2.6 Millimetre-wave receivers 

2.6.1 The MARS imaging array receiver 

The MARS phase I receiver consists of eight 230GHz mixers giving sparse sampling of the full 2- focal plane of the JCMT. Four central mixers will be used to look at the effects of mutual coupling (`crosstalk'), while four further mixers at the corner of the field will provide information on the aberrations. We hope to have this engineering test instrument operating on the JCMT in late 1995 or early 1996. During 1993 and 1994 we made progress in several areas:

1. We have built and tested a novel quasi-optical LO injection system, which permits high-efficiency coupling of the LO, with independent control of the LO power level for each individual mixer.
2. Experiments with scaled models at 15GHz show that very high aperture efficiencies can be obtained with diagonal horns that have a dielectric plug insert separated from the horn walls by a thin `airgap'. The performance is essentially identical to that of a corrugated conical horn, but the structure is much easier to realize, especially at short wavelengths.
3. The hybrid cryostat is now complete and awaiting testing. The cryostat incorporates many new ideas, and is expected to provide a hold-time of some days despite the high intrinsic heat-loads associated with the large vacuum window and complex cooled electronics.
4. Most importantly, testing of the antipodal finline mixer to date has been very encouraging. Our collaborators at the University Department of Materials Science and Metallurgy have supplied one batch of devices with excellent characteristics, and we hope to have our first noise temperature results within weeks. The new design requires only a very simple mixer block, with no tuners, and a much simpler coil structure; we expect it to operate at close to optimum performance over the full waveguide band.

2.6.2 Common-user receivers for JCMT 

Construction of the C and D band common-user receiver, RxW, is nearing completion. The optical system has been constructed and the mirrors and interferometers tested individually. The low-noise electronic modules, microprocessor control system, and software are finished. The performance of the Daikin 4K closed-cycle refrigerator has been investigated in detail. Emphasis now shifts towards completing the cryostat and the two local-oscillator systems. The intention is to install the receiver on the JCMT before the end of 1995.

Members of the group also participated actively in the design study for the JCMT B-band array, particularly with regard to the optical and RF design. This design study has now been published, and approved by the JCMT Board as the basis of the common-user array. MRAO will be responsible for the two 16-element arrays (one for each polarization), as well as for various other aspects of the instrument.

2.6.3 SIS mixers 

A considerable amount of work has gone into the development of 500GHz SIS mixers for RxW. The infrastructure for this type of work was enhanced during the year with the construction of a complete micromachining facility within the Radio Astronomy Group. We have now constructed 4 mixer blocks, and tested them, with a variety of Nb junctions, at 500GHz. The mixer blocks have two backshort tuners and are of a particularly rugged design. Using our computer-controlled measurement facility we are currently comparing the performance with the predictions of the quantum-theory of mixing (Kerr, Pan, & Withington, 1993). In particular, we have studied issues such as dc behaviour, IF noise mechanisms, multiparticle and Josephson effects, RF tuning, IF matching, stability, and system noise temperature. The main conclusion is that we are now able to guarantee system noise temperatures, in the laboratory, of less than 100K at 500GHz, and we have observed system noise temperatures as low as 70K (Withington, Isaak, Kovtonyuk & Panhuyzen, 1994). In connection with this work we are also collaborating closely with SRON and the University of Groningen on the development of a pair of 700GHz mixers for RxW. The first tests, at SRON, have resulted in system noise temperatures of about 400K. This first generation of mixers has been so successful that we are now working on the design of fixed-tuned, broad-band mixers for high-redshift extragalactic astronomy.

2.6.4 Quasi-optics 

We have continued to study in detail the behaviour of submillimetre-wave optical systems.

We have studied the behaviour of a variety of horns and mirrors, and we have demonstrated the first binary-phase grating at submillimetre wavelengths (Murphy and Withington, in press ). We have also considered theoretically a number of problems associated with the design and behaviour of single-beam quasioptical systems: we have addressed the problem of truncation in submillimetre-wave systems (Murphy, Withington & Egan 1993); we have considered the behaviour of off-axis mirrors (Withington, Murphy & Isaak in press; Murphy & Withington, submitted), and we have investigated the possibility of using phase-retrieval techniques to study the modal content of submillimetre-wave beams (Withington & Isaak 1993).