By means of a grid and three centered and astigmatic optical systems, it is possible to get an arbitrary orientation of the image of the slits of the grid with respect to the measuring direction. Such a device improves the accuracy of the observation of boundary layers encountered in aerodynamical, thermal, and mass transport phenomena. This process can also easily be applied to differential interferometry.
R. Grossin, M. Jannot, and S. Viannay, "Schlieren Visualization Device Allowing an Arbitrary Orientation of the Lines with Respect to the Scanning Direction," Appl. Opt. 10, 201-204 (1971)