3. The telescope
The VLT UT-1 had first light on the 25. May, 1998 (see The Messenger No. 92). A full description of the telescope can be found in the VLT White Book (1998; http://www.eso.org/outreach/info-events/ut1fl/whitebook). For SV the telescope was used at the Cassegrain focus which provides a plate scale of 528 microns per arcsecond at f13.6. The complete optical train was in place including the Linear Atmospheric Dispersion Compensator (LADC) which is located above the focal plane and is always in the beam. For the Science Verification observations described here the LADC was maintained in the closed position.
The telescope pointing during SV was of order 2-3 arcseconds RMS depending on the exact Alt-Az position of the main axes. The telescope was run in closed loop active optics mode including defocus corrections using the off-axis guide star as the reference. Field aberrations are removed before the residual aberrations are calculated. Typically a new correction is calculated and applied every 30 seconds to both the primary mirror and the secondary. The secondary corrects for residual decentering coma (the bulk of decentering coma is taken out by passively correcting the position of the primary mirror) and defocus. On the primary the rest of the first 15 Zernicke terms was corrected with the exclusion of tilt. Since focus is automatically corrected, no overheads for focusing the telescope are incurred. It should be noted that the image quality delivered by the telescope systematically surpassed the seeing as measured by the outside seeing monitor.
Light from the same guide star is used (with a dichroic) to feed the guide probe. Rapid guiding corrections are calculated from the CCD and sent to the secondary mirror unit, which provides the field stabilization through tip-tilt. The update frequency to the secondary was between 10 and 20 Hz depending the magnitude of the guide star.
With the exception of periods of strong wind (12 m s-1) the tracking performance with rapid guiding active was better than 0.2 arcseconds peak to peak. Operational overheads for the telescope permitted 180 seconds per preset before the telescope moved on to target. Following that the acquisition of the guide star and the first active optics correction typically require 30 to 60 seconds more. No other overhead is presented by the telescope itself. Offsetting the telescope is done during readout of the CCD. We note that the telescope SV phase was purposefully placed in the middle of the commissioning of the telescope. The figures presented here are indicative of the performance of the telescope at the time of SV. We expect improvements as a better understanding of the telescope develops in the second part of commissioning.
© European Southern Observatory (ESO) 1999
Online publication: March 1, 1999