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Astron. Astrophys. 362, 959-967 (2000)

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2. Observations and reductions

CCD BVI images in the fields of Ruprecht 119, NGC 6318 and BH 245 were obtained during three photometric nights in September 1994 and June-July 1995, respectively, using the 24-inch telescope of the University of Toronto Southern Observatory (Las Campanas Observatory, Chile). In both runs we employed the same PM 512x512 METACHROME coated CCD to give improved blue response. As the scale on the chip is 0".45 per pixel, the sky covered by a single frame is about 4´x4´. The observations were supplemented with a series of 10 bias and 10 skyflats during the sunsets and 10 domeflats at the end of each night to calibrate the CCD instrumental signature. During the two observing runs the typical seeing was 1".5. The sequence of observations per filter for the three objects is listed in Table 1, while their schematic finding charts in the V-passband are shown in Fig. 1. Observations of 10-14 stars in the Selected Area standard fields (Landolt 1992) covering a wide range in colour (0.2 [FORMULA] [FORMULA] [FORMULA] 1.9) were also taken nightly. The airmass values for both program and standard fields were always smaller than 1.2.

[FIGURE] Fig. 1. Schematic finding charts for the fields of Ruprecht 119 (top-left), NGC 6318 (top-right), and BH 245 (bottom). A cross indicates the adopted cluster centre. Two concentric circles of 22".5 and 45".0 radii are also shown. North is up and East is to the left.


[TABLE]

Table 1. Journal of observation.


The observations were reduced at the Astronomical Observatory of the National University of Córdoba, Argentina, using the IRAF 1 routines. The extinction corrected instrumental magnitudes b, v and i were obtained following the reduction procedure described by Piatti et al. (1999) and transformed to the standard system from the following relations:

[EQUATION]

where V, [FORMULA] and [FORMULA] are the standard magnitude and colours. Eqs. (1) to (3) were simultaneously solved by least squares for each night n, the rms errors being [FORMULA] 0.01 mag in all the passbands. The resulting coefficients listed in Table 2 were then used to derive magnitude and colours of all the stars measured for different independent combinations of BVI images. We thus generated three independent tables per program object containing a running star number, the X and Y coordinates, the V magnitude, the [FORMULA] and [FORMULA] colours, and the observational errors provided by the IRAF.INVERFIT task [FORMULA], [FORMULA], and [FORMULA]. These three tables were finally combined averaging the values for stars which appear in more than a single table. This procedure allowed us to obtain the individual photometric internal errors and to detect any anomalous value as well. The final tables are available upon request to the first author. A comparison of our CCD photometry with MV73 for 7 stars in common in the field of Ruprecht 119 yields [FORMULA] and [FORMULA]. However, Kozok (1985) obtained three accurate measurements for star # 6 of MV73 and the difference between both photometries yields [FORMULA] and [FORMULA], which suggests that the zero points of MV73's photometry could be slightly shifted. For this reason, we decided not to apply any zero point correction to our photometry of Ruprecht 119.


[TABLE]

Table 2. Transformation coefficients.


We also obtained integrated spectra for the three objects under discussion with a REOSC spectrograph and a Tek 1024x1024 CCD (1 pixel=24 µm) attached to the 2.15 m telescope at the Complejo Astronómico El Leoncito (CASLEO, Argentina) during a run in May 1995. The observations were carried out scanning the slit across the objects in the north-south direction in order to properly sample the cluster stellar content and the background regions. The total field along the slit was 4´.7. A grating of 300 lines/mm was used in two different set-ups, namely "blue nights" and "red nights". During the blue nights, we obtained spectra ranging from 3500 to 7000 Å with an average dispersion in the observed region of 140 Å/mm (3.46 Å/pixel). The slit width was 4".2 resulting in an average resolution of 14 Å, according to the FWHM of the He-Ar comparison lamps. During the red nights, we obtained spectra from 5800 to 9200 Å with a similar dispersion (3.36 Å/pixel) and a resolution of 17 Å (same slit width). To eliminate second order contamination an OG 550 filter was employed. Exposures of 15 minutes were taken until reach a total of 45 minutes for Ruprecht 119 and one hour for the remaining two objects in the blue and red ranges, respectively; the resulting S/N ratios ranging between 15 and 50. In addition, spectra of standard stars to derive flux calibrations were also obtained. In the blue range we used the standard stars LTT 4364, EG 274 and LTT 7379 (Stone & Baldwin 1983), whereas in the red range we added to the standard list the hot dwarf star HD 160233 (Gutiérrez-Moreno et al. 1988) with the aim of applying corrections for telluric emission bands. The integrated spectra were reduced at the Astronomical Observatory of Córdoba following the precepts outlined by Piatti et al. (1998a). For the reduction we used He-Ar comparison lamp spectra obtained between and after the object observations as well as bias, and dome, twilight sky and tungsten lamp flat-fields obtained nightly.

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© European Southern Observatory (ESO) 2000

Online publication: October 30, 2000
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