We report on the development of Argus, a 16-pixel spectrometer, which will enable fast astronomical imaging over the 85–116 GHz band. Each pixel includes a compact heterodyne receiver module, which integrates two InP MMIC low-noise amplifiers, a coupled-line bandpass filter and a sub-harmonic Schottky diode mixer. The receiver signals are routed to and from the multi-chip MMIC modules with multilayer high frequency printed circuit boards, which includes LO splitters and IF amplifiers. Microstrip lines on flexible circuitry are used to transport signals between temperature stages. The spectrometer frontend is designed to be scalable, so that the array design can be reconfigured for future instruments with hundreds of pixels. Argus is scheduled to be commissioned at the Robert C. Byrd Green Bank Telescope in late 2014. Preliminary data for the first Argus pixels are presented.
We present some detail of the waveguide probe and SIS mixer chip designs for a low-noise 180-300 GHz double-sideband receiver with an instantaneous RF bandwidth of 24 GHz. The receiver's single SIS junction is excited by a broadband, fixed-tuned waveguide probe on a silicon substrate. The IF output is coupled to a 6-18 GHz MMIC low-noise preamplifier. Following further amplification, the output is processed by an array of 4 GHz, 128-channel analog autocorrelation spectrometers (WASP II). The single-sideband receiver noise temperature goal of 70 Kelvin will provide a prototype instrument capable of rapid line surveys and of relatively efficient carbon monoxide (CO) emission line searches of distant, dusty galaxies. The latter application's goal is to determine redshifts by measuring the frequencies of CO line emissions from the star-forming regions dominating the submillimeter brightness of these galaxies. Construction of the receiver has begun; lab testing should begin in the fall. Demonstration of the receiver on the Caltech Submillimeter Observatory (CSO) telescope should begin in spring 2003.
New astronomical and remote-sensing instruments require microwave spectrometers with modest spectral resolution over many gigahertz of instantaneous bandwidth. Applications include millimeter-wave searches for distant objects with poorly known redshifts, submillimeter and far-infrared observations of Doppler-broadened spectral lines from galaxies, and observations of pressure-broadened atmospheric lines.
Wide bandwidths and the consequent stability requirements make it difficult to use general-purpose receiver and spectrometer architectures in these applications. We discuss analog auto- and cross-correlation lag spectrometers that are optimized for these observations. Analog correlators obtain their wide bandwidths by a combination of transmission line delays and direct voltage multiplication in transistor or diode mixers. We show results from a new custom transistor multiplier with bandwidth to 25 GHz. Stability becomes increasingly important as bandwidths broaden. We discuss system requirements for single-dish correlation radiometers, which have intrinsic high stability, and present results showing that analog cross-correlators are suitable backends for these receivers.
We present results from atmospheric model calculations for the design of an atmospheric phase correlation radiometer for the Berkeley-Illinois-Maryland Millimeter Array (BIMA). The radiometer will monitor the atmospheric path delay by observing the fluctuations in the emission from tropospheric water vapor which causes de-correlation of astronomical signals which are observed along different lines of sight. We discuss the applicability of monitoring the optically thick 183 GHz water line and the optically thin 22 GHz water line. We conclude that for the BIMA site, which is not extremely dry, optical depth effects make observations of the 183 GHz line unfavorable. We discuss possible observing schemes and conclude that a multi-channel radiometer provides the highest achievable accuracy, as it provides the possibility to fit a line shape model to the observed channels. Systematic errors due to the unknown altitude distribution of water vapor and optical depth effects can be significantly reduced by this scheme, compared to methods which only monitor the change of the peak brightness temperature of the water vapor line.
We have developed a novel wideband spectrometer for astronomical heterodyne spectroscopy. The spectrometer, WASP, has 3250 MHz bandwidth and 33 MHz resolution, a combination well matched to submillimeter spectroscopy of high-redshift objects, interacting galaxies, active galactic nuclei, and planetary atmospheres. The spectrometer is an autocorrelation spectrometer with analog microwave integrated circuit multipliers separated by microstripline transmission line delays. Our prototype spectrometer is compact, requires little power (75 W), and integrates stably for many hours.