Manufacturing method for monolithic freeform Offner-gratings for hyper-spectral imaging

The first diffraction gratings on curved substrates were manufactured by employing ruling engines by H. A. Rowland. Due to the ruling principle, these gratings are characterized by equidistant parallel lines if the line pattern is projected along the optical axis onto a tangential plane in the vertex of the grating substrate. The Offner spectrometer is based on such a classical grating on a convex spherical substrate. This spectrometer type shows very good field correction properties. Therefore, it is among the most promising spectrometer types to meet the demands of hyper-spectral imaging. A further improvement of the optical performance is based on the modification of the surface figure of the substrate to an aspherical shape while keeping the mentioned constant grating line distribution. There are many reasons to employ interference lithography/holography - and in particular the direct blazing approach - even for the generation of the specific Offner grating line distribution on the convex substrate. A main benefit of this method is the attainable nearly perfect angular orientation of the blaze facets for the whole grating aperture. Here the achievable well defined blaze structure leads to best diffraction efficiencies close to the theoretical optimum - independent from the local curvature of the substrate. To manage the more complex recording setups of direct blazed Offner gratings, reliable methods for testing the wave front quality are a necessary pre-condition. A corresponding test method based on holographic principles will be introduced in the following text. The aspherical Offner grating was designed for the application in the UV-1 spectrometer within the Sentinel 5 mission, which is part of the European Earth Observation Program Copernicus". The spectrometer is a passive grating imaging spectrometer with a swath width of 2.670km and a spatial resolution of 50x50km².