A Balloon-borne Michelson Interferometer for Solar Spectroscopy in the 10-100 Micron Region

-A Fourier spectrometer of the Michelson interferometer variety has been constructed for the study of the far infrared solar spectrum through the stratosphere at resolutions down to 0.5 cm-1. Details of the design, construction and performance of this balloon-borne system, both optical and electronic, are given. The instrument was first flown during August 1965. Despite some difficulties, the interferometer system performed as expected and some useful data was obtained. The system will be flown again as soon as possible. RBsumB. Un spectromktre de Fourier, du type Michelson a kt6 construit pour l'Ctude du spectre solaire dans l'jnfrarouge lointain a travers la stratosphkre, avec des rCsolutions allant jusqu'h 0,5 cm-1. Nous donnons des details sur le principe, la construction et les performances de ce systkme au point de vue optique et Clectronique. Le premier vol a eu lieu en aoht 1965. MalgrC quelques difficult& l'interferometre a fonctionnk suivant les previsions et quelques rCsultats utiles ont Cte obtenus. Un nouveau vol aura lieu dks que possible. Introduction. The infrared region beyond about 13 microns is unavailable for ground-based astronomical observations because of the low transparency of the atmosphere. Even a source as strong as the Sun is invisible between 40 and 300 microns [I]. The major cause of the absorption is atmospheric water vapour, which is, however, largely confined to the troposphere. Near infrared spectrometers lifted into the stratosphere (that is, above about 20 kilometers) by means of balloons have shown that the resultant atmospheric transparency is considerably enhanced [2, 31. One therefore has grounds for hope that a similar improvement would be observed in the far infrared. If this were found to be true, then the significant payload capacity of modern balloons could be exploited for astronomical purposes. However, before one can embark on an ambitious program of astronomical observations in the far infrared, the stratospheric transmission and the manner in which it varies must be fully explored. Such an exploration was the purpose of this experiment. The standard approach to such a measurement (which was followed in this case) is to employ the Sun as an extra-atmospheric light source and to determine how the received spectrum varies as a function of the air-mass. Where the absorption is weak, the spectra may be extrapolated to zero atmosphere and hence one may determine not only the atmospheric transmission as a function of zenith distance but also the solar spectrum itself. However, this can only be done satisfactorily if the signal/noise is good and if the spectral resolution is small enough to minimize line blending. Instrumental approach. To make such a measurement of the stratospheric transmission from solar spectroscopy in the far infrared we have constructed a Fourier spectrometer of the classical Michelson interferometer type. The instrumental aperture is 50 mm, the spectral range roughly 50-1 000 cm-I (10-200 microns) and the planned resolution a little better than 1 cm-I. i) INTERFEROMETER OPTICAL SYSTEM. The mechanical and optical design of the instrument was controlled by the fact that the instrument is required to operate in a remote environment whilst being both reasonably light and sufficiently rugged to withstand Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1967223