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Proceedings Paper

Design and preliminary tests of the VLT secondary mirror unit
Author(s): Stefano Stanghellini; Emmanuel Manil; M. Schmid; Karl H. Dost
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Paper Abstract

The mechanics of the secondary unit are located behind the secondary mirror as seen from the telescope focus. The secondary is undersized and defines the pupil of the telescope. There is a focusing and centering drive for slow adjustments of the secondary mirror. In addition, the secondary mirror has a fast beam steering mechanism for chopping and rapid guiding to remove atmospheric wavefront tilt during observations. The specified square wave chopping frequency is 5 Hz with a duty cycle larger than 80%. To achieve a high bandwidth, the secondary mirror is manufactured of light-weighted beryllium coated with nickel. The beam steering mechanism has a counter-vibrating mass to compensate for dynamic forces and moments. The chopping mechanism has been successfully tested. The code of the digital control used during the tests was generated using Matlab real time toolbox. The servos were implemented on a digital signal processor card equipped with a TMS 320C40. To compensate for resonances inside the bandwidth of the servos, a special filter is applied in the velocity loop. The design of the secondary unit is now completed and fabrication and assembly have begun.

Paper Details

Date Published: 21 March 1997
PDF: 12 pages
Proc. SPIE 2871, Optical Telescopes of Today and Tomorrow, (21 March 1997); doi: 10.1117/12.269001
Show Author Affiliations
Stefano Stanghellini, European Southern Observatory (Germany)
Emmanuel Manil, European Southern Observatory (Germany)
M. Schmid, Dornier Satellitensysteme GmbH (Germany)
Karl H. Dost, Dornier Satellitensysteme GmbH (Germany)

Published in SPIE Proceedings Vol. 2871:
Optical Telescopes of Today and Tomorrow
Arne L. Ardeberg, Editor(s)

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