After seven years of construction, the Gran Telescopio Canarias (Grantecan) on La Palma, Spain, began trial operation in April.
The telescope, the largest optical telescope in the Northern Hemisphere, is rotated about two axes for vertical and horizontal motion. In addition, the motion of Earth has to be compensated during hour-long observations. The azimuth axis angle is measured over a diameter of about 15 m, which requires a scale tape length of 48.48 m; 1,212,000 lines with the scale-tape grating period of 40 µm are available for the angular measurement.
Thanks to the high quality of the measuring signals, each grating period can be interpolated by a factor of 4,096 to provide 10-nm measuring steps for positioning the azimuth axis. This results in an angular resolution of 0.0003 arc sec. Calibration and compensation of systematic errors bring a system accuracy of 0.06 arc sec over 13.2°.
The system is now in a test phase lasting one year. The hyperbolic telescope mirror consisting of 36 hexagonal segments will be expanded segment-by-segment until it reaches the total diameter of 10.4 m. At the same time, all instruments will be adjusted. When ready for operation, the telescope will focus 4 million times more accurately than the human eye.
Heidenhain angle encoders are used to position the telescope.
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