OPTICAL DESIGN OF A FREEFORM OFFNER SPECTROMETER ENABLING A VERY LARGE FIELD OF VIEW

An Offner spectrometer with a freeform grating has been designed. The aim of the introduction of freeform technology on the spectrometer is to enhance the field of view of the instrument, while meeting specific performance requirements The Offner spectrometer comprises three concentric mirrors creating a telecentric imaging system, by introducing the freeform element, the spectrometer's layout is optimized without altering its fundamental structure. The design study started with a layout employing only standard optics, and the maximum operable field of view was investigated (51mm). The implementation of a freeform polynomial to the grating's surface resulted into a significant expansion of the field of view, up to 58mm. The chosen polynomials for the description of the new surfaces are Zernike FRINGE sag up to the 21st order coefficients. During this phase, all variables of the layout are free to be changed, including all positioning variables. Performance benchmarks such as smile and keystone distortion, Modulation Transfer Function (MTF), and dispersion grating specifications are consistently taken under control during the design development. This study underscores the efficacy of freeform optics in pushing the boundaries of optical technology, with promising implications for imaging systems and optical instrumentation optimization. The successful application of freeform optics in this Offner spectrometer design exemplifies its potential for advancing optical engineering in various fields.