Design and manufacture of a modified freeform Offner spectrometer with 0.6× magnification

Abstract

The Offner spectrometer is characterized by a lower F/#, excellent imaging quality, and small distortion, which are beneficial for symmetry and concentricity. However, its 1:1 magnification imposes a constraint on the design, manufacture, and alignment of foreoptics, especially with small F/# applications. The modified Offner spectrometer is hence proposed, and its design and manufacture are introduced in this paper. The initial structure parameters of the modified Offner spectrometer are solved with the wave aberration theory of a plane-symmetrical system. The primary and tertiary mirrors use the Zernike polynomial freeform surfaces to improve the imaging quality during optimization. The components of the designed modified Offner spectrometer are fabricated, and the spectral performance is measured after alignment. The measured spectral resolution is 3.91 nm, and the maximum smile and keystone are 1.37 and 1.8 µm, respectively, which are both less than 10% of the spatial and spectral dimension pixels.