Plane (flat) gratings are prone to coma and astigmatism. Using multiple optics, as in the Czerny-Turner configuration, also increases the stray light. To minimize these errors, the BLACK-Comet spectrometer uses a concave grating. This means the three optics in a Czerny-Turner configuration are combined into a single optic: the grating itself. Below is a picture of the inside of the BLACK-Comet.
These gratings are produced with grooves that are neither equispaced nor parallel, and are computer optimized to form near-perfect images of the entrance slit on the detector plane. Owing to their large optical numerical aperture and correction from aberrations, these concave gratings provide much better light collection efficiency and signal to noise ratio.
Read “Performance Advantages of Concave Gratings in Compact Fiber Optic Spectrometers”
