Original Source: European Southern Observatory

January 31, 2002

Sharp images of Saturn and Io with the new adaptive optics facility at the VLT.

Commissioning of NAOS-CONICA progresses

"First light" for the new NAOS-CONICA Adaptive Optics facility on the 8.2-m VLT YEPUN telescope at the Paranal Observatory was achieved in November 2001, cf. ESO PR 25/01. A second phase of the "commissioning" of the new facility began on January 22, 2002, now involving specialized observing modes and with the aim of trimming it to maximum performance before it is made available to the astronomers later this year.

Test images of excellent sharpness

During this demanding and delicate work, more test images have been made of various astronomical objects. Some of these show selected solar system bodies, for which the excellent image sharpness achievable with this new instrument is of special significance. In fact, the VLT photos of the giant planet Saturn and Io, the innermost of Jupiter's four large moons, are among the sharpest ever obtained from the ground. They even compare well with some photos obtained from space.

The NAOS adaptive optics corrector was built, under an ESO contract, by the Office National d'Etudes et de Recherches Aérospatiales (ONERA), Laboratoire d'Astrophysique de Grenoble (LAOG) and the DESPA and DASGAL laboratories of the Observatoire de Paris in France, in collaboration with ESO. The CONICA infra-red camera was built, under an ESO contract, by the Max-Planck-Institut für Astronomie (MPIA) (Heidelberg) and the Max-Planck Institut für Extraterrestrische Physik (MPE) (Garching) in Germany, in collaboration with ESO.

Saturn - Lord of the rings

Image copyright: ESO

This NAOS/CONICA image of Saturn (PR Photo 04a/02), the second-largest planet in the solar system, was obtained on December 8, 2001, at a time when Saturn was close to summer solstice in the southern hemisphere. The distance was 1209 million km. It is a composite of exposures in two near-infrared wavebands (H and K) and displays well the intricate, banded structure of the planetary atmosphere and the rings. At this moment, the tilt of the rings was about as large as it can be, allowing the best possible view of the planet's South Pole. That area was on Saturn's night side in 1982 and could therefore not be photographed during the Voyager encounter.

The dark spot close to the South Pole is a remarkable structure that measures approximately 300 km across. It was only recently observed in visible light from the ground with a telescope at the Pic du Midi Observatory in the Pyrenees (France) - this is the first infrared image to show it.

The bright spot close to the equator is the remnant of a giant storm in Saturn's extended atmosphere that has lasted more than 5 years.

The present photo provides what is possibly the sharpest view of the ring system ever achieved from a ground-based observatory. Many structures are visible, the most obvious being the main ring sections, the inner C-region (here comparatively dark), the middle B-region (here relatively bright) and the outer A-region, and also the obvious dark "divisions", including the well-known, broad Cassini division between the A- and B-regions, as well as the Encke division close to the external edge of the A-region and the Colombo division in the C-region. Moreover, many narrow rings can be seen at this high image resolution, in particular within the C-region - they may be compared with those seen by the Voyager spacecraft during the flybys.

This image demonstrates the capability of NAOS-CONICA to observe also extended objects with excellent spatial resolution. It is a composite of four short-exposure images taken through the near-infrared H (wavelength 1.6 μm) and K (2.2 μm) filters.

This observation was particularly difficult because of the motion of Saturn during the exposure. To provide the best possible images, the Adaptive Optics system of NAOS was pointed towards the Saturnian moon Tethys, while the image of Saturn was kept at a fixed position on the CONICA detector by means of "differential tracking" (compensating for the different motions in the sky of Saturn and Tethys).

Io - volcanoes and sulphur

Io has a diameter of 3660 km and orbits Jupiter at a mean distance of 422,000 km - one revolution takes 42.5 hours. Like the Earth's moon, it always turns the same side towards the planet. As shown by the Voyager spacecraft in 1979, its surface is covered by active volcanoes and lava fields - it is in fact the most volcanic place known in the solar system.

Due to this activity, Io's surface is continuously reshaped. The features now seen are all correspondingly young, with a mean age of the order of 1 million years only. The variations in appearance and colour are due to different volcanic deposits of sulphur compounds. The cause of all this activity is Jupiter's strong gravitational pull that leads to enormous stresses inside Io and related heating of the entire moon.

Image copyright: ESO

This photo shows Io as imaged with the VLT NAOS-CONICA Adaptive Optics instrument on December 5, 2001, through a near-infrared, narrow optical filter (Brackett-gamma at wavelength 2.166 μm). Despite the small angular diameter of Io, about 1.2 arcsec, many features are visible at this excellent optical resolution. Some of these are volcanoes, others correspond to lava fields between these.

Io has been observed with the NASA Galileo spacecraft since 1996 at higher resolution in the visible and infrared, especially during close encounters with the satellite. However, this NAOS image fills a gap in the surface coverage of the infrared images from Galileo.

The capability of NAOS/CONICA to map Io in the infrared at the present high image resolution will allow astronomers to continue the survey of the volcanic activity and to monitor regularly the related surface processes.

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see also:

• First Light of the NAOS-CONICA system (Dec 03, 2001)

• ESO's description of the NAOS-CONICA system

• Voyager images of Saturn

• Io website of the Galileo mission