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2 The survey

2.1 Goals

EIS-wide is a relatively wide-angle survey of four pre-selected patches of sky, 6 square degrees each, spanning the right ascension range $22^{\rm h} < \alpha < 9^{\rm h}$. The main science goals of EIS-wide are the search for distant clusters and quasars. To achieve these goals the original proposal envisioned the observation of 24 square-degrees in V and I, 6 square degrees in B over one of the patches, and 2 square degrees in U in a region near the South Galactic Pole.

Because of the slow start of the survey due to the unusually bad weather caused by El Niño, which dramatically affected the observations in the period of July-November 1997, some of these goals had to be reassessed by the WG (da Costa et al. 1998a). It was decided to limit the observations to the I-band, except for patch B, the region close to the South Galactic Pole, where observations were conducted in B, V and I over 1.5 square degrees. The current status of the observations for EIS-wide is summarized in Table 1, where the J2000 centers of the actually surveyed patches and the area covered in the different bands are given.

Table 1: Current sky coverage
\begin {tabular}{lcccccc}
Patch & $\alpha$\s...
 & - & - & 1.5 & 2.7 & 16.8 \\ \hline\end{tabular}\end{center}\end{table}

EIS-deep is a multicolor survey in four optical and two infrared bands covering 75 arcmin2 of the HST/Hubble Deep Field South (HDFS), including the WFPC2, STIS and NICMOS fields, and a region of 100 arcmin2 in the direction of the southern hemisphere counterpart of the Lockman Hole, to produce samples with photometric redshifts to find U-dropout candidates and galaxies in the redshift range 1 < z < 2. Observations for EIS-deep will start in August 1998 and therefore this part of the survey is not discussed in the present paper.

2.2 Field selection

The four EIS-wide patches were selected to have, in general, low optical extinction, low FIR ($\lambda=$ 100 $\mu$, AV < 0.05) emission and low HI column density ($\sim$2 1020 cm-2). They were also examined to guarantee that they would not include very bright stars or nearby clusters of galaxies. Preference was also given to fields that would overlap with other interesting datasets, especially recently completed or ongoing wide-angle radio surveys (e.g., NVSS, Westerbork in the Southern Hemisphere, Australia Telescope ESO Slice Project). The fields were also chosen to cover a range of galactic latitudes of possible interest for galactic studies. A map showing the position of the EIS patches and overlapping surveys can be found in the EIS web-pages ("'').

2.3 EIS filters

  Since the primary consideration in the selection of the filters was the desire for depth, the observations were conducted using wide-passband filters. EIS uses a special set of $BVI_{\rm c}$ filters (ESO WB430 # 795, WB539 # 796, WB829 # 797), which were designed to have higher transmission than the $BVI_{\rm c}$ passbands. The transmission curves for the filters and the full response of the NTT-EMMI red system with the EIS filters are shown in Fig. 1, and can be retrieved in electronic form from the World Wide Web at "''.

While the effective wavelengths of these filters are close to those of the Johnson-Cousins $BVI_{\rm c}$ filters, their passbands are broader and have sharper cutoffs. The measured passbands of these filters have been used to derive synthetic photometry using the Gunn & Stryker (1983) catalog of spectrophotometric scans of main-sequence and giant stars. For WB430 # 795 (B band) the throughput was determined to be 0.42 mag higher than Johnson B (at B-V=0), and for WB829 # 797 0.44 mag higher than Cousins $I_{\rm c}$ (at $V-I_{\rm c}=0$). However, the WB539 # 796 filter turned out to have a throughput slightly lower (0.18 mag) than Johnson V.

\resizebox {8.8cm}{!}{\includegraphics{7652f1.eps}}\end{figure} Figure 1: Transmission curves for the EIS filters (dashed lines) and total system throughput including the contribution from the filters, telescope and camera optics, and the detector (solid lines)

2.4 Survey strategy

The observations for EIS-wide started in July 1997 and are being conducted using the EMMI camera (D'Odorico 1990) mounted on the 3.5-m New Technology Telescope (NTT) at La Silla. The EMMI red channel is equipped with a Tektronix $2046 \times 2046$ chip with a pixel size of 0.266 arcsec and an effective field-of-view of about $9' \times 8.5'$,because of the strong vignetting at the top and bottom parts of the CCD. In order to cover a large area of the sky the observations are being conducted using a sequence of 150 s exposures shifted by half the size of an EMMI-field both in right ascension and in declination. This leads to an image mosaic whereby each position in the sky is observed twice for a total integration time of 300 s, except at the edges of the patch. The easiest way of visualizing the geometry of the EIS mosaic is to picture two independent sets of tiles (referred to as Pi,j, where i refers to the row along right ascension and j to columns in declination) forming a contiguous grid (normally referred to as odd and even depending on the value of j, and P is the patch name A-D) superposed and shifted in right ascension and declination by half the width of an EMMI frame. To ensure continuous coverage, adjacent odd/even frames have a small overlap at the edges ($\sim$20 arcsec). Therefore, a small fraction of the surveyed area may be covered by more than two frames. Such a mosaic ensures good astrometry, relative photometry and the satisfactory removal of cosmic ray hits and other artifacts.

2.5 Observations

\resizebox {8.8cm}{!}{\includegraphics{7652f2.eps}}\end{figure} Figure 2: Seeing distribution for patch A obtained from all observed frames (top panel) and only from the frames actually accepted for the survey (bottom panel). Vertical lines refer to 25, 50 and 75 percentiles of the distributions

Because of bad weather conditions, for patch A it was only possible to cover 3.2 square degrees in the I-band and 1.2 square degrees in the V-band. These observations were obtained in six different runs in the fall of 1997. The observations were carried out in standard visitor mode and data were taken in less than ideal conditions. A total of 400 science frames were obtained in I-band for patch A with the seeing varying from about 0.6 arcsec to over 2 arcsec. Regions observed under poor conditions were re-observed to maintain some degree of uniformity in the depth of the survey. In Fig. 2 the seeing distribution of all frames obtained in patch A (top panel) and of the frames actually accepted for the survey (bottom panel) are compared. This gives an idea of the area which required re-observations $(\!~\rlap{$<$}{\lower 1.0ex\hbox{$\sim$}}$1 square degree) and the impact that these had on the early progress of the survey. The median seeing for all frames is about 1.2 arcsec, while for the accepted frames it is about 1.1 arcsec. More importantly, the fraction of frames with seeing greater than 1.5 arcsec was greatly reduced. Note that the observing conditions have varied considerably within a run and from run to run during the period of observations of patch A, with only a small number of photometric nights. Therefore, depending on the application it may be necessary to further filter the object catalogs according to the characteristics of the data (Sect. 6.3). While this is trivial when dealing with single frames, proper pruning of the data requires more sophisticated tools when considering catalogs produced from the coadded image.

\resizebox {8.8cm}{!}{\includegraphics{7652f3.eps}}\end{figure} Figure 3: Seeing distribution for all the EIS patches. The plot includes all the data taken in 1997. Note the great improvement of the seeing distribution, especially for patches C and D

It is worth emphasizing that the data for patch A are by far the worst. This can be seen in Fig. 3, where the seeing distribution for patch A is compared with that of the other patches.

During EIS nights photometric and spectrophotometric standards are regularly observed (Landolt 1992a; Baldwin & Stone 1984; Landolt 1992b). Whenever possible, photometric solutions are derived to evaluate the quality of the night and to determine absolute zero-points for the tiles observed during photometric nights.

The photometric quality of the nights is also being assessed from the observations conducted by the Geneva Observatory at the 0.7 m Swiss telescope, which regularly monitors the extinction coefficients at La Silla. The information from the Swiss telescope is stored in a calibration database to provide the necessary reference with the survey nights. Whenever this information is available, images taken during photometric nights are flagged, and this information is used in the absolute photometric calibration of the patch (Sect. 3.9).

2.6 EIS magnitude system

The zero-points of the EIS magnitude system have been defined to give the same B, V, I magnitudes as in the Landolt system for stellar objects with $(B-V)=(V-I_{\rm c})=~0$. In other words, EIS magnitudes are by definition equal to the magnitudes in the Johnson-Cousins system for zero-color stars.

In Fig. 4 we show the observed transformation between the EIS system and Johnson-Cousins for the I-band, as a function of color. The data points are based on all the reduced observations of standard stars currently available (run 1-7). From a linear fit to the data points the color term between the EIS and Cousins system is found to be small ($0.014 \pm 0.004$). Note, however, that because of the limited amount of photometric nights up to run 7, color terms have not yet been taken into account in the photometric solutions, and the current I-band zero-point of the EIS system may be subject to small changes ($\sim$0.01 mag) at a later date, when more data becomes available in the final release.

\resizebox {8.8cm}{!}{\includegraphics{7652f4.eps}}\end{figure} Figure 4: Relation between the EIS and Johnson-Cousins system as a function of Johnson-Cousins color. Shown are all the standard stars observed under photometric conditions in the period July 97-January 98

2.7 External data


In order to provide additional constraints on the absolute calibration of the survey, data were obtained under photometric conditions from other telescopes at La Silla. Observations were conducted at the 2.2 m telescope (4 half-nights, out of which one half-night under photometric conditions) and one night at the 0.9 m Dutch telescope.

The observations with the 2.2 m telescope were carried out using EFOSC2 with a $2048\times2048$ CCD chip, with a pixel size of 0.27 arcsec and a seeing of about 1.5 arcsec. The filters used in these observations were ESO # 583, ESO # 584, ESO # 618. EFOSC2 has a field of view $8.3'
\times 7.7'$. During observations of the only photometric night, standard stars and frames over the original area of patch A were obtained in B, V and I. Unfortunately, only two I-band images overlap with the surveyed area, as shown in Fig. 5. The limiting magnitude of the frames is $I \sim 20$. Over 100 objects per frame were found in common with the overlapping survey fields.

\resizebox {8.8cm}{!}{\includegraphics{7652f5.eps}}\end{figure} Figure 5: Distribution of frames obtained at the 2.2 m and the 0.9 m Dutch (D) telescopes at La Silla within the covered region of patch A. Also shown is a part of a DENIS strip that crosses the field. The open rectangles represent regions containing EIS tiles observed under photometric conditions

The observations with the 0.9 m Dutch telescope were done in one photometric night using TK512CB (a $512\times 512$ CCD chip, with a pixel size of 0.47 arcsec) at a seeing of about 1.5 arcsec. The field of view approximates one quarter of an EIS survey tile, i.e. $3.1' \times
4.0'$. The filters used were ESO # 419, ESO # 420, and ESO # 465. The limiting magnitude of the science frames is $I \sim
22$. Approximately 200 objects per frame were found in common with the overlapping EIS survey tiles for the range 14 < I < 22 to define the zero-point.

Images from both telescopes were processed in a standard way using IRAF. One major problem was the severe fringing observed in the I-band frames from the 2.2 m. The fringing was removed by creating a combination of the 300 s science exposures and using IRAF's mkfringcor task, but preliminary results show that this correction may have affected the measurement of faint objects.

In addition to these scattered fields, a reduced strip of I-band data from the DENIS survey (Epchtein et al. 1996) is also available. Unfortunately, close inspection of the data showed that the strip was observed in non-photometric conditions.

Figure 5 shows the overlap of the external data with the surveyed region. Also shown are the regions covered by EIS tiles observed in photometric conditions.

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