2. Observations and reductions
The observations were carried out during several observing runs in 1992-1995 with the Danish 1.54 m telescope at ESO, La Silla, equipped with a direct camera and CCD #28 (a Tek device). This combination of telescope and CCD yields a field size of about , although the effective field size was somewhat smaller, typically , because we combined each image from many individual frames that were not perfectly aligned. The image scale is / pixel. Four fields centered on eclipsing variable stars were observed, two in the SMC and two in the LMC (Table 1). The HV12578 field is located close to the region E of the ESO Key Programme on Coordinated Investigations of Selected Regions in the Magellanic Clouds (de Boer et al. 1989) in the northern part of the LMC, whereas the HV982 field is located close to the Key Programme region F, in the northern outskirt of the LMC Bar, close to the 30 Doradus complex. The HV1433 and HV11284 fields are located on each side of the Key Programme B region, in the southern part of the SMC.
The observations were originally obtained for a study of eclipsing binary B stars (Clausen et al., in prep.). Therefore, the data set consists of a large number of short individual exposures (typically 5 minutes in vby and 20 minutes in u). Initial reductions, essentially flatfielding and bias subtraction, were carried out using standard IRAF 1 tools. For each field and each filter, exposures with seeing better than and with sky background levels lower than a certain threshold were selected, and these exposures were then summed using the IRAF task imcombine. Hence, equivalent integration times of typically 3-5 hours in and v, and 5-10 hours in u were obtained (see Table 2). The combination of many short exposures offered the advantage that cosmic ray events could be effectively eliminated, using the crreject option in imcombine.
Table 2. Data for the combined frames. The first two columns are self-explanatory, the third column gives the number of frames used in the combined frames, the total integration time (in minutes) is listed in the fourth column, and the last column gives the resulting seeing measured on the combined frames.
The calibration to the standard uvby system was based on observations of secondary standard stars defined by photoelectric observations with the Strömgren Automatic Telescope (SAT) at ESO, La Silla (see Clausen et al. 1997). The SAT observations were carried out simultaneously with the 1.54 m observations, and also provided extinction coefficients for the calibration of the CCD photometry. The rms difference between the transformed CCD magnitudes and SAT magnitudes of the secondary standard stars was less than 0.01 in and around 0.015 in . Although the individual CCD exposures were obtained during several observing runs, the instrumental system was stable enough that we did not find any reason to use separate transformations for the different runs, except for zero-point differences. This indicates that the properties of the filters and the CCD are quite stable, while the zero-point differences may be due to changes in the reflectivity of telescope mirrors, gain settings for the CCD camera electronics etc.
For the CCD photometry we used DAOPHOT II (Stetson 1987) running within IRAF. The internal accuracy of the CCD photometry was typically around 0.01 mag in and 0.03 mag in and for stars brighter than , while we find that systematic errors due to the determination of zero-points for the PSF photometry relative to the aperture photometry of the standard stars are on the order of 0.01 mag in all filters.
It is a notorious problem to get the zero-points of the crowded-field PSF photometry relative to the standard aperture photometry right, in particular in a case like ours where there are no bright, isolated stars present in the fields. We approached the problem as follows: First, a set of individual exposures in the four Strömgren filters were selected for each field. Photometry was obtained on all stars in the selected frames following the usual DAOPHOT procedure, and then all stars except for a few bright ones were subtracted using the task substar in the DAOPHOT package. The few stars that were not subtracted were then measured using standard aperture photometry, thus providing a set of possible "tertiary standard stars" in each frame. The final tertiary standard stars were selected from careful curves of growth analyses; see Larsen (1996) for further details. The PSF photometry of the combined frames was then tied into the aperture photometry system using the tertiary standards in each frame.
Finally Be stars were identified from exposures and only between 5 and 39 of these stars were found in the various fields. It was also tested that foreground stars cause no serious problems; see Larsen (1996) for details.
© European Southern Observatory (ESO) 2000
Online publication: January 29, 2001