We describe an aperture synthesis radio tele- scope optimized for studies of the Galactic interstellar medium (ISM), providing the ability to image extended structures with high angular resolution over wide elds. The telescope produces images of atomic hydrogen emis- sion using the 21-cm H i spectral line, and, simultaneously, continuum emission in two bands centred at 1420 MHz and 408 MHz, including linearly polarized emission at 1420 MHz, with synthesized beams of 1 0 and 3:4 0 at the respective frequencies. A full synthesis can achieve a con- tinuum sensitivity (rms) of 0.28 mJy/beam at 1420 MHz and 3.8 mJy/beam at 408 MHz, and the 256-channel H i spectrometer has an rms sensitivity of 3.5B 0:5 sin K per channel, for total spectrometer bandwidth B MHz and declination . The tuning range of the telescope permits studies of Galactic and nearby extragalactic objects. The array uses 9 m antennas, which provide very wide elds of view of 3.1 and 9.6 (at the 10% level), at the two frequencies, and also allow data to be gathered on short baselines, yielding extremely good sensitivity to extended structure. Single-antenna data are also routinely incorpo- rated into images to ensure complete coverage of emission on all angular scales down to the resolution limit. In this paper we describe the telescope and its receiver and corre- lator systems in detail, together with calibration and ob- serving strategies that make this instrument an ecient survey machine.

A new Auto-Correlation Spectral Imaging System (ACSIS) for the James Clerk Maxwell Telescope (JCMT) is being developed at the National Research Council of Canada, in collaboration with the Joint Astronomy Centre and the United Kingdom Astronomy Technology Centre. The system is capable of computing the integrated power-spectra over 1-GHz bandwidths for up to 32 receiver beams every 50 ms. An innovative, multiprocessor computer will produce calibrated, gridded, 3-D data cubes so that they can be viewed in real-time and are in hand when an observation is over. When connected to arrays of receivers at the Nasmyth focus of the telescope, the system will be able to rapidly make large-scale images with high spectral resolution and map multiple transitions. The ACSIS system will be mated initially with the multibeam 350-GHz receiver system, Heterodyne ARray Program (HARP), under development at the Mullard Radio Astronomy Observatory in Cambridge, England. In this paper we describe ACSIS, how it is designed and the results of key performance tests made.

A unique lag‐based VLBI correlator system has been developed for the purpose of supporting S2‐based space VLBI observations in both the Japanese‐led VSOP mission and the Canadian Geodetic VLBI program. The system architecture has been designed so that replication of a small number of modules can be used to construct systems with a wide range of sizes. Optimized for a large correlator, the design is "station based" in the sense that as many hardware and software functions as possible are performed before data are replicated and transmitted for baseline (station pair) processing. As well as delay compensation and generation of phase rotation coefficients, station‐based functions include autocorrelation, tone extraction, pulsar gating, signal‐statistics accumulation, and digital filtering. Doppler‐shift correction (fringe stopping) is performed on a baseline basis at each correlator lag so that there are no smearing effects (lag‐dependent loss of coherence) or frequency shifts that must otherwise be corrected after correlation. This is a key element that simplifies the baseline processing architecture when high accelerations associated with an orbiting antenna must be considered. Flexible, efficient distribution of data from station‐based hardware to baseline‐based hardware is accomplished by serializing the wide data paths to 1 Gbit s−1 signals and using high‐speed switches to route the signals to their final destinations where they are deserialized before cross‐correlation. This greatly reduces the size, wiring complexity, and cost of the system. The interval between updates of the delay models, integration times, and other important events is typically 10 ms but can be as short as 1 ms. Within this period, delay and fringe model generation is performed using linear hardware synthesizers. The correlator also contains a number of unique signal processing functions that extend its capability beyond a basic VLBI correlator: flexible Local Oscillator frequency switching for bandwidth synthesis; rapid (1 ms) correlator dump intervals (allowing, for example, the study of some single‐pulse pulsar characteristics on VLBI baselines); simple but powerful multirate digital signal‐processing techniques to allow correlation of signals at different but related sample rates; and a digital "zoom" filter for producing very high resolution cross‐power spectra. The correlator software, written almost entirely in C, is highly integrated into the system, supports all of the functions mentioned above, and is reconfigurable to support expansion of the correlator. The software schedules the use of hardware resources to enable correlation of multiple observations concurrently and automatically schedules the correlation of observations that require more than the available number of physical playback terminals. There is also substantial precorrelation consistency checking. The delay model is based on CALC for ground‐based antennas and NAIF for space‐based antennas. Output data are stored in the UVFITS format. The paper describes the design rationale, architecture, and function of the correlator and also provides specifications for the implemented system.