2. Observation, data reduction and calibration
Narrow and broad band imaging was carried out on April 12 and 13, 1999 with the 8.2m VLT Antu (UT1) using FORS in imaging mode. A narrow band filter was used which has a central wavelength of 3814 Å and a FWHM of 65 Å. For 1138-262 the emission of Ly at 1216 Å is redshifted to 3838 Å, which falls within the range of the narrow band filter. The broad band filter was a Bessel B with central wavelength of 4290 Å and a FWHM of 880 Å, which receives both continuum and redshifted Ly line emission. The detector was a Tektronix CCD with 20482 pixels and a scale of 0.2" per pixel. Eight separate 30 minutes exposures were taken in narrow band and six 5 minutes exposures in B, shifted by 20" with respect to each other to minimize flat fielding problems and to facilitate cosmic ray removal. The average seeing was 0.8" and the 1 limiting AB magnitude per square arcsecond was 27.8 for the narrow and 28.1 for the broad band image.
Image reduction was carried out using the IRAF reduction package. The individual images were bias subtracted and flat fielded by twilight flat fields for the narrow band and an average of the unregistered science exposures for the broad band. The images were then registered by shifting them in position by an amount determined from the location of several stars on the CCD. The registered images were co-added and cosmic rays removed. To improve the signal to noise and the sensitivity to faint extended objects, the resulting images were smoothed with a Gaussian function having a FWHM of 1". The spectrophotometric standard star GD108 (Oke 1990) was used to calibrate the fluxes in broad and narrow band. Calibration in broad band is accurate to 0.1 magnitude, but absorption lines and a small break in the part of the spectrum of GD108 which falls in the narrow band inhibits equally accurate calibration for this filter. Instead, it was assumed that the spectrum is flat in this wavelength range and the narrow band is calibrated relative to the broad band with an accuracy of 0.2 magnitude, as estimated from the spectrum of GD108. We assume that the median equivalent width (EW) of a random sample of objects is equal to zero. Since the median EW of our sample calibrated by the standard star is equal to 0.5 erg s-1 cm-2 Å-1, we subtract this number from the narrow band flux densities of the extracted objects to compute the EW. In this way, we do not rely on the absolute flux calibration.
© European Southern Observatory (ESO) 2000
Online publication: June 26, 2000