), will start. Although the NTE' was a very big success, we have used the time since 1988 to further
improve our measuring techniques. First we will report on the metrology applied during fabrication of the
NTT, then we will describe the tremendous improvements in resolution and speed now ready for the TNG.