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 23^h 31^m - 01^h 23^m and 22^h 32^m to 22^h 57^m(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 12^h run and each of the two observing bands were calibrated separately following the standard procedures for ATCA observations.