2 Data

2.1 Observations and reduction

Figure 1: Typical spectral windows for SAG (Upper panel) and DUO (lower panel).

The data discussed in this paper are based on two sets of photographic plates taken with the ESO 1 m Schmidt telescope at La Silla (Chile) between 1994 and 1996. The first set of observations consists of a series of 82 $B_{\rm J}$ photographic plates centred on (l,b)=(3.1$^{\circ}$, -7.1$^{\circ}$) and taken between June 11th 1994 and October 7th 1994. The limiting magnitude of these plates reaches $B_{\rm J}\sim20.5$. A first analysis of these data allowed the detection of microlensing events toward the Galactic bulge as part of the DUO project (Alard & Guibert 1997). The second set of data results from a series of 68 Kodak Tech-Pan 4415 films combined with a BG12 filter centred on (l,b)=(6.6$^{\circ}$, -10.8$^{\circ}$). These observations spanned 83 days between May 17th and August 9th 1996. The exposure times resulted in a limiting magnitude of $V\sim 20$. Together with the first set of plates, this series allowed us to reveal the shape of the Sagittarius dwarf galaxy near the Galactic Plane (Alard 1996; Cseresnjes et al. 2000, hereafter Paper I). For convenience we will refer to the first field as the DUO field whereas the second field will be called SAG field. Typical spectral windows for SAG and DUO are shown in Fig. 1.

The plates were scanned at CAI/Paris Observatory with the high-speed microdensitometer MAMA. The extraction of the sources were performed with the software Extractor written by Alard (Alard & Guibert 1997). The final database contains the light curve for $\sim$ $20\times 10^{6}$ stars.

2.2 Photometry

The $B_{\rm J}$ band is well known to astronomers and it suffices here to recall the photometric relation linking this band to the standard Johnson system $B_{\rm J}=B-0.28(B-V)$ (Blair & Gilmore 1982). On the other hand, the emulsion used in the second field may not be familiar to the reader so we shall say some words about it. The Tech-Pan 4415 emulsion is a fine grained, high resolution film with a sensitivity extending to 0.69 $\mu$m. Together with a BG12 filter it results in a band peaking at $\sim$0.39$~\mu$m with a width of $\sim$0.15 $\mu$m at half maximum transmission (see Fig. 1 in Paper I). We refer the interested reader to Phillipps & Parker (1993), Parker & Malin (1999) and the Kodak Web
site for a more thorough description of the emulsion. The band resulting from the combination 4415+BG12 has been calibrated with a sequence of 1638 stars located within our field and which were published by Sarajedini & Layden (1995). This calibration resulted in the relation B₄₄₁₅=V+1.47(V-I).