Astigmatism in the basic Offner spectrometer

Abstract

The in-plane configuration of the basic Offner spectrometer is revised. The locations of meridional and sagittal images of the slit center are analyzed when the slit is displaced in the axial direction from the usual configuration, whereas its center is kept on the Rowland circle of the concave mirror. This translates the position of the sagittal image plane and allows for the cancellation of astigmatism and meridional coma for two wavelengths while these aberrations are kept small over the whole spectral range. This is accomplished without splitting the concave mirror into two different mirrors, simplifying the design and its practical implementation. A design example is presented with excellent optical performance