The Correction of Keystone Distortion in Czerny–Turner Spectrometer Using Freeform Surface

In the past, conventional Czerny–Turner spectrometers were usually designed to achieve high resolution while often ignoring astigmatism in the sagittal direction. In contrast, by replacing the focusing mirror with a freeform surface in the structure, we can obtain a Czerny–Turner spectrometer with low keystone distortion by controlling the astigmatism. At the same time, the area sensor can receive all of the spectrum from the optical system. In this paper, we briefly describe the formation of keystone distortion and smile in a plane grating. Additionally, the validity of the method is verified through simulation. Finally, we evaluated the smile and keystone distortion of both the initial and final systems. The keystone and smile were reduced to 1.77 μm and 8.3 μm, respectively, over the wavelength range of 535 nm to 630 nm. Concurrently, the resolution achieved was 0.4 nm.