Off-axis surface tolerance analysis: Tips and tricks

Off-axis and eccentric pupil optical configurations are becoming more and more used in a variety of applications, in particular they are the most preferred solution for cameras devoted to study planetary surfaces, small solar system bodies (i.e. asteroids and comets) and also the atmospheres of the exoplanets. These optical designs, being devoid of central obstruction, are able to guarantee better PSF and MTF performance, and thus higher contrast imaging capabilities with respect to classical on-axis designs. In particular they are suitable for looking at extended targets with intrinsic low contrast features, or scenes where a high dynamic signal range is present. The tolerance analysis tools available in most of the commercial raytracing software packages are able to deal automatically with on-axis surfaces, but they have to be adapted for considering the off-axis cases. In particular, some tricks have to be considered in the definition of the off-axis surfaces to obtain the correct tolerance results when the way in which the surfaces are manufactured and/or mounted needs to be taken into account. For example, an off-axis section of a conic surface can be either obtained by cutting an off-axis piece from a large on-axis parent element or, as it is more common nowadays, by manufacturing it stand-alone. In this paper, a review of the tricks that the author has devised during her twenty-year experience in designing and tolerancing off-axis systems will be given. In particular the methods adopted to deal with the surface tolerance analysis of some off-axis instruments for space applications will be described in detail.