Design of a two-lens telephoto collimator for CMAG coronagraph

Abstract Solar coronagraphs often include narrowband filters located at an image of the telescope pupil. These filters are commonly illuminated with collimated light coming from an intermediate telecentric image of the corona. In this configuration, the intermediate image is located at the front focal plane of a collimator, while the telescope pupil is formed at its back focal plane. Collimators employed to illuminate the filter have long focal lengths and usually need to be designed as telephoto lenses for the purpose of minimizing the distance from the intermediate image to the pupil. This is particularly important for spaceborne instruments, whose dimensions are strictly constrained. However, conventional telephoto lenses cannot be employed in this case as they unavoidably increase the front-to-back focal length of the system. Reducing the number of optical elements in coronagraphs is also essential to minimize stray light and ghost images. In this work, we derive the equations for two-component telephoto lenses needed to minimize the front-to-back focal distance. We apply these equations to design a collimator for CMAG coronagraph. We study the image quality as a function of the telephoto ratio, we athermalize our design and we carry out a tolerance analysis. We demonstrate that the front-to-back focal length can be reduced by $$\textbf{45}\,\mathbf {\%}$$ for a field of view as large as 1.28° while keeping the root mean square wavefront error below $$\varvec{\lambda /20}$$ .