3. Observations and reduction
In order to obtain identifications and redshifts of our FSC sources we used the ESO Faint Object Spectrograph and Camera (EFOSC; Buzzoni et al. 1984) on the telescope of the European Southern Observatory at La Silla, Chile, on the nights of September 6-8, 1994. Conditions were clear but not photometric and the seeing (measured in Gunn i-band) was typically . The detector was a anti-reflection coated, thinned, back-illuminated Tektronix CCD with a pixel size of in imaging mode. Our observing strategy was as follows. First, a short (1-2 min) exposure of the field was taken in imaging mode using a B-band filter (and for some objects also in R and/or Gunn i), allowing the detection of objects several magnitudes below the COSMOS plate limit. Subsequently, long-slit spectra were taken of potential counterpart galaxies, in order of decreasing likelihood ratio L, and including galaxies above and below the COSMOS plate limit, until an emission line galaxy was found. The spectra were taken with integration times of 10 to , using the B300 and R300 grisms with a slit, providing a spectral resolution from 3640 to 6860Å (B300) or from 5970 to 9770Å (R300). Wavelength calibration was derived from exposures of a HeAr lamp. Photometric and spectroscopic calibration was achieved by observations of the spectrophotometric standard L. Flatfields were obtained from lamp exposures. Data reduction was performed using the standard long-slit reduction procedures as implemented in the IRAF package.
© European Southern Observatory (ESO) 1999
Online publication: February 23, 1999