Low-noise, high-efficiency optics design for ALMA receivers

The Atacama Large Millimeter Array (ALMA) will be a powerful new interferometer for millimeter and submillimeter astronomy on an excellent site in Chile. The design of the receiver optics plays a crucial role in the ultimate sensitivity of the ALMA instrument. We present the design concepts used in the development of the optical layout of the receivers. Properties of optical elements are reviewed, and the suitability of various types of optical components is discussed. Particular emphasis is placed on principles of high efficiency optical train designs. It is shown that the optical systems should be a simple as possible and that truncation losses have to be minimized. The areas which have the greatest uncertainty are the windows and IR filters where the tradeoffs between the radio frequency losses, IR blocking, pressure resistance and gas permeability have to be carefully evaluated. A sample design for the ALMA Band 3 (84-116 GHz) is given.