To avoid rotating the mirror about the vertex of the paraboloid rather than the centre of the clear aperture, the collimator is tilted from the point leaving the folding prism exit surface. An extra coordinate break is introduced to compensate for the shifts of the component that this method would otherwise introduce. In this way all the translation and rotations listed in table 1 were transferred into the ray tracing model. As explained before, these represent a rigid rotation of the whole spectrograph and are expected to produce no effect on the CCD image, both in terms of quality and position. The resulting spot diagram for the central wavelength is shown in figure 1, while centroid positions for key wavelengths are listed in table 2.
Residual errors are of the order of the ray tracing model accuracy.
Both the spot diagram and the centroid positions confirm that the Zemax model of this rigid rotation of the spectrograph is correct.