title: Asymmetric white-pupil collimators for high-resolution spectrographs.
abstract: We present the optical principle of a white-pupil collimator made up of two mirrors with unequal focal lengths. Analytic formulas for third-order aberrations are derived and compared with the results of ray-tracing experiments. It is shown that the image quality is good if the second mirror has a suitable conic coefficient and the F ratio is not too small. Furthermore, third-order coma and astigmatism can be zeroed simultaneously if the angles are also appropriately selected. When implemented in high-resolution echelle spectrographs, this design permits smaller cross dispersers and cameras, with considerable savings in cost.
authors.affiliation: Osservatorio Astronomico di Padova, Vicolo dell'Osservatorio 5, 35122 Padova, Italy. gratton@pd.astro.it
authors.forename: R G
authors.initials: RG
authors.lastname: Gratton
authors.forename: R
authors.initials: R
authors.lastname: Bhatia
authors.forename: A
authors.initials: A
authors.lastname: Cavazza