THORLABS GVS011 - Fig. D.2 Pillow-shaped Field Distortion Caused by the Arrangement of Mirrors

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Single- and Dual-Axis Scanning Galvo Systems for Large Beam Diameters
As a result, you will find the scanning field turn out to be a "pillow-shaped" image, see
Fig. D.2 below.
Fig. D.2 Pillow-shaped Field Distortion Caused by the Arrangement of Mirrors