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3 Observations and calibration

To cover the two areas of $22\hbox{$^\circ$ }\times 1\hbox{$^\circ$ }$ (region A) and $5\hbox{$^\circ$ }\times 1\hbox{$^\circ$ }$ (region B) of the ESP survey with uniform sensitivity, $69\times 4$ and $15\times 4$pointings at $20\hbox {$^\prime $ }$ spacing are needed respectively. An area of 1.3 sq. degs. ($4\times 4$ pointings) of region A was not observed, because of the presence of the strong radio source PMNJ2326-4027, which would have prevented us from reaching the deep noise level required. This reduced the total number of fields to be observed to $65\times 4$ (region A) and $15\times 4$ (region B), i.e. 320 pointings in total. The right ascension range actually covered by the ATESP survey is thus 23h 31m - 01h 23m and 22h 32m to 22h 57m(J2000).

An observing time of 1.2 hours per pointing was needed to reach a $3\sigma$ rms noise level of $\simeq 0.2$ mJy (with $2\times 128$ MHz bandwidth). To obtain good hour angle coverage we organized the snapshot observations as follows: the 320 fields were divided in 16 sets of 20 fields each ($5\times 4$). Every set was then observed for $2\times 12^{\mathrm{h}}$, that is, the 20 fields were observed in sequence, for 1 minute each, repeating this for 72 times and adding 3 minutes for calibration every hour.

The observing campaign started in November 1994 and was completed in January 1996. A log of the observations is given in Table 2 (dates, arrays used, observing time and frequencies). We stress that, for our purposes, the use of different 6 km arrays is not relevant in any way.


Table 2: Log of the observations
Date tobs Array $\nu_1$ $\nu_2$
  h   MHz MHz
18/11/94-21/11/94 $3\times 12$ 6D 1344 1452
23/12/94-04/01/95 $13\times 12$ 6A 1344 1452
15/12/95-01/01/96 $17\times 12$ 6C 1344 1448

The two 128 MHz observing bands were set in the most interference-free region of the 20 cm band (1.3 - 1.5 GHz).

The flux density calibration was performed through observations of the source PKS B1934-638, which is the standard primary calibrator for ATCA observations (S=14.9 Jy at $\nu = 1384$ MHz, Baars et al. 1977 flux scale). The phase and gain calibration was based on observations of secondary calibrators, selected from the ATCA calibrator list. Every single 12h run and each of the two observing bands were calibrated separately following the standard procedures for ATCA observations.

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