The observations were made at the ESO 3.5 m New Technology Telescope(NTT) with the new infrared imager/spectrometer SOFI (Moorwood, Cuby and Lidman, 1998, The Messenger, 91,9). SOFI is equipped with a 1024 x 1024 pixel Rockwell 'Hawaii' array. In its large fieldmode, used here, the pixel size is 0.292 ± 0.001 arcsec/pix and the field of view ~ 5 x 5 arcmin. The field was observed in thetwo near-infrared filters J (λc =1.247 µm; λ ~ 0.290 µm ) and Ks (λc = 2.162 µm; Δλ~ 0.297 µm) over several nights of good seeing (≤ 0.9 arcsec) in March 1998. The total exposure times are 624 minutes and 256 minutes in Ks and J respectively, resulting from thecoaddition of short jittered observations. The jittering was controlled by the automatic jitter template (described in the SOFI User Manual) which produces a set of frames slightly offset intelescope position from the starting point. The observed positions were randomly generated (Poisson) within a box of 30 x 30 arcsec centered on the first pointing. Thus the deepest exposure is reachedin the central 4.45 x 4.45 arcmin of the final image. The effective exposure time of each individual frame was 1 min in Ks, (resulting from the average of 6 exposure of 10 s each), and 2 min inJ (resulting from the average 6 exposures of 20 s each). A single measurement normally comprised 60 and 30 frames in Ks and J respectively i.e total integrations of 1 hour which were then repeated. Table. 1 summarizes the observations and gives the expected 3-sigma limiting magnitudes within an aperture of 2 x FWHM (1.5arcsec) in the coadded final frames.

Filter Number of texp FWHM Zero-point µ mlim
  frames (s) (arcsec)   mag/arcsec2 (3 σ)
Ks 624 37440 0.75 22.469 23.03 22.87
J 128 15240 0.75 23.072 24.82 24.66