1 Introduction

There are two main properties which make the study of the Galactic globular clusters (GGC) particularly interesting: 1) each cluster (with possible rare exceptions) is made up by a single population of stars, all born at the same time, in the same place, and out of the same material; 2) GGC stars have the oldest measurable age in the Universe, and therefore we believe they are the oldest fossil records of the formation history of our Galaxy.

Among the many tools we have to investigate the properties of a stellar population, the color-magnitude diagrams (CMD) are the most powerful ones, as they allow to recover for each individual star its evolutionary phase, giving precious information on the age of the entire stellar system, its chemical content, and its distance. This information allows us to locate the system in the space, giving a base for the distance scale, study the formation histories of the Galaxy, and test our knowledge of stellar evolution models.

In particular, the study of a large sample of simple stellar systems, as the GGCs, provides important clues to the Milky Way formation history. Recently, many studies on the relative ages of the GGCs have been presented with results at least controversial: while some authors find a notable age spread ($\sim5$ Gyrs) among the clusters, others find that the bulk of GGCs is coeval. This controversy is surely mainly due to the heterogeneity of the data used in each study, where the combination of photographic and/or CCD data from the early epochs of solid state detectors has been frequently used. For this reason, a survey of both southern and northern GGCs has been started two years ago by means of 1-m class telescopes, i.e. the 91 cm European Southern Observatory (ESO)/Dutch telescope and the 1m Isaac Newton Group (ING)/Jacobus Kapteyn telescope (JKT). We were able to collect the data for 52 of the 69 known GGCs with $(m-M)_V\leq 16.15$. Thirty-nine have been observed with the Dutch telescope (data that are presented in this paper, hereafter Paper I), and the remaining ones with the JKT (the corresponding CMDs will be presented in a companion paper, Rosenberg et al. [2000], hereafter Paper II).

As a first exploitation of this new data base, we have conducted a GGC relative age investigation based on the best 34 CMDs of our catalog (Rosenberg et al. [1999], hereafter Paper III), showing that most of the GGCs have the same age. We have also used our data base to obtain a photometric metallicity ranking scale (Saviane et al. [2000], hereafter Paper IV), based on the red giant branch (RGB) morphology. We measured a complete set of metallicity indices, based on the morphology and position of the RGB. Using a grid of selected RGB fiducial points, we defined a function in the (V-I)₀, $M_{\rm I}$, [Fe/H] space which is able to reproduce the whole set of GGC RGBs in terms of a single parameter (the metallicity). The use of this function will improve the current determinations of metallicity and distances within the Local Group.

There are many other parameters that can be measured from a homogeneous, well calibrated CMD data base: the horizontal branch (HB) level, homogeneous reddening and distances, etc. We are presently working on these problems. However, we believe it is now the time to present to the community this data base to give to anyone interested the opportunity to take advantage of it.

In the next section, we will describe the observations collected at the ESO/Dutch telescope during two runs in 1997. The data reduction and calibration is presented in Sect. 3, while in Sect. 4 a cross check of the calibration between the two runs is given. In order to facilitate the reader's work, we have included the main parameters characterizing our clusters in Sect. 5. Finally, the observed fields for each cluster, and the obtained CMDs are presented and briefly discussed in Sect. 6.

Figure 1: Heliocentric distribution of all GGCs with $(m-M)_V\leq 16.15$ mag. In the upper panel, the GGCs projection over the Galactic plane is presented. The open circles represent the clusters studied in the present paper (Paper I); the open squares, the GGCs of Paper II; and the asterisk, the GGC Pal 1 (Rosenberg et al. [1998]). The clusters which are not included in our catalog are marked by open triangles. In the lower panel, the XZ projection is shown. The Milky Way is schematically represented

 

 

Table 1: Observed clusters at the DUTCH in 1997

ID	Cluster	Other	Obs.	Obs.	Seeing	Long.
	(NGC)	Name	date	fields	$V/I(\hbox{$^{\prime\prime}$ }$)	Exp.(s)
1	104	47 Tuc	23/Dec.	2	1.4/1.3	1800
2	288	-	24/Dec.	3	1.4/1.4	1800
3	362	-	26/Dec.	3	1.6/1.5	1200
4	1261	-	24/Dec.	3	1.3/1.3	1800
5	1851	-	23/Dec.	2	1.3/1.2	1800
6	1904	M 79	24/Dec.	3	1.3/1.2	1800
7	2298	-	23/Dec.	2	1.3/1.2	1800
8	2808	-	11/Apr.	2	1.3/1.2	1500
8	2808	-	26/Dec.	2	1.5/1.4	1500
9	-	E3	23/Dec.	2	1.5/1.4	1800
10	3201	-	12/Apr.	2	1.5/1.4	900
10	3201	-	24/Dec.	3	1.3/1.2	900
11	4372	-	13/Apr.	2	1.3/1.2	1500
12	4590	M 68	14/Apr.	2	1.2/1.2	1500
13	4833	-	15/Apr.	2	1.3/1.2	1500
14	5139	$\omega$ Cen	11/Apr.	2	1.2/1.2	900
15	5897	-	12/Apr.	1	1.4/1.4	1500
16	5927	-	13/Apr.	1	1.3/1.2	1500
17	5986	-	14/Apr.	1	1.3/1.3	1500
18	6093	M 80	12/Apr	2	1.3/1.2	1500
19	6101	-	15/Apr.	1	1.8/1.7	1500
20	6121	M 4	13/Apr.	2	1.3/1.2	900
21	6171	M 107	14/Apr.	2	1.4/1.3	1500
22	6266	M 62	14/Apr.	1	1.7/1.6	1500
23	6304	-	15/Apr.	1	1.5/1.3	1500
24	6352	-	11/Apr.	1	1.4/1.3	1500
25	6362	-	12/Apr.	1	1.4/1.3	1200
26	6397	-	13/Apr.	2	1.3/1.2	900
27	6496	-	14/Apr.	1	1.4 1.2	1200
28	6541	-	11/Apr.	2	1.3/1.2	1200
29	6544	-	15/Apr.	1	1.4/1.4	1500
30	6624	-	12/Apr.	1	1.3/1.2	1500
31	6626	M 28	13/Apr.	1	1.2/1.1	1500
32	6637	M 69	14/Apr.	1	1.2/1.1	1200
33	6638	-	13/Apr.	1	1.2/1.2	900
34	6656	M 22	15/Apr.	2	1.2/1.2	1500
35	6681	M 70	11/Apr.	1	1.3/1.2	1500
36	6717	Pal 9	12/Apr.	1	1.3/1.2	1500
37	6723	-	13/Apr.	1	1.2/1.1	1200
38	6752	-	14/Apr.	1	1.3/1.2	1200
39	6809	M 55	15/Apr.	1	1.3/1.1	900