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Anyone know what the tower is for? The HET website doesn't say. --BjKa 10:20, 29 January 2007 (UTC)
The tower contains instrumentation for monitoring and adjusting the mirror segments. Periodically the telescope is pointed toward the tower, measurements are made, and the mirror segments moved to place them in optimal positions. Bill Jefferys 23:09, 16 April 2007 (UTC)
In the early years of operation, the tower was not used because it was only capable of "capturing" the positions of the misaligned mirrors if they were quite close to where they should be. The mirror drift was much larger than the engineering specification, so it was easier to point the telescope to a star and "stack" the images, and then the telescope would provide somewhat decent images for an hour or so before more drift required restacking. A sensor system which measured the positions of the mirrors against one another was added after the telescope was declared "operational" -- at a cost of about 30% of the original design cost of the telescope. I do not know if the tower is currently used to recenter the mirrors: its purpose is to some extent served by the sensors, though measurement of the overall shape of the mirror, not just the relations between the individual mirrors to each other, is still necessary. To a large extent, the telescope's optical instability has crippled it over its life, leading to a paucity of scientific impact compared to other 9m class telescopes, and motivated the conversion of the HET over to a "light bucket" for cosmological surveys -- something it can do well even with poor optical performance. 220.127.116.11 (talk) 22:55, 11 November 2012 (UTC)