2 Observation preparation

2.1 Be star catalogs for NGC 330

For the present study we selected Be stars mainly from two catalogs: Grebel (1995) and Keller et al. (1999). The distance modulus of the SMC is $m-M_{V} = 18\hbox{$.\!\!^{\rm m}$ }9$ (Grebel 1995) and Keller et al. (2000) used $m-M_{V} = 18\hbox{$.\!\!^{\rm m}$ }85$. The 116 Be stars detected by Grebel (1995) range from $V=14\hbox{$.\!\!^{\rm m}$ }4$ to $V=18\hbox{$.\!\!^{\rm m}$ }9$ which corresponds to spectral types B0 to B6 using the calibration of Zorec & Briot (1991). Be stars detected by Keller at al. (1999) range from $V=14\hbox{$.\!\!^{\rm m}$ }4$ to $V=17\hbox{$.\!\!^{\rm m}$ }5$ corresponding to spectral types B0 to B3. This means that all targets of Keller et al. (1999) and those targets of Grebel (1995) with $V^{{\rm m}}<$ 18 $.\!\!^{\rm m}$25 (G1-G103) belong to the spectral classes B0-B4 under consideration.

2.2 Astrometry

Our preparation image used for the observations with FORS2 was obtained a year earlier with FORS1 (Hummel et al. 1999) with a field of view of 6.8 $\times$ 6.8 arcmin. Four targets of the astrometric catalog of Tucholke et al. (1996) were in the field of the preparation image and were used to calibrate the astrometric scale. The catalogues of Grebel (1995) and of Keller et al. (1999) were cross-correlated using the finding charts and a new astrometric target list was created. The image distortion of FORS1 was taken into account when deriving astrometric positions for the Be stars in the field of NGC 330. Our astrometry shows a mean offset of $\Delta \alpha = 0\hbox{$.\!\!^{\prime\prime}$ }8$ and $\Delta \delta = 0\hbox{$.\!\!^{\prime\prime}$ }9$ with respect to the catalog of Keller et al. (1999).

2.3 Selection effects, and completeness

Only one setup was prepared, meaning a subset of targets was selected from the catalogs on the basis of an optimum geometrical solution for the slit positions in the field of view. Since we limited the fully available spectral range of the 600R+24 grism by the H$\alpha$ interference filter H_Alpha+59 which provides a bandwidth of $\Delta \lambda=60$ Å, we could place several slits side by side without overlapping the spectra on the CCD.

The completeness of this study can be estimated to be around 40% for the early-type Be stars B0-B4 in NGC 330.

2.4 Mask production

Slit positions for the mask were defined using FIMS (Hummel 2000b) and the mask was produced together with technical masks for the commissioning some weeks before the observations with the VIRMOS Mask Manufacturing Unit (Conti et al. 1999). We used a common slit width of 0.26 mm corresponding to 0.5 arcsec.