The European Large Area ISO Survey - I. Goals, definition and observations

We describe the European Large Area ISO Survey (ELAIS). ELAIS was the large st single Open Time project conducted by ISO, mapping an area of 12 deg(2) at 15 mu m with ISOCAM and at 90 mu m with ISOPHOT. Secondary surveys in ot her ISO bands were undertaken by the ELAIS team within the fields of the pr imary survey, with 6 deg(2) being covered at 6.7 mu m and 1 deg(2) at 175 m u m. This paper discusses the goals of the project and the techniques employed i n its construction, as well as presenting details of the observations carri ed out, the data from which are now in the public domain. We outline the EL AIS 'preliminary analysis' which led to the detection of over 1000 sources from the 15 and 90-mu m surveys (the majority selected at 15 mu m with a fl ux limit of similar to 3 mJy), to be fed into a ground-based follow-up camp aign, as well as a programme of photometric observations of detected source s using both ISOCAM and ISOPHOT. We detail how the ELAIS survey complements other ISO surveys in terms of de pth and areal coverage, and show that the extensive multi-wavelength covera ge of the ELAIS fields resulting from our concerted and on-going follow-up programme has made these regions amongst the best studied areas of their si ze in the entire sky, and, therefore, natural targets for future surveys. T his paper accompanies the release of extremely reliable subsets of the 'pre liminary analysis' products. Subsequent papers in this series will give fur ther details of our data reduction techniques, reliability and completeness estimates and present the 15- and 90-mu m number counts from the 'prelimin ary analysis', while a further series of papers will discuss in detail the results from the ELAIS 'final analysis', as well as from the follow-up prog ramme.