Abstract: It is the objective of the present invention to provide a spectrometer of which the modulation transfer function (MTFs) in a spectral direction and in a direction perpendicular to the spectral direction is enhanced for all wavelengths ranging from the short to the long. For a spectrometer according to the present invention, a spherical face is employed for collimating and condensing light in a spectral direction, instead of the spherical collimator/condenser that is used for a conventional spectrometer, and troidal faces in a non-spherical shape, such as in a paraboloidal shape, are provided for collimating and condensing in a direction perpendicular to the spectral direction. The focusing distance on the non-spherical face is a distance so calculated that the modulation transfer function is improved for all the wavelengths ranging from the short to the long.

Abstract: An infrared interferometer, which includes a moving mirror, a fixed mirror, a beam splitter for delivering an interference light and a sensor for sensing the interference light, for observation of atmospheric spectra is equipped with a cloud observation arrangement for determining the cloud coverage ratio and the altitude of the top surface of a cloud, comprising a reflecting mirror for extracting a part of the interference light, a narrow-band-pass filter for A and B bands of oxygen molecules and a sensing element, the cloud observation arrangement enabling simultaneous observation with the atmospheric spectra observation to eliminate harmful influence of the cloud therefrom.