High Resolution Balloon-Borne Spectrograph for the Near Solar Ultraviolet

In order to obtain the solar spectrum of the Mgii lines at 2795.5 Å and 2802.7 Å, we built a balloon instrument which consisted of a Cassegrain telescope, with an aperture ratio of F/20 and a focal length of 300 cm to give a solar image on the slit of a spectrograph. The theoretical spectral resolution is better than 10 mÅ. In the laboratory, we measured a spatial resolution of 10 μ in the image plane of the spectrograph, which corresponds to 1 arc sec in this mounting. During a balloon flight of 1969, April 30, which used a biaxial pointing system, we obtained a spatial resolution better than 10 arc sec, limited by the accuracy of the pointing system. Lastly, on 1970 June 24, the same instrument with a servosystem inside the telescope, was launched and spectral and spatial resolutions of 25 mÅ and about 3 arc sec, respectively, were obtained.