Introduction

The 870 $\mu$m atmospheric window (see Figure 1) has the highest transmission of any submillimeter band, and due to prevailing physical conditions in the interstellar medium is also one of the richest spectroscopically. Figure 1 shows some of the important molecular transitions in this atmospheric window. The noise performance of submillimeter receivers is improving dramatically every year and is approaching limits set by quantum mechanics and/or the sky background, especially in the lower end of the submillimeter frequency band. A large increase in the speed of spectroscopic astronomical observations can be obtained by using a heterodyne array receiver. There are two approaches to obtaining a real improvement in the speed of imaging using arrays. It is possible to develop an array with a large number of elements each of which has less than optimum performance in terms of noise temperature and optical coupling. A better approach in terms of cost and speed is to build a moderate sized array without compromising mixer performance.

 

Figure 1: Model Atmospheric transmission in the 870 $\mu$m window (Mount Graham 1mm water). The frequencies of important molecular transitions are marked.

The seven element 345 GHz focal plane array receiver we are building for the Heinrich Hertz Telescope (HHT) will have a tuning range between 315 and 380 GHz and will make excellent use of the telescope and available atmospheric transmission. The HHT is a joint development between Steward Observatory, University of Arizona, and the Max-Planck-Institut of Radioastronomie. The overall design of the optics and cryogenic systems of the array will be outlined in a future paper [1]. In this paper we describe the design of individual mixer elements that will make up the final array.

Technical, scientific and budgetary issues dictate the following objectives for the mixer design:

• State-of-the-art performance of the individual elements. In particular we require receiver temperatures $T_R \le 100$ K over the band, a tuning range of 315 to 380 GHz to make full use of the atmospheric band, an IF bandwidth of 4 - 6 GHz, and good antenna efficiencies and clean beams.
• Since the array will be a facility instrument, the mixers should be robust and easy to use. Therefore, no mechanical tuners will be used.
• To keep the costs down, the mixer blocks should be designed for easy fabrication.