2 Observations and reductions

CCD BVI images for the fields of Melotte 105, Hogg 15 and Pismis 21 were taken during an observing run in June/July 1995 with the University of Toronto Southern Observatory 0.6 m telescope (Las Campanas Observatory, Chile) and with a PM 512$\times$512 METACHROME chip 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{$^\prime$ }$. Ruprecht 140 was observed during a previous observing run in September 1994 with the same telescope and instrument. The air-masses of the observed fields were always less than 1.35, just lying within the air-mass range of the standard stars, and the seeing was typically 1.6 $\hbox{$^{\prime\prime}$ }$. The observations were supplemented with nightly exposures of 10 bias, 10 dome- and 10 twilight sky-flats to calibrate the CCD instrumental signature. We also observed nightly an average of 12 standard stars of the Selected Areas # 104, 105, 107, 108, 110, 111, 112 and 113 (Landolt 1992) covering a wide range in colour ( $-0.1 \le V-I \le 2.0$) to transform the instrumental magnitudes into the standard system. Table 1 lists the exposure sequences and seeing for each object and filter, while Fig. 1 shows the schematic finding chart built with all the stars measured in the V band.

The IRAF/DAOPHOT package was used to reduce the observations in the standard way at the Astronomical Observatory of the National University of Córdoba, Argentina. The b, v and i instrumental magnitudes (corrected to 1 s integration using a zero point of 25.0 mag) for a total of 1311 stars were transformed into the standard system using the following equations:

$$b_{j,n} = b_1 + V + (B-V) + b_2\times (B-V) + b_3\times X_{j,n},$$ (1)

$$v_{k,n} = v_1 + V + v_2\times (B-V) + v_3\times X_{k,n},$$ (2)

$$i_{l,n} = i_1 + V - (V-I) + i_2\times(V-I) + i_3\times X_{l,n},$$ (3)

where V, (B-V) and (V-I) are the standard magnitude and colours and X the corresponding air-mass for the j, k, lth measured standard star. We solved the equations for all coefficients simultaneously for each night n with the PHOTCAL package in IRAF. The nightly rms errors from the transformation into the standard system ranged from 0.004 to 0.014 mag in B, 0.005 to 0.028 mag in V and 0.009 to 0.029 mag in I, with means of 0.007, 0.012 and 0.015 mag, respectively, 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 cluster of the sample (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 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 5 provide this information and are available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr.

Figure 2: Magnitude and colour photometric errors provided by DAOPHOT as a function of V for the richest cluster of the sample (Melotte 105). They are typical in our sample

 

 

Table 1: Journal of observations

Object	Date	Filter	Exposures	seeing
	(UT)			( $\hbox{$^{\prime\prime}$ }$)
Melotte 105	1995 June 29	B	$1\times 60$ s, $2\times300$ s	2.0
		V	$1\times 60$ s, $1\times120$ s, $1\times150$ s	2.0
		I	$2\times30$ s, $1\times40$ s	2.0
Hogg 15	1995 June 27	B	$1\times 60$ s, $2\times900$ s	1.9
		V	$1\times 60$ s, $1\times600$ s, $1\times900$ s	1.7
		I	$1\times 60$ s, $2\times300$ s	1.6
Pismis 21	1995 July 2	B	$1\times 60$ s, $2\times300$ s	1.3
		V	$1\times20$ s, $1\times120$ s, $1\times180$ s	1.5
		I	$1\times2$ s, $2\times10$ s	1.9
Ruprecht 140	1994 September 4	B	$1\times 60$ s, $2\times300$ s	1.3
		V	$1\times 60$ s, $2\times100$ s	1.3
		I	$1\times10$ s, $2\times15$ s	1.3

A comparison of our CCD photometry with previous photoelectric measurements carried out in Melotte 105 and Hogg 15 shows good agreement. For three stars observed in common with Sher (1965) in Melotte 105, the mean differences ($\Delta$ = Sher-CCD) are: $\Delta V =-0.03\pm0.03$ and $\Delta (B-V) = -0.01 \pm 0.05$, while for 16 stars measured in common with Moffat (1974) in Hogg 15, these differences are: $\Delta V = +0.06 \pm 0.12$ and $\Delta (B-V) =+0.00 \pm 0.07$. These values show that there are no significant offsets in our CCD zero-point magnitude and colour scales. We have also compared our CCD photometry with that of Kjeldsen & Frandsen (1991) for 125 stars in common, the mean raw differences being: $\Delta V = 0.02 \pm 0.05$ and $\Delta (B-V)$ = 0.01 $\pm$ 0.07. These authors noted an observational offset of 0.07 between their B-V values and those of Sher (1965). However, there is not only a zero-point offset in their photometry with respect to that of Sher (and therefore with respect to our CCD photometry), but also a peculiar behaviour of the mean differences and dispersions with V and B-V, as shown in Fig. 3.

Figure 3: Magnitude and colour differences between Kjeldsen & Frandsen (1991) and our values for Melotte 105 as a function of V and B-V