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The Osservatorio Astronomico Capodimonte Deep Field (OACDF) is a
multi-colour imaging survey project that is opening a new window towards
the distant universe.

It is conducted with the ESO Wide Field Imager (WFI), a 67-million pixel
advanced camera attached to the MPG/ESO 2.2-m telescope at the La Silla
Observatory (Chile).

As a pilot project at the Osservatorio Astronomico di Capodimonte (OAC)
[1], the OACDF aims at providing a large photometric database for deep
extragalactic studies, with important by-products for galactic and
planetary research. Moreover, it also serves to gather experience in the
proper and efficient handling of very large data sets, preparing for the
arrival of the VLT Survey Telescope (VST) with the 1 x 1 deg2 OmegaCam

PR Photo 15a/01: Colour composite of the OACDF2 field.

PR Photo 15b/01: Interacting galaxies in the OACDF2 field.

PR Photo 15c/01: Spiral galaxy and nebulous object in the OACDF2 field.

PR Photo 15d/01: A galaxy cluster in the OACDF2 field.

PR Photo 15e/01: Another galaxy cluster in the OACDF2 field.

PR Photo 15f/01: An elliptical galaxy in the OACDF2 field.

The Capodimonte Deep Field

ESO PR Photo 15a/01

Caption: This three-colour image of about 1/4 of the Capodimonte Deep
Field (OACDF) was obtained with the Wide-Field Imager (WFI) on the
MPG/ESO 2.2-m telescope at the la Silla Observatory. It covers “OACDF
Subfield no. 2 (OACDF2)” with an area of about 35 x 32 arcmin2 (about
the size of the full moon), and it is one of the “deepest” wide-field
images ever obtained. Technical information about this photo is
available below.

With the comparatively few large telescopes available in the world, it is
not possible to study the Universe to its outmost limits in all directions.
Instead, astronomers try to obtain the most detailed information possible
in selected viewing directions, assuming that what they find there is
representative for the Universe as a whole.

This is the philosophy behind the so-called “deep-field” projects that
subject small areas of the sky to intensive observations with different
telescopes and methods. The astronomers determine the properties of the
objects seen, as well as their distances and are then able to obtain a map
of the space within the corresponding cone-of-view (the “pencil beam”).
Recent, successful examples of this technique are the “Hubble Deep Field”
(cf. ESO PR Photo 26/98) and the “Chandra Deep Field” (ESO PR 05/01).

In this context, the Capodimonte Deep Field (OACDF) is a pilot research
project, now underway at the Osservatorio Astronomico di Capodimonte (OAC)
in Napoli (Italy). It is a multi-colour imaging survey performed with the
Wide Field Imager (WFI), a 67-million pixel (8k x 8k) digital camera that
is installed at the 2.2-m MPG/ESO Telescope at ESO’s La Silla Observatory
in Chile.

The scientific goal of the OACDF is to provide an important database for
subsequent extragalactic, galactic and planetary studies. It will allow
the astronomers at OAC — who are involved in the VLT Survey Telescope
(VST) project — to gain insight into the processing (and use) of the
large data flow from a camera similar to, but four times smaller than the
OmegaCam wide-field camera that will be installed at the VST.

The field selection for the OACDF was based on the following criteria:

* There must be no stars brighter than about 9th magnitude in the field,
in order to avoid saturation of the CCD detector and effects from
straylight in the telescope and camera. No Solar System planets should
be near the field during the observations;

* It must be located far from the Milky Way plane (at high galactic
latitude) in order to reduce the number of galactic stars seen in this

* It must be located in the southern sky in order to optimize observing
conditions (in particular, the altitude of the field above the
horizon), as seen from the La Silla and Paranal sites;

* There should be little interstellar material in this direction that may
obscure the view towards the distant Universe;

* Observations in this field should have been made with the Hubble Space
Telescope (HST) that may serve for comparison and calibration purposes.

Based on these criteria, the astronomers selected a field measuring about
1 x 1 deg2 in the southern constellation of Corvus (The Raven). This is
now known as the Capodimonte Deep Field (OACDF). The above photo (PR Photo
15a/01) covers one-quarter of the full field (Subfield No. 2 – OACDF2) —
some of the objects seen in this area are shown below in more detail. More
than 35,000 objects have been found in this area; the faintest are nearly
100 million fainter than what can be perceived with the unaided eye in the
dark sky.

Selected objects in the Capodimonte Deep Field

ESO PR Photo 15b/01

Caption: Enlargement of the interacting galaxies that are seen in the
upper left corner of the OACDF2 field shown in PR Photo 15a/01. The
enlargement covers 1250 x 1130 WFI pixels (1 pixel = 0.24 arcsec), or
about 5.0 x 4.5 arcmin2 in the sky. The lower spiral is itself an
interactive double.

ESO PR Photo 15c/01

Caption: Enlargement of a spiral galaxy and a nebulous object in this
area. The field shown covers 1250 x 750 pixels, or about 5 x 3 arcmin2
in the sky. Note the very red objects next to the two bright stars in
the lower-right corner. The colours of these objects are consistent
with those of spheroidal galaxies at intermediate distances (redshifts).

ESO PR Photo 15d/01

Caption: A further enlargement of a galaxy cluster of which most members
are located in the north-east quadrant (upper left) and have a reddish
colour. The nebulous object to the upper left is a dwarf galaxy of
spheroidal shape. The red object, located near the centre of the field
and resembling a double star, is very likely a gravitational lens [2].
Some of the very red, point-like objects in the field may be distant
quasars, very-low mass stars or, possibly, relatively nearby brown dwarf
stars. The field shown covers 1380 x 1630 pixels, or 5.5 x 6.5 arcmin2.

ESO PR Photo 15e/01

Caption: Enlargement of a moderately distant galaxy cluster in the
south-east quadrant (lower left) of the OACDF2 field. The field measures
1380 x 1260 pixels, or about 5.5 x 5.0 arcmin2 in the sky.

ESO PR Photo 15f/01

Caption: Enlargement of the elliptical galaxy that is located to the west
(right) in the OACDF2 field. The numerous tiny objects surrounding the
galaxy may be globular clusters. The fuzzy object on the right edge of
the field may be a dwarf spheroidal galaxy. The size of the field is
about 6 x 5 arcmin2.

Technical Information about the OACDF Survey

The observations for the OACDF project were performed in three different
ESO periods (18-22 April 1999, 7-12 March 2000 and 26-30 April 2000). Some
100 Gbyte of raw data were collected during each of the three observing
runs. The first OACDF run was done just after the commissioning of the

The observational strategy was to perform a 1 x 1 deg2 short-exposure
(“shallow”) survey and then a 0.5 x 1 deg2 “deep” survey.

The shallow survey was performed in the B, V, R and I broad-band filters.
Four adjacent 30 x 30 arcmin2 fields, together covering a 1 x 1 deg2 field
in the sky, were observed for the shallow survey. Two of these fields were
chosen for the 0.5 x 1 deg2 deep survey; OACDF2 shown above is one of these.

The deep survey was performed in the B, V, R broad-bands and in other
intermediate-band filters.

The OACDF data are fully reduced and the catalogue extraction has started.
A two-processor (500 Mhz each) DS20 machine with 100 Gbyte of hard disk,
specifically acquired at the OAC for WFI data reduction, was used. The
detailed guidelines of the data reduction, as well as the catalogue
extraction, are reported in a research paper that will appear in the
European research journal Astronomy & Astrophysics.


[1]: The team members are: Massimo Capaccioli, Juan M. Alcala’, Roberto
Silvotti, Magda Arnaboldi, Vincenzo Ripepi, Emanuella Puddu, Massimo
Dall’Ora, Giuseppe Longo and Roberto Scaramella.

[2]: This is a preliminary result by Juan Alcala’, Massimo Capaccioli,
Giuseppe Longo, Mikhail Sazhin, Roberto Silvotti and Vincenzo Testa, based
on recent observations with the Telescopio Nazionale Galileo (TNG) which
show that the spectra of the two objects are identical.

Technical information about the photos

PR Photo 15a/01 has been obtained by the combination of the B, V, and R
stacked images of the OACDF2 field. The total exposure times in the three
bands are 2 hours in B and V (12 ditherings of 10 min each were stacked to
produce the B and V images) and 3 hours in R (13 ditherings of 15 min each).
The mosaic images in the B and V bands were aligned relative to the R-band
image and adjusted to a logarithmic intensity scale prior to the
combination. The typical seeing was of the order of 1 arcsec in each of the
three bands. Preliminary estimates of the three-sigma limiting magnitudes
in B, V and R indicate 25.5, 25.0 and 25.0, respectively. More than 35,000
objects are detected above the three-sigma level. PR Photos 15b-f/01
display selected areas of the field shown in PR Photo 15a/01 at the
original WFI scale, hereby also demonstrating the enormous amount of
information contained in these wide-field images. In all photos, North is
up and East is left.