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Astron. Astrophys. 346, 359-368 (1999)

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2. Observational data

2.1. Photometry

The photometric data were obtained during a number of runs since 1988, at the 3.6m Canada-France-Hawaii Telescope, the 3.5m telescope of Calar Alto (CAHA, Spain) and the 2.5m Isaac Newton Telescope (Spain). We have selected the images obtained under photometric conditions. The total number of filter bands considered is 10 when we add the two F555W ([FORMULA]) and F814W ([FORMULA]) WFPC2 images from our HST program (PI. B. Fort), although the two B bands have very similar central wavelengths (but different widths). Table 1 provides with the central wavelengths and widths of the different filters. In the case of the 2 pairs (R and r, I and [FORMULA]), the difference between the central wavelengths is of the order of [FORMULA], and the filter widths are different. We use all these data as complementary information in the following discussion, and we consider different independent sets of filters to constrain the properties of the stellar population (see Sect. 4). All magnitudes are given in the Vega system, and we use the SED of Vega and the detailed filter and detector responses to set the zero points when converting magnitudes into fluxes. As most of these data have been published elsewhere, we give only a summary of the main characteristics of the images in Table 1, including the detection levels and references. The [FORMULA] magnitude is defined here as the magnitude of an object with 4 connected pixels [FORMULA] above the sky level.


Table 1. Characteristics of the images and detection levels. [FORMULA] magnitudes correspond to objects with 4 connected pixels, each 2 [FORMULA] above the sky level. Observations were carried out at the CFHT (1), the INT (2), HST (3) and CAHA (4).
a) Pelló et al. 1991
b) Le Borgne et al. 1991
c) Kneib et al. 1999
d) Miralles et al. 1999, in preparation.

2.2. Spectroscopy

The spectroscopic data set used in this paper comes from three different runs: one at the CFHT and two at the 4.2m William Herschel Telescope (WHT). H3 was one of the targets of the spectroscopic survey performed by Bézecourt and Soucail (1997) at CFHT in August 1995. The spectrograph used was MOS/SIS (Le Fèvre et al., 1994), with the V150 grism, providing a low dispersion of 7.3 Å /pixel. H5a and H5b were observed during two separate runs at the WHT, in September and June 1996 respectively, with the LDSS-2 multiobject spectrograph (Allington-Smith et al. 1994). The grism used was the medium blue, with a dispersion of 5.3 Å /pixel. The total exposure times were 6.7 ksec ([FORMULA] sec + 1700 sec) on H5b and 9.9 ksec ([FORMULA] sec + 2700 sec) on H5a. The reduction was performed using standard IRAF procedures as well as our own software packages.

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© European Southern Observatory (ESO) 1999

Online publication: May 21, 1999