2. Observations and data reduction

The OGLE project was conducted using the 1-m Swope telescope at Las Campanas Observatory. A single pixels Loral CCD chip, giving a scale of 0.435 arcsec/pixel was used as the detector. The initial processing of the raw frames was done automatically in near-real time. Details of the standard OGLE processing techniques were described by Udalski et al. (1992).

In 1993 we monitored fields 104A and 104B located west and east of the cluster center, respectively. In 1994 we monitored field 104C located north of the cluster center. In 1995 we monitored fields 104D and 104E covering southern part of the cluster. A condensed summary of the data used in this paper is given in Table 1 (click here). Detailed logs of the observations can be found in Udalski et al. (1993, 1995, 1997). The equatorial coordinates of centers of fields 104A-E are given in Table 2 (click here). A schematic chart with marked locations of all of the monitored fields is shown in Fig. 1 (click here). Most of the monitoring was performed through the Johnson V filter. Some exposures in the Kron-Cousins I band were also obtained. Most of observations in the V-band were collected with an exposure time ranging from 300 to 600 seconds (420 seconds was the most common value). The I-band exposures lasted 300 seconds. For the majority of the analyzed frames the seeing was better than 1.6 arcsec. The reduction techniques as well as the algorithms used for selecting potential variables are described in Paper I. Profile photometry was extracted with the help of DoPHOT (Schechter et al. 1993). The total number of stars contained in data bases with V band photometry ranged from 18397 to 33014. Table 3 (click here) gives condensed information about the numbers of stars analyzed for variability and about the quality of the derived photometry. The useful data were obtained for stars with 14.0<V<20.25.

  
Figure 1: A schematic chart showing location of fields 104A-E. The cluster center is marked with a cross. Each field covers armin². North is up and east is to the left

   

Field N_V Dates of observations

104A 286 Jun. 17 - Sep. 7, 1993

104B 270 Jun. 17 - Sep. 7, 1993

104C 288 Jun. 16 - Sep. 15, 1994

104D 125 Jun. 8 - Aug. 22, 1995

104E 120 Jun. 8 - Aug. 22, 1995

Table 1: Summary of observations collected for fields 104A-E. N _V is the number of V-band images included in the data bases

   

Field RA(1950) DEC(1950) h:m:s

104A 0:19:52.7 -72:22:45

104B 0:23:53.1 -72:21:01

104C 0:21:47.9 -72:10:35

104D 0:20:14.7 -72:31:14

104E 0:23:10.4 -72:31:22

Table 2: Equatorial coordinates for the centers of fields 104A-E

  

104A 104B 104C 104D 104E

V N N N N N

14.25 0.014 214 0.014 196 0.021 100 0.016 112 0.012 86

14.75 0.017 118 0.015 112 0.014 65 0.012 54 0.011 69

15.25 0.016 109 0.015 115 0.014 62 0.012 63 0.013 57

15.75 0.015 153 0.016 165 0.012 117 0.017 93 0.012 67

16.25 0.018 240 0.019 244 0.016 152 0.018 122 0.017 111

16.75 0.026 434 0.023 499 0.018 261 0.019 222 0.020 213

17.25 0.027 2057 0.027 2181 0.022 1273 0.023 1064 0.022 1017

17.75 0.032 3149 0.032 3248 0.025 1922 0.026 1734 0.026 1584

18.25 0.041 3973 0.039 4089 0.031 2486 0.034 2292 0.033 2179

18.75 0.052 4574 0.054 4884 0.042 3193 0.044 2697 0.044 2622

19.25 0.072 4515 0.071 5000 0.055 3746 0.058 3029 0.061 2984

19.75 0.102 3803 0.100 4248 0.076 3727 0.081 2537 0.086 2930

20.25 0.148 2986 0.144 3029 0.114 3456 0.119 2118 0.127 2579

Table 3: Basic statistical data for stars in fields 104A-E examined for variability. The data are given in bins 0.5 mag wide. Columns 2, 4, 6, 8 and 10 give median value of rms for a given bin. Columns 3, 5, 7, 9 and 11 give the numbers of stars examined for variability