Astron. Astrophys. 332, 939-957 (1998)

3. WET observations and data reductions

AM CVn was one of the two main targets in a WET run organized in March/April 1990. WET (the Whole Earth Telescope) (Nather et al. 1990) is an international organization of observers who participate in coordinated campaigns in order to eliminate periodic gaps in the observing coverage. Such gaps introduce aliases into the Fourier transforms (FT's), greatly complicating the analysis of multiperiodic pulsators. Because of the object's northern declination, only observatories in the northern hemisphere, listed in Table 1, participated in this run.

Table 1. Participating sites.

All telescopes were equipped with two-star photometers, except at the CFHT and Haute-Provence observatories where three star photometers were used. Integration times were 10 s for all the individual runs listed in Table 2, comprising a total of 143.2 hrs spanning 12 days. All our photometers use blue-sensitive bi-alkali phototubes without filters to maximize signal-to-noise ratio. The response of the system is in the wavelength range -6000 Å (Kalytis et al. 1997). The details of our data reduction procedure are presented in Nather et al. (1990), we summarise it here:

Table 2. Journal of observations for WET run in 1990 of AM CVn

Both three-star photometers continuously monitored the sky background. For these data, we subtracted the sky point by point after cross-calibration and smoothing. Observers with two-channel photometers interrupted the observations of the target and comparison star for about 1 minute sky observations at irregular intervals - approximately every hour. We interpolated linearly between sky observations and subtracted the result from the data. The effects of extinction and other slow transparency variations were accounted for by fitting a third-order polynomial to each sky-subtracted data set and then dividing by this fit for normalising the data. We then subtracted the number one to obtain a mean of zero for all the data sets, producing light curves which show modulations in intensity (mi) ³ as a fraction of the total intensity, allowing us to combine light curves from different telescopes without further processing. The reduced WET light curve is shown in Fig. 1, where each panel spans 24 hours.

Fig. 1. The normalized light curve for the entire WET run. Each panel shows one day of data. The tickmarks on the ordinate scale represents 10 mmi. Each panel represents the light curve for one day. Time is in hours from midnight of the BJD of each day of the run

© European Southern Observatory (ESO) 1998

Online publication: March 30, 1998
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