Mm-wave Radio Astronomy Instrumentation for the Onsala Space Observatory and SEST

Doctoral thesis, 2000

The thesis presents work done in developing instrumentation for mm- and submm-wave radio astronomy. The work described is in several fields. The greater part of the thesis is concerned with sensitive heterodyne receivers and in particular the cryogenically cooled mixers. In paper A the SEST telescope and its observing system are described. The author was mainly involved with developing the initial Schottky-based receivers for the 3 mm and 1.3 mm bands. These mixers were an extension of earlier Schottky mixer work for the Onsala Space Observatory. We have built a 345 GHz SIS receiver for the Swedish-ESO Submillimeter Telescope. The SIS mixer uses a Nb-Al2O3-Nb junction with an integrated tuning structure implemented with superconducting microstrip transmission lines. The junction, which was fabricated in Chalmers, has demonstrated excellent sensitivity. Also incorporated into the mixer block is a special biasing circuit to reduce the risk of static discharge from damaging the sensitive SIS junction. The mixer itself incorporates a diagonal horn antenna directly machined into the mixer block to avoid any losses associated coupling two waveguides together. The properties of the diagonal horn antenna have been investigated and its suitability for submm-wave applications evaluated. The antenna is simple to fabricate and gives good coupling to the fundamental mode Gaussian beam. It also has the feature that its physical foot-print in the focal plane is rather small compared to its gain or effective collecting area. This makes it an attractive choice for focal plane imaging applications and was chosen as the feed antenna for the SISYFOS imaging array which has been built for OSO. Paper D describes the further development of the technique of phase-retrieval holography for the measurement of radio telescope surface alignment. The novel feature of this application is the use of a satellite transmitted beacon signal instead of ground based one. The use of the satellite signal enabled us to make surface measurements at the more relevent elevation angles of 30 – 60°. Finally in Paper E I review the systems considerations for focal plane imaging arrays.