5.6.4 Optical Observations

ELAIS follow-up in the optical wavelength range has been particularly massive, with several projects probing different flux ranges and wavebands over different areas, the two most ambitious projects aiming at covering the whole ELAIS fields in the southern and northern hemisphere, respectively.

The S1 field was surveyed in $R$ down to $R \sim 23.0$ by Franca et al. (2004) using the 1.5 Danish/ESO telescope. The N1 and N2 fields were surveyed in $U$, $g'$, $r'$, $i'$ and $Z$ down to 23.4, 24.9, 24.0, 23.2 and 21.9 respectively, as part of the Wide Field Survey (McMahon, 2001), using the Wide Field Camera at the Isaac Newton Telescope. Optical identification of N1 and N2 15 $\mu$m sources based on maximum likelihood were obtained by (Gonzalez-Solares, 2004), and is described in greater detail in Chapter 7.

In view of the increasing role of ELAIS fields as privileged "cosmic windows" where most large-area surveys will be carried out by both space and ground-based facilities (e.g. with Spitzer, as described in Chapter 8), deep optical imaging within ELAIS regions is a continuing effort. The ESO/Spitzer Wide-Angle Imaging Survey (ESIS, P.I. Franceschini) is an ongoing ESO Large Programme aimed at providing optical imaging over 6  $\square^\circ$ in 5 bands based on WFI 2.2 m and VIMOS down to 25-26 mag in the S1 field.