Because of its excellent resolving power, the Hubble Space Telescope offers an exceptional opportunity to study the crowded centers of Galactic globular clusters (GGC). We therefore began several years ago a study of color-magnitude diagrams (CMD) of clusters, using HST's WFPC2 camera.

Our first program, GO-6095, examined 10 clusters in the HST B(F439W) and V (F555W) bands (Sosin et al. 1997a; Piotto et al. 1997), with accompanying ultraviolet images for better study of the extended blue tails that occur on the horizontal branches (HB) of a number of clusters. That program gave some quite interesting results, like the first discovery of extended horizontal branches (EHB) in the metal-rich clusters NGC 6388 and NGC 6441 (Rich et al. 1997), and the discovery in NGC 2808 of an EHB extending down the helium-burning main sequence, with a multimodal distribution of stars along it (Sosin et al. 1997b). GO-6095 also persuaded us that it would be more profitable to explore a larger number of clusters more rapidly in B and V alone, in order to delineate the upper parts of their CMDs, and especially their HBs, with the intention of returning to the more interesting clusters for more intensive follow-up studies. We therefore changed our program to a snapshot study (GO-7470) aimed at producing CMDs down to a little below the main-sequence (MS) turnoff for all Galactic globular clusters with apparent B distance modulus $\leq$ 18.0 and whose centers had not yet been observed by HST in a comparable way - 53 clusters in all. Since snapshot programs have no guarantee of being completed in a given year, we have resubmitted each year the list of clusters not yet observed (GO-8118, GO-8723). By the time we are writing this paper, only one of the GGCs in our original list remains to be observed: NGC 6779. There were also in the HST archive images of a number of clusters that could be treated similarly. We have measured these in the same way as our own images, and present here CMDs of a total of 74 GGCs, measured and reduced in a uniform way, all observed with WFPC2 with the same filter set, and with the PC centered on the cluster center. For all of the 74 GGCs, we ran artificial star experiments to measure the completeness of the star counts in all the relevant branches of the CMD. In this paper we describe the observations, the reduction procedures, the artificial star tests, and the steps followed to transform the instrumental magnitudes into magnitudes in both the HST flight and standard Johnson photometric systems.

Table 1: Observation log. Asterisks indicate clusters reduced by using Stetsons PSFs.
Cluster Program ID Exp. Time Exp. Time

F555W [s] F439W [s]

NGC 104 GO6095 1, 7 7, $2\times(50)$

NGC 362 GO6095 3, 18 18, $2\times(160)$

NGC 1261^* GO7470 10, 40 40, $2\times(160)$

NGC 1851 GO6095 6, 40 40, $2\times(160)$

NGC 1904 GO6095 7, 40 40, $2\times(160)$

NGC 2419^* archive 10, $2\times(100)$ $2\times(260)$

NGC 2808 GO6095 7, 50 50, $2\times(230)$

NGC 3201^* GO8118 3, $2\times(30)$ 40, $2\times(100)$

NGC 4147^* GO7470 10, 40 40, $2\times(160)$

NGC 4372 GO8118 3, $2\times(30)$ 40, $2\times(100)$

NGC 4590^* GO7470 5, 40 40, $2\times(100)$

NGC 4833 GO8118 3, $2\times(30)$ 40, $2\times(100)$

NGC 5024 GO8118 5, 40 40, $2\times(160)$

NGC 5634 GO7470 10, 40 40, $2\times(160)$

NGC 5694 archive 10, $4\times(60)$ 120, $3\times(500)$

IC 4499^* GO8723 10, 40 40, $2\times(160)$

NGC 5824 archive $2\times(10)$ , $5\times(60)$ 120, $3\times(500)$

NGC 5904 GO8118 3, $2\times(30)$ 40, $2\times(100)$

NGC 5927 GO6095 10, 50 50, $2\times(160)$

NGC 5946^* GO7470 10, 40 40, $2\times(160)$

NGC 5986^* GO7470 10, 40 ec 40, $2\times(160)$

NGC 6093^* archive $2\times(2)$ , $4\times(23)$ $2\times(30)$

NGC 6139 GO7470 10, 40 40, $2\times(160)$

NGC 6171^* GO7470 5, 40 40, $2\times(100)$

NGC 6205^* archive $4\times(1)$ , $4\times(8)$ $2\times(14)$

NGC 6229 GO8118 10, 40 40, $2\times(160)$

NGC 6218 GO8118 3, $2\times(30)$ 40, $2\times(100)$

NGC 6235 GO7470 10, 40 40, $2\times(160)$

NGC 6256^* GO7470 10, 40 40, $2\times(160)$

NGC 6266 GO8118 3, $2\times(30)$ 40, $2\times(100)$

NGC 6273 GO7470 10, 40 40, $2\times(160)$

NGC 6284^* archive 4, $4\times(40)$ 30, $6\times(160)$

NGC 6287^* archive $2\times(5)$ , $3\times(50)$ 60, 230

$3\times(1000)$

NGC 6293 archive 2, $5\times(40)$ 20, $6\times(160)$

NGC 6304^* GO7470 10, 40 40, $2\times(160)$

NGC 6316^* GO7470 10, 40 40, $2\times(160)$

NGC 6325^* GO7470 10, 40 40, $2\times(160)$

NGC 6342 GO7470 10, 40 40, $2\times(160)$

NGC 6356 GO7470 10, 40 40, $2\times(160)$

NGC 6355^* GO7470 10, 40 40, $2\times(160)$

IC 1257^* GO8723 30, 100 160, $2\times(300)$

NGC 6362^* GO7470 5, 40 40, $2\times(100)$

NGC 6380 GO7470 10, 40 40, $2\times(160)$

NGC 6388 GO6095 12, 50 50, $2\times(160)$

NGC 6402 GO8118 7, 40 40, $2\times(160)$

NGC 6401 GO7470 10, 40 40, $2\times(160)$

NGC 6397^* archive 1, 8, $6\times(40)$ $2\times(10)$ , $2\times(80)$

$16\times(400)$ , $4\times(500)$

NGC 6440^* GO8723 10, 40 40, $2\times(160)$

NGC 6441 GO6095 14, 50 50, $2\times(160)$

NGC 6453 GO8118 10, 40 40, $2\times(160)$

NGC 6517^* GO8723 10, 40 40, $2\times(160)$

NGC 6522 GO6095 10, 50 50, $2\times(160)$

NGC 6539 GO8118 10, 40 40, $2\times(160)$

NGC 6540 GO8118 5, 40 40, $2\times(160)$

NGC 6544 GO8118 3, $2\times(30)$ 40, $2\times(100)$

NGC 6569 GO8118 5, 40 40, $2\times(160)$

NGC 6584 GO8118 5, $2\times(30$ ) 40, $2\times(100)$

**Table 1:** Observation log. Asterisks indicate clusters reduced by using Stetsons PSFs.
Cluster	Program ID	Exp. Time	Exp. Time
		F555W [s]	F439W [s]
NGC 104	GO6095	1, 7	7, $2\times(50)$
NGC 362	GO6095	3, 18	18, $2\times(160)$
NGC 1261^*	GO7470	10, 40	40, $2\times(160)$
NGC 1851	GO6095	6, 40	40, $2\times(160)$
NGC 1904	GO6095	7, 40	40, $2\times(160)$
NGC 2419^*	archive	10, $2\times(100)$	$2\times(260)$
NGC 2808	GO6095	7, 50	50, $2\times(230)$
NGC 3201^*	GO8118	3, $2\times(30)$	40, $2\times(100)$
NGC 4147^*	GO7470	10, 40	40, $2\times(160)$
NGC 4372	GO8118	3, $2\times(30)$	40, $2\times(100)$
NGC 4590^*	GO7470	5, 40	40, $2\times(100)$
NGC 4833	GO8118	3, $2\times(30)$	40, $2\times(100)$
NGC 5024	GO8118	5, 40	40, $2\times(160)$
NGC 5634	GO7470	10, 40	40, $2\times(160)$
NGC 5694	archive	10, $4\times(60)$	120, $3\times(500)$
IC 4499^*	GO8723	10, 40	40, $2\times(160)$
NGC 5824	archive	$2\times(10)$ , $5\times(60)$	120, $3\times(500)$
NGC 5904	GO8118	3, $2\times(30)$	40, $2\times(100)$
NGC 5927	GO6095	10, 50	50, $2\times(160)$
NGC 5946^*	GO7470	10, 40	40, $2\times(160)$
NGC 5986^*	GO7470	10, 40 ec	40, $2\times(160)$
NGC 6093^*	archive	$2\times(2)$ , $4\times(23)$	$2\times(30)$
NGC 6139	GO7470	10, 40	40, $2\times(160)$
NGC 6171^*	GO7470	5, 40	40, $2\times(100)$
NGC 6205^*	archive	$4\times(1)$ , $4\times(8)$	$2\times(14)$
NGC 6229	GO8118	10, 40	40, $2\times(160)$
NGC 6218	GO8118	3, $2\times(30)$	40, $2\times(100)$
NGC 6235	GO7470	10, 40	40, $2\times(160)$
NGC 6256^*	GO7470	10, 40	40, $2\times(160)$
NGC 6266	GO8118	3, $2\times(30)$	40, $2\times(100)$
NGC 6273	GO7470	10, 40	40, $2\times(160)$
NGC 6284^*	archive	4, $4\times(40)$	30, $6\times(160)$
NGC 6287^*	archive	$2\times(5)$ , $3\times(50)$	60, 230
		$3\times(1000)$
NGC 6293	archive	2, $5\times(40)$	20, $6\times(160)$
NGC 6304^*	GO7470	10, 40	40, $2\times(160)$
NGC 6316^*	GO7470	10, 40	40, $2\times(160)$
NGC 6325^*	GO7470	10, 40	40, $2\times(160)$
NGC 6342	GO7470	10, 40	40, $2\times(160)$
NGC 6356	GO7470	10, 40	40, $2\times(160)$
NGC 6355^*	GO7470	10, 40	40, $2\times(160)$
IC 1257^*	GO8723	30, 100	160, $2\times(300)$
NGC 6362^*	GO7470	5, 40	40, $2\times(100)$
NGC 6380	GO7470	10, 40	40, $2\times(160)$
NGC 6388	GO6095	12, 50	50, $2\times(160)$
NGC 6402	GO8118	7, 40	40, $2\times(160)$
NGC 6401	GO7470	10, 40	40, $2\times(160)$
NGC 6397^*	archive	1, 8, $6\times(40)$	$2\times(10)$ , $2\times(80)$
			$16\times(400)$ , $4\times(500)$
NGC 6440^*	GO8723	10, 40	40, $2\times(160)$
NGC 6441	GO6095	14, 50	50, $2\times(160)$
NGC 6453	GO8118	10, 40	40, $2\times(160)$
NGC 6517^*	GO8723	10, 40	40, $2\times(160)$
NGC 6522	GO6095	10, 50	50, $2\times(160)$
NGC 6539	GO8118	10, 40	40, $2\times(160)$
NGC 6540	GO8118	5, 40	40, $2\times(160)$
NGC 6544	GO8118	3, $2\times(30)$	40, $2\times(100)$
NGC 6569	GO8118	5, 40	40, $2\times(160)$
NGC 6584	GO8118	5, $2\times(30$ )	40, $2\times(100)$

Table 1: continued.
Cluster Program ID Exp. Time Exp. Time

F555W [s] F439W [s]

NGC 6624 archive $4\times(10)$ $2\times(50)$

NGC 6638 GO8118 5, 40 40, $2\times(160)$

NGC 6637 GO8118 3, $2\times(30)$ 40, $2\times(100)$

NGC 6642 GO8118 3, 40 40, $2\times(160)$

NGC 6652^* GO6095 10, 50 50, $2\times(160)$

NGC 6681^* GO8723 3, $2\times(30)$ 40, $2\times(100)$

NGC 6712^* archive 60 160

NGC 6717 GO8118 3, $2\times(30)$ 40, $2\times(100)$

NGC 6723 GO8118 3, $2\times(30)$ 40, $2\times(100)$

NGC 6760^* GO8723 7, 40 40, $2\times(160)$

NGC 6838 GO8118 3, $2\times(30)$ 40, $2\times(100)$

NGC 6864^* GO8723 7, 40 40, $2\times(160)$

NGC 6934^* GO7470 10, 40 40, $2\times(160)$

NGC 6981 GO7470 10, 40 40, $2\times(160)$

NGC 7078 archive $4\times(8)$ $2\times(40)$

NGC 7089 GO8118 3, $2\times(30)$ 40, $2\times(100)$

NGC 7099 archive $4\times(4)$ $2\times(40)$

**Table 1:** continued.
Cluster	Program ID	Exp. Time	Exp. Time
		F555W [s]	F439W [s]
NGC 6624	archive	$4\times(10)$	$2\times(50)$
NGC 6638	GO8118	5, 40	40, $2\times(160)$
NGC 6637	GO8118	3, $2\times(30)$	40, $2\times(100)$
NGC 6642	GO8118	3, 40	40, $2\times(160)$
NGC 6652^*	GO6095	10, 50	50, $2\times(160)$
NGC 6681^*	GO8723	3, $2\times(30)$	40, $2\times(100)$
NGC 6712^*	archive	60	160
NGC 6717	GO8118	3, $2\times(30)$	40, $2\times(100)$
NGC 6723	GO8118	3, $2\times(30)$	40, $2\times(100)$
NGC 6760^*	GO8723	7, 40	40, $2\times(160)$
NGC 6838	GO8118	3, $2\times(30)$	40, $2\times(100)$
NGC 6864^*	GO8723	7, 40	40, $2\times(160)$
NGC 6934^*	GO7470	10, 40	40, $2\times(160)$
NGC 6981	GO7470	10, 40	40, $2\times(160)$
NGC 7078	archive	$4\times(8)$	$2\times(40)$
NGC 7089	GO8118	3, $2\times(30)$	40, $2\times(100)$
NGC 7099	archive	$4\times(4)$	$2\times(40)$

The data set that we present has already been shown to be extremely valuable in attacking a number of still-open topics on evolved stars in GGCs. In particular, these data have been used by Piotto et al. (1999a) to investigate the problem of the EHBs and by Raimondo et al. (2002) to study the properties of the red HB in metal-rich clusters; in Zoccali et al. (1999) and Bono et al. (2001) we have throughly discussed the red giant branch (RGB) bump, and compared the predicted position and dimension of this feature with the observed ones; in Zoccali et al. (2000) we have used the star counts on the HB and on the RGB to gather information on the helium content and on the dependence of the helium content on the cluster metallicity; in Zoccali & Piotto (2000) we have made the most extensive comparison so far available between the model evolutionary times away from the main sequence and the actual star counts on the subgiant branch (SGB) and RGB; in Cassisi et al. (2001) we have compared the observed and theoretical properties of the asymptotic giant branches; in Piotto et al. (2000) we have used the CMDs in our database for the study of the GGC relative ages, and, finally, in Piotto et al. (1999b), we have started to investigate the GGC blue straggler (BS) population, and have shown how the BS CMD and luminosity function can differ in clusters with rather different morphologies. Future papers will carry out other detailed studies based on the same data. Among these, we are presently working on the derivation of the relative ages, following the strategy already delineated in Rosenberg et al. (1999) on a similarly photometrically homogeneous set of CMDs, but from ground-based data, and by Piotto et al. (2000) on a small subsample of the present HST database. We are also working on the large BS database that came from the CMDs presented in the following sections (Piotto et al. 2002, in preparation).

As better described in Sect. 3, all the CMDs, the star positions, the magnitudes in both the F439W and F555W flight system, and the B and Vstandard Johnson system will be made available to the astronomical community immediately after the publication of the present paper.

1 Introduction