The Design of Re-Imaging Optics for Passing Several Beams Through Small Cryostat Windows

In this paper, we present a method to design re-imaging optics for receivers under heavy physical and mechanical constraints introduced by an antenna and cryostat. Multibeam receivers will face many challenges to fit optics inside compact cryostats and to pass multiple beams through small windows. The optics can be solved and filtered to obtain solutions that work under these strong mechanical constraints. We utilized the properties of a pupil to create an image of the sub-reflector at a different position to which additional tertiary optics were solved to re-direct beams through small windows. This method was used to design a two-beam multibeam receiver for the ALMA 12-m antenna and cryostat. The optics were successfully able to pass a beam through the small cryostat window and achieve a frequency-independent illumination on the sub-reflector.