2 Observations and data reductions

The data presented here were obtained at the European Southern Observatory at La Silla (Chile) using a 1.5 m telescope equipped with a Boller and Chivens spectrograph and a CCD $2048\times2048$ with a readout noise of $\pm$7 electrons. A 225 gr/mm grating with a dispersion of 330 Å /mm in the first order was used. The CCD has a square 15 $\mu$m pixel, that gives a dispersion of about 5 Å/pixel in the wavelength direction. The spectral range is about $4900 <\lambda < 9200$ Å with a FWHM of 10 Å. The spectra were taken through a 5 arcsec slit oriented in the East-West direction, in order to eliminate possible loss of light due to atmospheric differential refraction. All of the observations were made as near as possible to the meridian.

The observations have been carried out between November 1996 and September 2001, and the observational circumstances are listed in Table 1 for the Mars-Crossers and in Table 2 for the Near-Earth objects. These tables show the date of observation, the number of exposures in each night, the heliocentric and geocentric distances, the solar phase angle and the visible magnitude for each asteroid, as well as the solar analog star used to obtain the final spectrum of each asteroid. Whenever more than one spectrum was available, the least noisy one was chosen to represent the referred asteroid, and the night when it was obtained is indicated in these tables by an asterisk. In Table 2 we give in addition the orbital classification of each Near-Earth asteroid as an Apollo, Amor or Aten object.

The spectral data reduction was performed using the Image Reduction and Analysis Facility (IRAF) package. Pixel-to-pixel variations were removed by subtracting the bias level from each frame and dividing it by a "flat field''. The IRAF apsum package was used to sum the pixel values within a specified aperture and to subtract the background level. Wavelength calibration was performed using a He-Ar lamp and the spectra were corrected for airmass through the mean extinction curve of La Silla (Tüg 1977). Solar-type stars (Hardorp 1978) have been observed in order to compute reflectivities and to estimate the quality of the night. All asteroid spectra are normalized around 5500 Å by convention.