Non-approximate method for designing Offner spectrometers

The so-called non-approximate method was adopted to design two-mirror three-reflection concentric Offner configuration for an dispersive spectrometer, which was using geometry and ray tracing method to derive the deviation between the arbitrary ray's image point and the principal ray's image point, the system aberration was evaluated based on this deviation, then each parameter was optimized and designed to meet the using conditions by computer software. The optimal object's position with minimal aberration, which also is known as the optimal imaging annular field, could be found in a different condition. Moreover, it presented that the ratio of the radius of convex grating to the radius of concave mirror ought to be close to 0.5∼0.6, the ratio r/R changed slightly with different wavelengths, but the bigger the wavelength was, the larger the ratio r/R would be.