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Observing continously and for a long period of time

The continuity of the observations on long time intervals is absolutely necessary.

  • In sismology, this prevents the introduction of "parasitic" frequencies in the oscillation spectra due to breaks (called secondary lobes).
  • For the research of planetary transits, the absence of breaks in the observations minimizes the risk of missing the transits.

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.

  • The polar sites and the multi-site networks are the only alternative from the ground, but they only give discontinous and limited periods.
  • Observations from space are the only way to ensure a sufficient continuity during a long period of time.
figures/texteBM.png
  • Until now, these oscillations have been detected in dozens of stars.
  • When the length of the breaks is reduced, the spectrum becomes more legible, as shown by this animation.
  • The breaks "blur" this spectrum.
figures/frequences-parasites.gif

Parasitic frequencies

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.

figures/sites-polaire.jpg

Multi-site networks

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 GONG network observes the Sun continously from 6 sites spread out on Earth.

The STEPHInetwork, dedicated to the observation of deltascuti variable stars, has3 sites in Mexico, in the Canary Islands and in China.