3. The observations
As it has been done in the other STEPHI campaigns, we observe from three observatories aproximately equally-spaced in longitude: San Pedro Mártir Observatory (SPM) in Baja California, Mexico; Xing Long Station (XL), Beijing, China; and Teide Observatory (OT) in Tenerife, Spain. The longitude difference between these three observatories (6.6 h between OT and SPM; 8.5 h between SPM and XL and 8.9 h between XL and OT) gives us the possibility to have at winter time, a set of almost continuous data during photometric periods. In fact, overlappings between different sites are usually encountered. This is one of the important considerations of this collaborative observational effort.
We have followed the same observational technique as it has been described earlier (Michel et al. 1995), using similar four-channel photometers at every site to simultaneously observe the program stars, the chosen comparison star and the sky close to them. An interferometric blue filter () has been used in order to avoid color effects. At the end, we have a continuous data set that we average to a resolution of one point per minute.
Relative calibration between efficiency of the four detectors is done by illuminating them with bright sky, as a uniform light source, at the beginning and at the end of the daily observations during several minutes. Mean values of these relative calibration factors are applied each night to each one of the star light curves, in order to put all of them at the same level of efficiency. This calibration has shown to be specially important in full-moon periods, where sky subtraction to the star channels without this calibration, could leave residuals of the luminosity changes due to the moon in the light curve.
In Table 2, the log of the observations is displayed. The best three-site coverage was from January 29th. to February 9th. (12 nights), where we obtained 153.3 hours of observations. There were 7 nights with some overlapping between Tenerife (OT) and San Pedro Mártir (SPM) observations, (these dates are marked with b in Table 2). For this period, the efficiency of the observations was 51.7% of the cycle. For the full period of 29 nights of observations from the three observatories, we have a 31.1% efficiency for the cycle and there was one more night with an overlapping between XL and OT (marked with in Table 2). A global run of 210.2 hours was finally obtained.
Table 2. Log of observations. Observing time is expressed in minutes for each site.
© European Southern Observatory (ESO) 1998
Online publication: November 3, 1998