Joseph Nunn designed an altitude-altitude-azimut mount to carry the astrographic film camera designed by Baker. The combination of both became well known as the Baker-Nunn Camera system (BNC). In 1955, the USA announced its intention to launch a series of satellites as contribution to the International Geophysical Year 1957. By determining the precise position of a satellite from several observatoria, it was possible to do precise geodetic measurements of the Earth. These measurements could give humanity information like the differences of gravity and the distribution of mass in the Earth, the figure of the Earth and precise distances of continents. For a network of observatoria, the Smithsonian Astrophysical Observatory ordered 12 BNC’s.

The primary task of the Baker-Nunn Cameras was to track the satellites and to take pictures of them. The USSR surprised the world by being the first country to bring the Sputnik as a satellite into orbit. The BNC proved to be succeful to take pictures of the Sputnik. This also stimulated to develop a BNC version for military usage like detection of rockets and reentry vehicles. In total five BNC’s were built for the US Armed Forces. Some were very much similar in design, although there was a version of the mount with an extra feature. It could be operated in alt-alt-az mode and by tilting the azimuthal base perpendicular to the polar axis it could be adapted to operate in equatorial mode. There was also a version with a radical different optical design, equiped with a slitless spectrograph.

The typical BNC-mount is a rather simple design with a large triangular azimuthal base with a vertical axis that carries two gimbals with the two altitude axes. All axes all are perpendicular to one other and can be driven independently. These mounts were designed to find satellites, in a time in which their trajectories were not well known. Tracking celestial objects like stars was not a design requirement. Because the trajectories where only known with a large uncertainty it was needed to design a very sensitive camera with a large field of view. The camera consisted of a 20 inch aperture, three element corrector lens in front of a 30 inch f/0.75 spherical mirror. The focal plane is approximately half way between the corrector and mirror. The camera had a field of view of 5° by 30°. Cinemascope film was used, it had dimensions of 55mm wide and 254mm long, the longer dimension aligned with the expected movement of the taget.

Boller & Chivens built approximately 20 Baker-Nunn camera’s, in a few different variaties for the different tasks. Tracking satellites, rockets and reentry vehicles demands much faster angular velocities (in the order of a few degrees per second) compared to tracking stars (approx. 15 seconds of arc). The fast optics, short integration times and the large field of view allowed for a relative low grade of precision for the controlled rotation of the axes. Tracking objects that culminated above 60° was relatively easy, objects that didn’t reach an altitude above 30° were hard to track with three-axis mounts. Soon the demand for accurate tracking grew. Photo-electric devices did provide instantly results with an increase of sensitivity. The drawback is a much smaller field of view, a smaller part of the sky could be observed. High cadence photometry was demanded to record light curves of rotating satellites.

To improve tracking precision, Kissel filed in 1967 his proposal of a four-axis satellite-tracking mount. The fouth axis enabled an improved way to track a satellite once its orbit was known. The three-axis of the orignal BNC were oriented and fixed in such a way that the satellite could be tracked by rotating only the fourth axis.

Space surveillance grew as a special branch in the world of celestial observation, now often refered to as Space Surveillance and Awareness. (SSA). As far as I know, the orignal Baker-Nunn camera’s are decomissioned since the nineties. Successors of the original task of the BNC’s are programs like the Ground-Based Electro-Optical Deep Space Surveillance (GEODSS), Space Surveillance Telescope (SST), ESA Space Debris Telescope, ROSACE, Passive Imaging Metric Sensor (PIMS) and the Zimmerwald Telescope. These succesors are all mounted on two axes telescope mounts. Baker-Nunn Camera’s were donated to public organizations. Of some it is known that they were refurbished to do further scientific work. Known at the time of this writing are the Telescope Fabra ROA Montsec (TFRM) consortium, the Rothney Astrophysical Observatory (NESS-T BNC program), University of New South Wales and the Aryabhatta Research Institute of Observational Sciences (2009) in India. Most of the BNC-mounts were retrofitted to a version of an equatorial mount, losing some of its flexibility in change for simplified operation.