Amazingly sharp image taken of lunar landscape
EUROPEAN SOUTHERN OBSERVATORY NEWS RELEASE
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.
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
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.
Testing the NAOS-CONICA instrument
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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