When scientific experiments require transmission of powerful laser or radio beams through the atmosphere, the Federal Aviation Administration (FAA) requires that precautions be taken to avoid inadvertent illumination of aircraft. At present, the FAA requires that laser operators use human spotters to protect against accidental illumination. Here, we describe a simple, inexpensive, and highly reliable electronic system for detecting aircraft entering the vicinity of a laser beam that makes use of the air traffic control (ATC) radio transponders required on most aircraft. The radio system uses two antennas, both aligned with the laser beam. One antenna has a broad beam and the other has a narrow beam. The ratio of the transponder power received in the narrow beam to that received in the broad beam gives a measure of the angular distance of the aircraft from the axis that is independent of the range or the transmitter power. This ratio is easily measured and can be used to shutter the laser when the aircraft is too close to the beam. Comparisons of prototype systems operating at both the Apache Point and W. M. Keck Observatory with an FAA database indicate successful identification of commercial airplanes passing near the telescope boresight.

Focal ratio degradation is the decrease in focal ratio (or equivalently the increase of numerical aperture) of a beam of light that has been transported through a multimode optical fibre. Multimode fibres are frequently used in astronomical instruments such as multi-object and integral field spectrographs due to the poor coupling efficiency inherent to single mode fibre use. Focal ratio degradation has potentially serious consequences for these instruments as it means that they require faster, more difficult to produce optics than they would otherwise and any focal ratio degradation above that allowed for in the design will result in loss of light. Focal ratio degradation is generally a small effect when the fibres are fed with light close close to the material numerical aperture of the fibre however in practice this is rarely done due to the technical difficulties and instead the fibres are used at slower focal ratios (lower numerical apertures) where focal ratio degradation is more significant. Consequently considerable effort has been expended in identifying and mitigating the various causes of focal ratio degradation in fibre fed instruments, however the issue of fibre core geometry is a relatively unexplored area.

Focal ratio degradation (FRD) in multimode optical fibres is an important constraint on the design of fibre fed astronomical instrumentation. We have investigated the effect on FRD of fibre core geometry, including square and hexagonal cross-sections.