An improved suspension system for the observatory variometer

The observatory variometers have been used for long-term observation of the Earth’s magnetic field for years and more. Because of this, it is necessary to exclude the influence of every external factors at the sensors of these magnetometers. One of the most often ones are tilts of their sensors because of the seasonal tilts of the pears at which they are installed. These variometers are usually equipped with sensors installed on brass or titanium platforms, pendulously suspended for tilt compensation. Here we present a new fiber-suspended sensor that better fits the observatory demands.

Usually, the tilt compensation ratio is defined as a relationship between the pendulum and base rotations in the same vertical plane (around the same horizontal axis). We found that at certain conditions a pendulum may rotate around two other axes perpendicular to the base rotation line.  The first effect appears as a pendulum rotation around the horizontal line belonging to the vertical plane in which the sensor base is tilted.  This effect can be seen not only at the inclination of the base but also at the rotation of the sensor around its vertical axis. We used such a rotation in a horizontally directed magnetic field H to match the alignment of the mechanical axes of suspension and the magnetic axes of the sensor.

The second detected effect was manifested as the pendulum rotation around the vertical axis during the inclination of the platform in the vertical plane where the upper pair of the fibers lies. The cause of the second effect is the imbalance of the lower part of the pendulum - either due to the uneven distribution of masses or due to different lengths of the lower pair cords.

To keep both effects at the lowest possible or negligible level, a new version of the fiber-suspended sensor is designed. This sensor has three supporting feet and a worm drive for fine adjustment of the magnetic components’ orientation and the magnetic axes leveling possibility included in the firmware. The following parameters were obtained: tilt range ±4°, tilt compensation ratio (including off-axis effects) >2000, and thermal factor <0.2 nT/°C.