2 Observations and reductions

We carried out CCD observations in the regions of Ruprecht 103, 124, 129, 146 and 166 through the B, V, and I passbands during four photometric nights in September 1994 and June-July 1995. The direct images were obtained with the 24-inch telescope of the University of Toronto Southern Observatory (Las Campanas Observatory, Chile), equipped with a Photometric $512\times 512$ Metachrome CCD coated to give improved blue response. The scale on the chip was 0.45 $\hbox{$^{\prime\prime}$ }$ per pixel, yielding an area covered by a frame of $4\hbox{$^\prime$ }\times 4\hbox{$^\prime$ }$. Columns 4 and 5 of Table 1 list the exposure sequences and the typical seeing for each object and filter, while Fig. 1 shows the schematic finding-charts built with all the measured stars in the V-band. Observations of 12-17 stars in the Selected Area standard fields (Landolt 1992) covering a wide range in colour ( $-0.12 \leq V-I \leq 2.05$) were also taken nightly. Furthermore, a series of 10 bias exposures and 10 flat-field frames of both the twilight sky and dome were taken for CCD calibration purposes. The airmass values for the observed fields were always smaller than 1.35, just lying within the airmass range of the standard stars.

   

Table 1: Journal of observation.

Object	Date	Filter	Exposures	seeing
	(UT)			( $\hbox{$^{\prime\prime}$ }$)
Ruprecht 103	1995 June 28	B	$1\times 60$ s, $2\times 300$ s	1.3
		V	$1\times 60$ s, $2\times 200$ s	1.3
		I	$1\times 30$ s, $2\times 60$ s	1.3
Ruprecht 124	1995 July 2	B	$2\times 900$ s	1.5
		V	$2\times 600$ s	1.5
		I	$1\times 60$ s, $2\times 300$ s	1.5
Ruprecht 129	1994 September 3	B	$1\times 80$ s, $1\times 400$ s, $1\times 500$ s	1.4
		V	$1\times 80$ s, $1\times 250$ s	1.4
		I	$1\times 5$ s, $2\times 30$ s	1.4
Ruprecht 146	1994 September 3	B	$1\times 80$ s, $1\times 600$ s	1.4
		V	$1\times 100$ s, $2\times 250$ s	1.4
		I	$1\times 20$ s, $2\times 50$ s	1.4
Ruprecht 166	1995 June 30	B	$1\times 60$ s, $2\times 600$ s	1.6
		V	$1\times 60$ s, $2\times 300$ s	1.6
		I	$1\times 30$ s, $2\times 150$ s	1.6

Figure 1: Schematic finding charts for the fields of Ruprecht 103 (top-left), Ruprecht 124 (top-right), Ruprecht 129 (middle-left), Ruprecht 146 (middle-right), and Ruprecht 166 (bottom). Two circular and elliptical extractions are also shown. North is up and East is to the left. The sizes of the filled circles are proportional to the brightness of the stars.

The observations were reduced at the Observatorio Astronómico de la Universidad Nacional de Córdoba (Argentina), using the IRAF routines. The b, v and i instrumental magnitudes were obtained following the reduction procedure described by Piatti et al. (1999) and transformed into the standard system using the following relations:

b_j,n = b₁ + V + (B-V) + b₂ (B-V) + b₃ X_j,n, (1)

v_k,n = v₁ + V + v₂ (B-V) + v₃ X_k,n, (2)

i_l,n = i₁ + V -(V-I) + i₂ (V-I) + i₃ X_l,n, (3)

where V, (B-V) and (V-I) are the standard magnitude and colours and X the corresponding airmass for the j, k, lth measured standard star.

 

Table 2: Magnitudes and colours of stars in the field of Ruprecht 103.

ID	$X_{\rm pixels}$	$Y_{\rm pixels}$	V	$\sigma(V)$	B-V	$\sigma(B-V)$	V-I	$\sigma(V-I)$	n
1	331.916	10.242	18.383	0.051	1.045	0.144	1.308	0.064	2
2	361.558	15.393	17.552	0.032	1.314	0.089	1.398	0.034	2
3	504.436	20.719	16.418	0.016	0.962	0.031	1.017	0.022	1
4	50.748	23.507	16.929	0.020	1.591	0.057	1.625	0.023	2
5	210.538	24.632	17.651	0.032	0.933	0.072	1.137	0.037	2
.	.	.	.	.	.	.	.	.	.
.	.	.	.	.	.	.	.	.	.
.	.	.	.	.	.	.	.	.	.

NOTE: Tables 2 to 6 are available in their entirety in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/379/453. A portion of Table 2 is shown here for guidance regarding its form and content.

Equations (1) to (3) were solved for all coefficients simultaneously for each night n with the PHOTCAL package in IRAF. The mean rms errors range between 0.005 mag and 0.020 mag in all the passbands, which indicate that the nights were all of good photometric quality. Figure 2 shows the trend of the photometric magnitude and colour errors with V provided by DAOPHOT for the richest observed field (see Sect. 3). For each selected field, we generated a master table containing a running star number, the X and Y coordinates, the V magnitude, the B-V and V-I colours, the observational errors provided by the IRAF INVERTFIT task $\sigma(V)$, $\sigma(B-V)$ and $\sigma(V-I)$, and the number of observations n. These tables were built by combining all the independent measurements using the stand-alone DAOMATCH and DAOMASTER programmes kindly provided by Peter Stetson. Tables 2 to 6 provide this information and are available in electronic form at the CDS. A portion of Table 2 is shown here for guidance regarding its form and content. A comparison of our CCD photometry with Moffat & Vogt (1973) for 3 stars in common in the field of Ruprecht 166 yields $\Delta(V_{\rm MV73} - V_{\rm CCD}) = 0.022 \pm 0.045$ and $\Delta((B-V)_{\rm MV73} - (B-V)_{\rm CCD})= 0.010 \pm 0.031$.

Figure 2: Magnitude and colour photometric errors provided by DAOPHOT as a function of V for the richest cluster of the sample (Ruprecht 166). They are typical in our sample.