Amazingly sharp image taken of lunar landscape
Posted: August 12, 2002

Thirty-three years after the first manned landing on the Moon, the ESO Very Large Telescope (VLT) has obtained what may be the sharpest image of the lunar surface ever recorded from the ground. It was made with the NAOS-CONICA (NACO) adaptive optics camera mounted on the ESO VLT 8.2-m YEPUN telescope at the Paranal Observatory.

A small area of the lunar surface, on the rim of the 56-km crater Taruntius. Photo: ESO
The photo shows an area about 700 km from the Apollo 11 landing site. The location is in the Eastern hemisphere, just North of the lunar equator, and right between two of the major "seas", Mare Tranquillitatis (Sea of Tranquillity) and Mare Foecunditatis (Sea of Fertility).

The field-of-view measures about 60 x 45 km^2 (taking into account the foreshortening because of the viewing angle), with part of a sunlit, 10-km wide crater named Cameron surrounded by a comparatively level terrain, bordered by some hills and, not least, with an incredible number of smaller craters.

The site of this NACO photo is situated at the rim of an older, rather eroded 56-km crater, Taruntius. A small part of the multiple walls of that crater are seen in the upper right corner and also to the left of the bottom centre of the above image. The centre of Taruntius is near the lower right corner of the photo. The rather flat terrain to the left in the photo corresponds to an "opening" in the crater walls.

At the time of the exposure, the Sun was approximately 7deg above the Western horizon to the left, and the shadows are therefore quite prominent, approx. 8 times longer than the elevation of the corresponding peaks and hills.

The nominal image sharpness is 0.07 arcsec, or about 130 metres on the lunar surface (in the N-S direction). Elevation differences of a few tens of metres only are therefore visible by the shadows they cast. The VLT image represents what an astronaut (with normal eye acuity of 1 arcmin) would see from 400 km above the surface.

Lunar surface formations
Located at 46deg East lunar longitude, 6deg North lunar latitude, this area is viewed from the VLT at an inclined angle and the craters therefore all appear as ellipses in the NACO image. However, taking into account the direction of the line-of-sight at the time of the observation, this view can be "rectified" by simple image processing. The corresponding "view from above" is shown below; most of the craters in the field now appear quite round.

A computer-processed version of the top image, in which the lunar surface is now viewed directly "from above". Photo: ESO
Many different types of lunar surface formations are visible in the VLT photo. In addition to the numerous impact craters of all sizes, there are also hills and ridges of a great variety of shapes, as well as a prominent "valley" (a "Rima", or fissure) that stretches nearly 50 km through the photo in East-West direction. It has been identified on earlier photos and as it is situated inside that crater, it was given the name "Rimae Taruntius" in 1985. It is very well resolved in this photo and resembles "Rima Hadley" that was visited by the Apollo 15 astronauts in 1971, but is much smaller. The mean width is about 600 metres (12 pixels). The bottom is in the shadows and the depth is therefore unknown. It is overlapped by several smaller craters that must have been caused by impacts after this depression was formed.

Measuring the length of the shadows, it is possible to infer the height of some of the formations. For instance, the shadows of the two peaks at the lower centre of the photo are about 4 km long, indicating that these formations are about 500 metres tall.

The surroundings
This area around Taruntius was imaged in 1994 by the NASA Clementine spacecraft when it mapped the entire lunar surface at 125-250 metres per pixel. The data led to the first complete map of the mineralogy (rock types) of the Moon.

Where is the NACO field at the Taruntius crater located on the Moon? A 400 x 400 km2 area surrounding this crater is shown in the right panel; it has been reproduced from a photo mosaique with 500-metre resolution based on exposures made in 1994 by the "Clementine" spacecraft in lunar orbit. Taruntius, Cameron and other craters in this area are identified in the diagram at the lower left. The area covered by the Clementine photo is outlined on a photo of the entire Moon (upper left), obtained at nearly the same phase as when the NACO image was made. Photo: ESO
The Clementine image shown here helps to identify the small area depicted by NACO. It is part of the Clementine Basemap Mosaic and has been observed with the onboard Ultraviolet/Visible camera through an optical filter centred at 750 nm. It covers a field-of-view of about 400 x 400 km^2, with a nominal resolution of about 500 metres. Many craters are well visible, including Taruntius with Cameron on the upper left sector of the multiple rim.

Testing the NAOS-CONICA instrument
This splendid VLT image is a by-product of the ongoing, thorough testing of the NAOS-CONICA (NACO) Adaptive Optics facility, recently installed at the 8.2-m YEPUN telescope, the fourth untit of the Very Large Telescope (VLT) at the ESO Paranal Observatory. This major astronomical instrument has already delivered other impressive views of the Universe.

Normally, NACO functions by "locking" on a point-like guide star, correcting the image smearing caused in the turbulent terrestrial atmophere by measuring the deformation of the recorded image of that star.

However, in the morning of April 30, 2002, shortly before sunrise, the astronomers and engineers working with NACO decided to do a test of wavefront sensing on an extended celestial object. For this, the giant telescope was turned towards the Moon, at that moment seen in the southern constellation of Ophiuchus, high above the western horizon at Paranal.

Guiding the advanced instrument on a sunlit lunar peak in the area between Mare Tranquillitatis and Mare Foecunditatis, a short exposure (0.22 seconds) was made through a narrow-band near-infrared filter (at wavelength 2.3 micrometers), with the adaptive optics (AO) activated in closed-loop mode. The telescope was set to track on that lunar mountain and the flexible AO mirror in NACO superbly "refocussed" the 25 x 25 arcsec2 field-of-view.

Although the atmosphere above Paranal was rather turbulent that morning - the nominal seeing was measured as 1.5 arcsec - and despite the use of an extended feature for the guiding, the NACO adaptive optics compensation achieved nearly theoretical image sharpness, about 0.068 arcsec for this waveband.

Images of other areas on the lunar surface may be attempted in the future with the VLT and NACO.

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