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Paschen Mounting

The common form of mounting used for a concave reflection grating is shown in fig. It is called the Paschen mounting.

In this mounting, the slit S is set on the circumference of the Rowland circle as shown. The slit is perpendicular to the plane of the paper. GG’ is the concave reflection grating and OC’ is the diameter of the Rowland circle. The spectra of different orders are imaged on the circumference of the circle. In the fig. C is the central image, A₁B₁ is the first order spectrum. A₂B₂ is the second order spectrum and A₃B₃ is the third order spectrum. With this mounting, several orders of the spectrum can be photographed simultaneously. The photographic plates are hold in a frame which can give the plates the proper curvature coincident with the Rowland circle. For any particular order, the dispersion is minimum when θ = 0. The disadvantages in this mounting is that the spacing of the spectral lines in the different regions of the spectrum is not proportional to the difference in wavelength between the lines.