EXES: The Echelon-cross-echelle Spectrograph for SOFIA

The Echelon-cross-echelle spectrograph (EXES) is a high spectral resolution, mid-infrared spectrograph designed for and operated on the Stratospheric Observatory for Infrared Astronomy (SOFIA). EXES has multiple operational modes, but is optimized for high spectral resolution. The heart of the instrument is a one meter long, diamond-machined echelon grating. EXES also uses a 10242 Si:As detector optimized for low-background flux. We will discuss the design, operation and performance of EXES.

The hyperfine structure of the lines of the arc spectrum of rubidium

The structure of the lines of the arc spectrum of rubidium was investigated with a reflexion echelon grating of high resolving power, the light source being a tube containing helium neon mixture at about ½ mm. pressure and a small quantity of rubidium; this tube was fitted with external electrodes and excited with an oscillator of very high frequency. The two resonance lines—7947 and 7800—were found to possess the same fine structure, and each possessed four components: at 0·00, —0·09, —0·19 and —0·23 cm. -1 ; the two outer components, 0·00 and —0·23 cm. -1 , are weak and the two inner components are strong. The lines 4215 and 4201 were found to possess three components; at 0·00, —0·09 and —0·20 cm. -1 . The line at the latter point was slightly broadened. This was presumed to correspond to the same structure as that observed in the resonance lines, the broadened line corresponding to —0·19 and —0·23 unresolved. This observed structure is explained as arising from the 5S 1/2 level; the two outer, weak, components arising from the isotope 87 and corresponding to a separation into two levels, 0·23 cm. -1 apart, in the 5S 1/2 level; while the two inner, stronger, components arise from the other isotope, 85, and correspond to a similar separation of 0·10 cm. -1 . The centre of gravity of the levels of Rb 85 is displaced about 0·02 cm. -1 from that of the levels of Rb 87. Measurements on the intensities of the components of 4201 show that the probable value for the nuclear moment of Rb 85 is 3/2, but the accuracy is not sufficiently high to enable the value for the scarcer isotope Rb 87 to be determined with certainty; the probable value is, however, 5/2. The magnetic moment of the nucleus of Rb 87 is shown to be approximately two and a half times as great as that of Rb 85.

The hyper-fine structure of the arc spectrum, and the nuclear rotation of indium

The are spectrum of indium was investigated with the object of finding whether any of its lines possessed hyperfine structure, resulting from a quantized nuclear spin, and a corresponding magnetic moment of the nucleus. The spectrum of cæsium, which was investigated by the author, closely resembles that of indium, both spectra being peculiarly suitable for investigation in respect of hyperfine structure on account of their remarkable simplicity. In the case of cæsium it was found possible to resolve the structure arising from the S-levels, but not that due to the P-levels. In indium, however, it was hoped, on account of the far greater separation of the P-levels and the correspondingly greater interaction between the nuclear spin and the electron orbit, to achieve a resolution of the structure due to the P-levels. This was found possible. The apparatus used for high resolving power was a reflection echelon grating; the instrument was made of fused silica, platinized. It had 25 plates each 7 mm. thick. The grating was made by Adam Hilger, Ltd., and possessed its theoretical resolving power of about 800,000 at a wave-length of 4500 A. U. The mounting and method of using the echelon grating are fully described. The source of light was a cooled vacuum tube containing helium and indium chloride, excited with external electrodes by means of a high-frequency alternating current. The analysis of the structure of the lines shows that the nucleus must be assumed to possess one quantum of rotation. The comparison of the deduced structures of the P ½ and the P 3/2 levels agrees quantitatively with Fermi’s theory of the interaction of the nuclear and the electron spins.