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 fields, covering a 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 753, 770, 791, 815, 837, 884 and 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. 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.
|Field||obs. run||Filter||Exp. time||# diths||Total exp.||Seeing||RMS||k||X||ZP||mlim|
the reported zero points are those for the first and third observing runs, as the second run was partially non-photometric.
* 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.