Observing continously and for a long period of time
The continuity of the observations on long time intervals is absolutely necessary.
The rotation of the Earth, around itself and around the Sun, makes it so that a given sky area is visible only by night and during a few months.
When the period of observations is fragmented and interrupted, by the alternation of day and night for instance, the Fourier spectrum of a periodic signal does not contain the lines associated with the oscillations corresponding to the period of this signal.
The polar sites
The polar sites are difficult to get to, and only allow for the observation of stars with a high declination and during the polar night, i.e. approximately 4 months.
The French-Italian project CONCORDIA should provide for the setting up of a telescope at C dome in 2008.
Some multi-site networks of observatories, spread out in longitude around the Earth, allow, in principle, for the following of a star of intermediary declination during 24 hours. However, the continuity of the observations is compromised by the weather conditions, and the maximal length of a continuous multi-site observation does not exceed two months (in a given place, a star rises and sets two hours sooner each month).
The STEPHInetwork, dedicated to the observation of scuti variable stars, has3 sites in Mexico, in the Canary Islands and in China.