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VLTs
The image shown (Figure 1) here was obtained with the VLT on May 16, 1998, in red light (R band), i.e. while the mirror was still uncoated. It is a 10-minute exposure of the center of Omega Centauri and it demonstrates that the telescope is able to track continuously with a very high precision and thus is able to take full advantage of the frequent, very good atmospheric conditions at Paranal. The images of the stars are very sharp (full-width-at-half-maximum (FWHM) = 0.43 arcsec) and are perfectly round, everywhere in the field. This indicates that the tracking was accurate to better than 0.001 arcsec/sec during this observation.
At a distance of about 17,000 light years, this cluster is barely visible to the naked eye as a very faint and small cloud. When Omega Centauri is observed through a telescope, even a small one, it looks like a huge swarm of numerous stars, bound together by their mutual gravitational attraction.
Most globular clusters in our Galaxy have masses of the order of 100,000 times that of the Sun. With a total mass equal to about 5 million solar masses, Omega Centauri is by far the most massive of its kind in our Galaxy.
The VLT active optics system fully controls the primary and secondary mirrors. In figure 3, images of stars are shown that were obtained during tests in which the control system forced the optical elements to produce three different image aberrations, namely triangular, round (defocus) and linear (astigmatism). This is done by applying different forces to each of the 150 individual active actuators on which the 8.2-m main mirror rests and to the position of the secondary mirror. In the case of the triangular aberration, the mirror was made to resemble Napoleon's hat. It is worth noting that the deviation of the mirror from its optimum shape is only 0.015 millimeters.
The great power of this system is demonstrated by the fact that the resulting stellar images can take on (nearly) any desired form. The optical system is also consistently brought back to its optimal form, producing the sharp images of a real star, shown in the lower row.
The control of the two mirrors is such that no significant aberrations remain after the corrections are applied. The accuracy of the control of shape of the primary mirror results in an average error of order 0.00005 millimeters. The telescope is only limited by the Earth's atmosphere. In space, the optical quality of the mirrors under active control could be diffraction limited.