Optical properties of a 5 m echelle vacuum spectrometer with an approximately diffraction-limited resolving power in the order of 10⁶

Abstract

The image formation and the spectral resolution of a 5m echelle spectrometer were investigated. As shown in this study, the spectrometer operates approximately diffraction-limited in the visible and uv, whereas in the vacuum uv (120.0 nm) the resolving power is still >54% of the diffraction-limited value. The spectrometer was designed according to the results of an analytical imaging theory. Residual optical aberrations were investigated by measuring the image distributions at four wavelengths between 120.0 and 632.8 nm (grating replaced by a plane mirror), in order to experimentally test the reliability of these theoretical results in combination with the influence of the imperfections of the optical components. The ratios of 40%-widths of the measured image distributions in the exit slit plane to the corresponding theoretical values were determined to be 0.56 at 120.0 nm, 0.68 at 174.0 nm, 0.74 at 257.3 nm, and 0.99 at 632.8 nm. Due to a lack of suitable light sources in the vacuum uv with sufficiently small band widths, the instrumental functions of the spectrometer could only be measured at 257.3, at 514.5, and at 632.8 nm, using stabilized and frequency doubled laser lines. The resolving powersR, which were deduced from the 40%-widths of the measured instrumental functions, were determined to be 1.42×10⁶ [0.80] at 257.3 nm, 0.86×10⁶ [0.97] at 514.5 nm, and 0.71×10⁶ [0.98] at 632.8nm (ratiosR ^exp/R ^theor in brackets). The comparatively high resolving powerR≳0.02×10⁶ [0.54] at 120.0 nm was estimated from the 40%-width of the measured image distribution.