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Field selection and observations

The field selection for the OACDF was based on the following criteria:
i)
 lack of stars brighter than V = 9 and lack of Solar System planets nearby to the field, in order to avoid CCD saturation and ghost effects;
ii)
 high galactic latitude in order to avoid high stellar crowding;
iii)
 negative declination in order to minimise atmospheric extinction effects;
iv)
 interstellar (IS) extinction: the maps of Hydrogen column density by Burstein and Heiles (1982) were used to obtain information on the distribution of IS extinction. The IS extinction E(B-V) is less than 0.03 mag in the selected region;
v)
  possible presence of HST observations in the field for comparison and calibration purposes.
These constraints led us to identify a region at RA=12:25:10, DEC=-12:48:31 for this survey.

The observations for the OACDF project were performed in three different periods (18-22 April 1999, 7-12 March 2000 and 26-30 April 2000), using the WFI mosaic camera attached to the ESO 2.2m telescope at La Silla, Chile. This camera consists of eight 2k x 4k CCDs forming a 8k x 8k array. The scale is 0.238''/pix. Some 100 Gbyte of raw data were collected in each one of the three observing runs. The first OACDF run was done just after the commissioning of the ESO-WFI. The observational strategy was to perform a 1 deg2 shallow survey and a 0.5 deg2 deep survey. Four adjacent $30'\times30'$ fields, covering a $1^{\circ} \times 1^{\circ}$ field in the sky, were observed in the B, V, R and I bands for the shallow survey. We call these fields OACDF1, OACDF2, OACDF3 and OACDF4. The fields 2 and 4 were chosen for the 0.5 deg2 deep survey, which was performed in the B,V,R broad-bands and in the $\lambda$753, $\lambda$770, $\lambda$791, $\lambda$815, $\lambda$837, $\lambda$884 and $\lambda$914 nm intermediate-band filters. These intermediate band filters represent the best tools to trace the 4000 Å brake in a typical SED for an old stellar spheroid in the intermediate redshift range, from z=0.2 to z=1.0.

In Table 1 a summary of the OACDF observations is reported.

Several standard fields, selected from the Landolt (1992) E-regions were observed in order to transform the B V R I instrumental magnitudes to the standard Johnson/Kron-Cousins system. Likewise, several spectrophotometric standard stars were observed in the intermediate-band filters for absolute flux calibration purposes.

The first and third run were photometric, while the second one was partially non-photometric.

Being obtained with a mosaic imager, the WFI data are multi-extension fits files (MEFs). Each extension corresponds to one of the eight CCDs of the mosaic. The physical separation between the different CCDs with respect to each other is about 100$\mu$. A sequence of telescope displacements following a rhombus-like pattern projected on the sky were performed in order to cover the CCD gaps. These pointings are the so called ditherings. For the shallow survey, 5 ditherings were done, while at least 8 ditherings were done for the deep survey.


Table 1: OACDF observing log and 3$\cdot \sigma $ limiting magnitudes.
Field obs. run Filter Exp. time # diths Total exp. Seeing RMS k X ZP$^\dagger$ mlim$^\ddagger$
      (s)   time arcsec ADU/pix        
OACDF2 deep 1st B 600 12 2.0h 1.24 4.0 0.27 1.287 24.94 25.7
OACDF2 deep 1st V 600 10 1.7h 1.07 5.0 0.13 1.184 24.34 25.3
OACDF2 deep 1st R 900 13 3.3h 1.11 11.0 0.09 1.482 24.64 25.4
OACDF2 deep 1st-2nd 753nm 1113-1350 9+10 6.5h 0.88 5.0 0.08 1.512 21.95 24.1
OACDF2 deep 1st-2nd 770nm 1200-1075 9+10 6.0h 0.86 4.5 0.08 1.544 21.88 24.1
OACDF2 deep 1st-2nd 790nm 1284-1200 9+10 6.5h 0.99 7.0 0.08 1.084 21.80 23.7
OACDF2 deep 1st-2nd 815nm 1284-1450 9+9 6.8h 0.79 4.0 0.08 1.044 21.59 24.1
OACDF2 deep 1st-2nd 837nm 1113-1700 9+8 6.6h 0.95 7.5 0.08 1.468 21.54 23.3
OACDF2 deep 2nd 884nm 1600 9 4.0h 0.99 7.0 0.08      
OACDF2 deep 1st-2nd 914nm 1008-1000 10+10 5.6h 0.90 3.5 0.08 1.046 21.15 23.5
OACDF4 deep* 3rd B 900 8 2.0h 0.99 5.5 0.27 1.095 24.94 25.9
OACDF4 deep 2nd-3rd V 900 7 1.8h 0.95 6.0 0.13 1.042 24.28 25.5
OACDF4 deep 2nd-3rd R 600-900 4+14 4.2h 0.99 9.5 0.09 1.410 24.55 25.6
OACDF4 deep 2nd-3rd 753nm 750-1350 5+10 4.8h 0.95 4.0 0.08 1.042 21.81 24.1
OACDF4 deep 3rd 770nm 1075 7 2.1h 1.33 4.5 0.08 1.250 21.72 23.5
OACDF4 deep 3rd 790nm 1200 9 3.0h 0.95 6.0 0.08 1.380 21.65 23.4
OACDF4 deep 2nd-3rd 815nm 750-1450 5+6 3.5h 0.95 4.5 0.08 1.110 21.42 23.7
OACDF4 deep 3rd 837nm 1700 7 3.3h 1.20 7.5 0.08 1.100 21.44 23.1
OACDF4 deep 3rd 914nm 1000 9 2.5h 0.81 3.5 0.08 1.110 21.05 23.0
OACDF shallow 1st B 240 5 20min 0.86 5.0 0.27 1.078 24.93 24.3
OACDF shallow 1st V 120 5 10min 0.83 4.5 0.13 1.237 24.34 23.5
OACDF shallow 2nd R 120 5 10min 1.38 6.0 0.09 1.320    
OACDF shallow 1st I 120 5 10min 0.81 10.0 0.08 1.650 23.41 22.3

$\dagger$ the reported zero points are those for the first and third observing runs, as the second run was partially non-photometric.

$\ddagger$ AB magnitudes.

* due to technical problems it was not possible to calibrate the B-band during the third run. Stars on the OACDF4, calibrated during the first observing run, were used as secondary standards.

Figure: Raw (top) vs. pre-reduced (bottom) OACDF image in the R band. The exposure time of this image is 900 seconds. Note the large scale structures due to non-uniform illumination in the raw image. The gaps between each one of the CCDs are clearly seen.
\begin{figure}
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\special{psfile=fig1a_alcala.ps vscale=38 hscale=...
...fig1b_alcala.ps vscale=38 hscale=38 angle=0 voffset=-60
hoffset=10}
\end{figure}


next up previous
Next: Data reduction Up: The Capodimonte Deep Field Previous: Introduction
Juan Alcala
2002-02-05