and Data Reduction \[s:observations\]
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Near-infrared imaging \[s:observations:ss:nir\]
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We obtained near-infrared images of four fields in W3, covering a large area ($0.58\deg\times$$0.46\deg$). Images were obtained with the Omega 2000 camera at the 3.5m telescope in the Calar Alto Observatory of the Centro Astronómico Hispano Alemán (CAHA) in Almería, Spain. Omega 2000 provides a $15^\prime \times 15^\prime$ field of view (FOV). Observations were made in $J$, $H$ and $K$ (1.1, 1.6 and 2.2 $\mu$m, respectively). A list of all fields observed is listed Table \[tab:obs\], which gives the field identification, the center of field positions, observation date, filter, seeing (estimated from sigma-clipped average full width half-maximum (FWHM) of the stars in each field), and the peak values for the brightness distribution, which is a good estimate of the sensitivity limits achieved. The brightness distribution peaks are in all cases at or above $J=20.5$ mag, $H=19.75$ mag and $K=19.25$ mag.

### Image reduction \[s:observations:ss:nir:sss:reduction\]

The Omega 2000 images were reduced with modified versions of the FLAMINGOS near-infrared reduction and photometry/astrometry pipelines, which are built in the standard `IRAF` Command Language environment. One pipeline [see @Roman-Zuniga:2006aa] processes all raw frames by subtracting darks and dividing by flat fields, improving signal to noise 