Wonders of active optics: corrections during observations

Each of the four ESO VLT Unit Telescopes has a thin primary mirror with 8.2-m diameter. When the telescope points in different directions, the gravity-induced changes of this giant mirror's shape and position are much larger than the wavelength of visible light. Active optics corrects this in the following way. The images of stars at the focal plane are continuously monitored during the observations by an "image analyser" that detects even the smallest deviations from the optimum. Based on the corresponding signals from this device, the telescope mirrors are then automatically adjusted at regular intervals. In this way, the stellar images always remain optimal, i.e. as round and sharp as possible.

In the VLT, this correction is done, both by deforming the primary mirror and by moving the smaller, rigid 1.1-m secondary mirror of beryllium. The first is done by changing the forces exerted by the 150 individual mirror supports on the rear side of the 8.2-m Zerodur mirror. The outcome of such a correction, which in practice is done at least once a minute, is a perfectly round stellar image. The size of this image is entirely determined by the current atmospheric conditions (referred to as the seeing).

The video initially shows some short-exposure images taken with (intentionally) strongly distorted telescope optics. The stellar images at the focal plane are recorded as strange-looking figures with diameters of the order of 8 arcsec. It may be interesting to note that this is about four times larger than the image size (2 arcsec) recorded already during the very first attempts to obtain a stellar image with VLT ANTU in April 1998, just after the installation of the mirrors in the telescope structure. At that time, apart from some prior mechanical adjustment and alignments and crude focussing, no further corrections had yet been done.

In the clip a so-called open loop correction is made. It does not use any information from the image analyser. In this case, the forces that are applied to the primary mirror, as well as the optimum position of the secondary mirror, are automatically computed from a table that contains pre-determined values for different inclinations of the telescope tube.