Image quality of the VLT

Superb image quality is the prime requirement for the VLT. The VLT should take full advantage of the exceptionally good ``seeing" conditions of the Paranal site, i.e. moments of a particularly stable atmosphere above the site, with a minimum of air turbulence.

In this diagram, the measured image quality of the VLT UT1 astronomical images is plotted versus the "seeing", as measured by the Seeing Monitor, a small telescope also located on top of the Paranal Mountain.

The dashed line shows the image quality requirement, as specified for the VLT at First Light. The dotted line shows the specification for the image quality, three years after First Light, when the VLT will be fully optimized. The fully drawn line represents the physical limit, when no further image distortion is added by the telescope to that introduced by the atmosphere.

The diagram demonstrates that First Light specifications have been fully achieved and, impressively, that the actual VLT performance is sometimes already within the more stringent specifications expected to be fulfilled only three years from now.

Various effects contribute to degrade the image quality of a telescope as compared to the local seeing, and must be kept to a minimum in order to achieve the best scientific results. These include imperfections in the telescope optical mirrors and in the telescope motion to compensate for Earth rotation during an exposure, as well as air turbulence generated by the telescope itself. The tight specifications shown in this figure translate into very stringent requirements concerning the quality of all optical surfaces, the active control of the 8.2-m mirror, the accuracy of the telescope motions, and, in the near future, the fast ``tip-tilt" compensations provided by the secondary mirror, and finally the thermal control of the telescope and the entire enclosure.

The only way to achieve an image quality that is "better than that of the atmosphere" is by the use of Adaptive Optics devices that compensate for the atmospheric distortions. One such device will be operative on the VLT by the year 2000, then allowing astronomers to obtain images as sharp as about 0.1 arcsec.

In this diagram, both seeing and telescope image quality are measured as the full-width-at-half-maximum (FWHM) of the light-intensity profile of a point-like source. The uncertainty of the measurements is indicated by the cross in the lower right corner.