ALMA Band 9 Optical Layout

In this report a mm wave optics prototype for an Atacama Large Millimeter Array (ALMA) receiver cartridge is discussed. Mainly the ALMA frequency band 9 (602-720 GHz) is considered. However the same design approach is applicable to the receiver optics of any other ALMA bands. The high frequency cartridges (bands 7-10) have been decided to have no intermediate optics between the telescope secondary mirror and the cartridge cold optics. (See notes of Tucson optics meeting). Consequently, in the optical design of band 9 cartridge full signal path with all focusing elements are considered as free design parameters. In this report a concept of choosing and optimizing these parameters is discussed. The design starts from an optimal corrugated and diagonal horn field representation. A possibility of distortion compensation by a pair of ellipsoidal mirrors is discussed. A concept of frequency independent radiation coupling between the horn and the Cassegrain telescope system is presented. An expression for parameters of two elliptical mirrors giving rise to frequency independent matching of the horn to the telescope is given. A practical corrugated horn for signal path of mixer and a practical diagonal horn for local oscillator (LO) part of mixer are proposed. Based on the horn and the ALMA prototype antenna design a two-mirror layout for ALMA band 9 is proposed. The alignment accuracy of optical components within the cartridge is considered. The alignment accuracy between the cartridge and the secondary mirror is also considered