Twelve CNSFRs were observed with the 4.2-m WHT at the Roque de los Muchachos Observatory using the ISIS double spectrograph, with the EEV12 and TEK4 detectors in the blue and red arms respectively, with the dichroic filter at A7500Á. Gratings R300B in the blue arm and R600R in the red arm were used, covering the spectral ranges A3650-A7000Á in the blue and A8850-A9650Á in the near infrared, yielding spectral dispersions of 1.73 Á per pixel in the blue arm and 0.79 Á per pixel in the red arm. A slit width of 1 arcsec was used. The nominal spatial sampling was 0.4 arcsec per pixel in each frame and the average seeing for this night was ~ 1.2 arcsec.
The data were reduced using the Image Reduction and Analysis Facility (IRAF) package following standard procedures. The high spectral dispersion used in the near infrared allowed the almost-complete elimination of the night-sky OH emission lines and, in fact, the observed A9532/A9069 ratio is close to the theoretical value of 2.48 in all cases. Telluric absorptions were removed from the spectra of the regions by dividing them by a relatively featureless continuum of a sub-dwarf star observed during the same night.
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