A spectrograph comprising: a housing that includes a wall having an inner surface facing an interior of the housing, the wall including first , second and third openings , the wall including projections extending inwardly of the second opening ; an entrance slit located at the first opening and configured to direct light along a first portion of a light path (LP1) in the interior of the housing ; a dispersive element located at the second opening and configured to receive light from the entrance slit along the first portion of the light path (LP1) and direct light along a second portion of the light path (LP2) in the interior of the housing ; and a detector located at the third opening and configured to receive light from the dispersive element along the second portion of the light path (LP2); characterized in that the dispersive element has a contour dimensioned to contact the projections which extend into the second opening , the projections and the contour of the dispersive element having complementary dimensional parameters and complementary tolerance parameters so that the contour of the dispersive element contacts all of the projections when the dispersive element is at least partially located in the second opening , and so that an orientation of the dispersive element relative to the entrance slit is fixed wherein the second opening includes inner and outer openings , the inner opening being located closer than the outer opening to the interior of the housing, the projections including a first group of projections extending inwardly of the inner opening and a second group of projections extending inwardly of the outer opening., wherein at least one of the first group of projections or the second group of projections includes at least three projections. a housing that includes a wall having an inner surface facing an interior of the housing, the wall including first , second and third openings , the wall including projections extending inwardly of the second opening ; an entrance slit located at the first opening and configured to direct light along a first portion of a light path (LP1) in the interior of the housing ; a dispersive element located at the second opening and configured to receive light from the entrance slit along the first portion of the light path (LP1) and direct light along a second portion of the light path (LP2) in the interior of the housing ; and a detector located at the third opening and configured to receive light from the dispersive element along the second portion of the light path (LP2); characterized in that the dispersive element has a contour dimensioned to contact the projections which extend into the second opening , the projections and the contour of the dispersive element having complementary dimensional parameters and complementary tolerance parameters so that the contour of the dispersive element contacts all of the projections when the dispersive element is at least partially located in the second opening , and so that an orientation of the dispersive element relative to the entrance slit is fixed wherein the second opening includes inner and outer openings , the inner opening being located closer than the outer opening to the interior of the housing, the projections including a first group of projections extending inwardly of the inner opening and a second group of projections extending inwardly of the outer opening., wherein at least one of the first group of projections or the second group of projections includes at least three projections. <EOS>