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A new class of apodizing functions suitable for Fourier spectrometry (and similar applications) is introduced. From this class, three specific functions are discussed in detail, and the resulting instrumental line shapes are compared to numerous others proposed for the same purpose.

Some considerations on the design and construction philosophy of balloon-borne, high-resolution, Fourier interference spectrometers are presented in the context of a far ir Michelson interferometer and a near ir double-passed cat’s-eye interferometer. The far ir instrument has already flown, with some success. The other is currently under development for a future flight.

A retroreflector (a device that reflects an incident beam of light through exactly 180°) constructed from a primary concave mirror and a small secondary mirror near its paraxial focus, the so-called cat’s-eye retroreflector, has been investigated. Ray-tracing of systems with both spherical and parabolic primaries suggests that the latter are considerably superior to the former and comparable to a cube-corner retroreflector. The investigation has also enabled mechanical tolerances for the construction of a cat’s-eye retroreflector to be deduced. The predicted tolerances may be much easier to attain than those for a cube-corner retroreflector.

SOME previous absolute measurements of the solar brightness temperature in the infra-red1,2have suggested that the Sun is deficient in infra-red radiation. The result reported here lends support to this idea and has allowed a first approximation to a power law for the decrement to be computed.