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Diffraction grating made by Paton Hawksley, Star Analyser 100 (SA100). This is a 100 lines/mm blazed transmission grating.


For spectroscopy the diffraction grating is screwed into the front of a 1.25 camera adaptor nosepiece. The diagram shows the arrangement of grating and camera set up for spectroscopy.


The optical configuration shown in the diagram constitutes a slitless spectrograph. A slit is not required as the objects being observed are point sources. The blazed transmission grating is placed in the path of light near the detector (CCD chip) and the image of stars in the field of view are dispersed (smeared) forming elongated images constituting the spectrum of component wavelengths of the source. The angle of dispersion β is given by the grating equation mλ = σ sin α+ σ sin β where m is the order number (m=1 in practice for blazed grating), λ wavelength, σ is groove pitch (0.01mm), α is angle of incidence.