In summer 2006, Plateau de Bure received its new hydrogen maser for very long baseline interferometry (VLBI). The EFOS-38 maser was constructed over a 15 month period in Neuchâtel, Switzerland.
In March 2006, a major change occurred in the Neuchatel maser production: all maser activities (including service for existing EFOS masers, staff, production and R&D) were transferred from the Observatoire de Neuchâtel to the newly founded T4Science corporation. EFOS-38 was completed at T4Science and underwent a rigorous testing program.
The specifications were quite demanding because this maser shall allow Plateau de Bure to do VLBI up to its highest foreseen frequency band at 350 GHz, which requires not only an excellent long-term stability but also a very clean signal on timescales shorter than one second. EFOS-38 met and surpassed the specifications.
On acceptance by IRAM, the maser was transported to the Plateau de Bure on August 23rd (Fig. 2) and connected to the LO system. Since then, the 5 MHz signal generated by the maser has been monitored against a GPS reference, and has proven to be linear within the precision of the fit over a period of about three months. This corresponds to a deviation of about one second in six million years.
In recent years, Plateau de Bure could not participate in two Global VLBI sessions due to maser failures. With the new EFOS-38, we are confident that the reliability and quality of VLBI campaigns at Bure will be much improved.
Active hydrogen masers are one of the most precise frequency references in operation today. Their basic reference is the hyperfine transition of atomic hydrogen (i.e. the famous 21 cm line), but the precision is further improved by the mechanical precision of the resonating cavity of the active maser. The decreasing signal-to-noise ratio of the maser signal on small time scales requires the presence of a high quality quartz resonator, which dominates the phase stability below timescales of 1-2 seconds. EFOS-38 is equipped with a BVA 8607 from Oscilloquartz, with an extra low phase noise.
The new Bure maser is a model of the EFOS-C series, which is a combination of a Russian physics package (by the VREMIA-CH J.S. company based in Nihzny Novgorod) and the electronics and monitoring package developed in Neuchâtel. On top of the standard no-break power supply of the Bure correlator room, the maser has a dedicated UPS which provides an additional 15 hours autonomy in case of power failure. It is installed in a temperature controlled, vibration-dampened rack to provide an optimum input signal to the Interferometer.
Local high-quality frequency standards are essential at observatories participating in Global millimeter VLBI. A common frequency reference cannot be shared world-wide with the required precision in real time, therefore each observatory has to provide individual frequency standards with an exactness and stability that will allow to detect the same photons across the world.