Late Afternoon at Taruntius: Amazingly Sharp VLT Image of Lunar Landscape

Images online at: http://www.eso.org/outreach/press-rel/pr-2002/phot-19-02.html

Caption: PR Photo 19a/02 shows a small area of the lunar surface, on the
rim of the 56-km crater Taruntius. This formation is located some 6 deg North
of the lunar equator and near the intersection of Mare Tranquillitatis and
Mare Foecunditatis in the Eastern hemisphere. It is late in the lunar
“day” and the Sun is low above the western horizon. The long shadows
greatly enhance the visibility of shallow surface formations. The largest
crater in the field (at the top) is named Cameron and measures about 10 km
across [1]. This photo (in near-infrared light at wavelength 2.3 micron) was
obtained in the morning of April 30, 2002, with the NAOS-CONICA (NACO)
adaptive optics (AO) camera mounted on the ESO VLT 8.2-m YEPUN telescope
at the Paranal Observatory. The on-line AO image corrections were achieved
by guiding on an illuminated lunar peak, just outside the field shown.
This image is “raw”, i.e. it has only been subjected to “background
subtraction” and cosmetic removal of a few “bad” pixels. Some of the
slopes in direct sunlight are overexposed. With angular resolution of
about 0.07 arcsec – corresponding to 130 metres at the distance of the
Moon – it is undoubtedly one of the sharpest images of the lunar surface,
ever obtained with a ground-based telescope. In this reproduction, the
North direction is about 10deg to the left of the top, with West to the
left. The size of the field is about 25 x 25 arcsec^2; when taking into
account the viewing angle, this corresponds to approximately 60 x 45 km^2.
The pixel size is that recorded by NACO, 0.027 arcsec, or approx. 50
metres on the Moon.

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, cf. PR Photo
19a/02. 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 photo shows an area about 700 km from the Apollo XI 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 [2]), with part of a
sunlit, 10-km wide crater named Cameron [1] 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 [1]. 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 PR Photo 19a/02. 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 [2], 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.

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 [2], this view can be “rectified” by simple image
processing. The corresponding “view from above” is shown in PR Photo
19b/02; most of the craters in the field now appear quite round.

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.

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.

The Clementine image shown here (PR Photo 19c/02) 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 [3]. 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, cf. ESO PR 25/01 and ESO PR Photos 04a-c/02.

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 [2].

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.

Other lunar images

An impressive ESO photo of the waning Moon was obtained in 1999 with
the WFI camera at the La Silla Observatory, cf. ESO PR 02/99.

Many websites display fine images of the Moon, obtained with professional
and amateur telescopes. Many links are available at the dedicated page
maintained by the Centre de Donnes Planetaires at the Institut
d’Astrophysique Spatiale (Paris, France).

The Hubble Space Telescope (HST) did not photograph the Taruntius
area, but an excellent photo of the Copernicus crater was published in
1999.

Notes

[1]: The lunar crater Taruntius (lunar co-ordinates: 5.6deg N; 46.5deg
E) was named in 1935 by the International Astronomical Union (IAU)
after the Roman philosopher Lucius Firmanus Taruntius (? – 86
B.C.). It measures about 56 km across. The 10-km crater Cameron
(6.2deg N; 45.9deg E) was named by the IAU in 1972 after the American
astronomer Robert Curry Cameron (1925 – 1972). Names of surface
features on planets and their natural satellites, including the
Earth’s Moon, are allocated by the “IAU Working Group for Planetary
System Nomenclature” and published on the web in the “Gazetteer of
Planetary Nomenclature”.

[2]: The NACO image was exposed on April 30, 2002, at 09:42 hrs
UT. The geometrical circumstances of this observation were the
following: the Moon was located at (Azimuth Az = 266 deg Elevation h = +62deg)
in the sky above the VLT at the Paranal Observatory; the Earth
(Paranal) was located at (Az = 263 deg h = +50 deg) and the Sun at (Az =
268 deg h = +7 deg) in the lunar sky above the Cameron crater. The
distance from Paranal to the Moon was about 370,000 km.

[3]: Acknowledgment: The Clementine Basemap Mosaic was compiled for
the National Aeronautics and Space Administration (NASA) by the United
States Geological Survey (USGS) under the direction of Dr. Alfred
S. McEwen, principal Investigator (now with the University of
Arizona). The DoD/BMDO Clementine spacecraft was built and operated by
the Naval Research Laboratory, with remote-sensing instruments from
the Lawrence Livermore National Laboratory. The field shown in PR
Photo 19c/02 was reproduced from a 0.5-km full resolution frame
(BM14N045) for which a browse page is available on the web; the file
itself is at:
http://pdsimage.wr.usgs.gov/CDROMS/cl_3015/bm90_90/bm14n045.img. Another
image of the Taruntius area with 100-metre pixels is available at
http://pdsimage.wr.usgs.gov/CDROMS/cl_3003/bi00_35n/bi03n045.img. A
comprehensive collection of data gathered by the instruments onboard
Clementine may be found via the Clementine Navigator of the Jet
Propulsion Laboratory Planetary Data System. Clementine also obtained
images of a small fraction of the lunar surface by means of a High
Resolution Camera (HRC) with a nominal resolution of 7 to 20
metres. However, none of these covered the area shown in the NACO
photo.

[4]: Acknowledgment: The image of the entire Moon shown at the upper
left of PR Photo 19c/02 was obtained with a 12-inch refractor when the
Moon was “aged” 17.9 days, i.e. almost the same phase as when the NACO
image was taken. It is reproduced from the Berliner Mond-Atlas (3rd
edition, 1989), published by the Wilhelm-Foerster-Sternwarte – Berlin
(Germany).