ALMA band 2 optics: Preliminary results

The National Radio Astronomy Observatory is currently developing the ALMA Band 2 cartridge, to be completed by mid-2016. The receiver will cover the 67–90 GHz band, with dual linear polarization. An orthomode transducer (OMT) based on turnstile junction has been developed and measured. The measured input return loss is better than 20dB. The optics comprises of a linear taper corrugated horn and a bi-hyperbolic lens that matches the waist of the horn to that of the telescope. The lens mounts on the 300K plate and serves as the vacuum barrier for the cryostat. The window for band 2 is offset from the center of the cryostat by 255mm, which results in a feed/ lens tilt of 2.48° to the cryostat axis. The feed is cooled to 15K, while the lens is at 300K. The minimum distance between the lens and the feed aperture is 83 mm and the maximum size of the lens is 92mm.