eso9807 — Organisation Release
Pointing the First 8.2-m VLT Telescope
17 February 1998
Now that the big mechanical pieces of the first 8.2-metre unit telescope (UT1) of ESO's Very Large Telescope (VLT) have been assembled, the work areas have shifted towards the many other components that are needed to make the first giant optical telescope in the southern hemisphere operational. Although most of these items may be of less impressive dimensions, they are equally indispensable to make a telescope of this size and weight point towards a given direction and follow the motion of the celestial objects to be observed with superior accuracy.
Some the most crucial components for the orientation of the UT1 have just been installed and are now being tested with success. They include the encoders which allow to read the position of the two telescope axes with the highest possible precision. The telescope turns around the vertical azimuthal axis on circular hydrostatic tracks, while the horizontal altitude axis tilts the telescope tube between zenith and the horizon.
Due to some difficulties in getting the originally foreseen encoder on time, ESO decided late last year to mount a steel band LIDA Heidenhain encoder on the first VLT telescope.
This encoder is an improved version of the serially produced LIDA encoder with a 40 micron grating; it uses 8 reading heads on each axis to detect the absolute angular position of the telescope.
Beginning in September 1997, ESO designed and produced the mechanical parts needed to retrofit this encoder on the existing telescope. At the same time, the Heidenhain company (Germany) built and calibrated the altitude encoder tape in record time to an absolute accuracy of 0.025 arcsec and also produced the azimuth tape. This small angle corresponds to that subtended by a human hair seen at a distance of 200 metres or by a 50 metres long object on the surface of the Moon.
In December 1997, the azimuth encoder seat (7 metres diameter!) was machined by Atelier de la Meuse (Belgium), directly on the telescope. By using the telescope fork structure as a `tool machine', a run out of the seat (deviation from a circle) with respect to the moving part of only 0.050 mm was achieved, thereby ensuring an absolute accuracy of the azimuth tape reading better than 0.1 arcsec, as requested.
The position loops of the telescope are now closed and, already during the first approximation tuning, the estimated measured repeatability of the encoder is within 0.04 arcsecond. The final tuning of the position loops will take place later this week, after the phasing of the drives by means of the encoder have been carried out.
This series of new images from Paranal shows the encoder strips and heads and also some other views that illustrate the current progress.
Preliminary information about the UT1 First Light Event, now rapidly approaching, is available on the ESO web.