Charged Particle Sources
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Flat field and imaging gratings are aberration-reduced concave gratings. The spectral images of the entrance slit are focused on a flat plane, so a flat detector array can detect the spectral intensity information simultaneously. This type of gratings is introduced by the holographic reading techniques. They have the important advantage of near stigmatic imaging, and hence are able to achieve maximum resolution and high photometric efficiency. Today, holographic reading is the most common solution for flat-field imaging. This is because classically ruled gratings are not capable of good imaging outside Rowland circle, and ruling of specially curved grooves by computer controlled engines or electron beams are much more expensive. Additionally improvement could be from delicate substrate design, and by having two different radii of curvature in the vertical and horizontal plane, astigmatism can be significantly reduced.