Asymmetric white-pupil collimators for high-resolution spectrographs

We present the optical principle of a white-pupil collimator made up of two mirrors with unequal focal lengths. Analytic formulas for third-order aber rations are derived and compared with the results of ray-tracing experiment s. It is shown that the image quality is good if the second minor has a sui table conic coefficient and the F ratio is not too small. Furthermore, thir d-order coma and astigmatism can be zeroed simultaneously if the angles are also appropriately selected. When implemented in high-resolution echelle s pectrographs, this design permits smaller cross dispersers and cameras, wit h considerable savings in cost. (C) 2000 Optical Society of America OCIS co des: 120.6200, 220.1000, 120.1680.