3 Low resolution spectroscopy

Slitless spectroscopic observations of a circular area of $\sim 13$ arcmin diameter centered on the cluster have been obtained with the AFOSC spectrograph at the Asiago 1.8-m telescope on July 29 1998 between 20:00 and 22:30 UT (see Fig. 2). We used grism No. 7 (100 Å/ mm), resulting in a dispersion of $\sim 2~$Å/pix and a resolution (seeing/guide dominated) of $\sim 7~$Å. The night was photometric with a seeing better than 1.5 arcsec. Altogether 10 spectral frames bracketed by 17 V-band images were obtained at different orientations (0$^\circ$, 45$^\circ$ and 90$^\circ$position angles) and with different placements on the spectrograph focal plane. This way the spectral overlap of stars near the clusters center can be disentangled. All spectral frames have been exposed for 60 s, except for a 300 s one.

The observations were reduced using IRAF 2.11 running on a laptop PC under Linux operating system. One dimensional spectral tracings were wavelength calibrated using the positions of the stars in the adjacent V-band images as a reference. Once the spectral lines have been identified, their centroids have been used for refinement of wavelength calibration. Spectral information could be extracted for 32 stars, most of them observed in multiple exposures. The wavelength calibration was generally good to 2 Å. The covered wavelength range for most stars was $\lambda\lambda 3700 - 6500$ Å. The flux calibration was obtained using standard stars observed during the same night. Inter comparison of tracings of the same star observed at different placements in the field and at different spectrograph orientations shows that the relative flux calibration (colours) are accurate to within 5% redwards of 4500 Å. The resolution and the sometime low S/N ratio of the spectra has prevented in most cases the determination of the luminosity class. Our spectral types, V magnitudes (from Table 2 when appropriate, or from the AFOSC V band frames in the other cases), B-V colors (from Table 2 when appropriate, and in the other cases from convolution with the appropriate photometric band profile on the flux calibrated spectra) and other derived data are listed in Table 3, where comparison with the scarce spectral classification from literature is also provided (Stephenson & Sanduleak 1971; Fitzgerald et al. 1979).