Simultaneous dual polarization observations at multiple frequencies are crucial for understanding time-varying astronomical phenomena. To achieve this, it is necessary to separate the main radio telescope beam into different frequency components to feed various receivers. This paper presents our innovative optical diplexer design, based on the layering of dielectric materials. By stacking nine periodic layers of low-loss High Resistivity Silicon (HR-Si) and Low-Density Polyethylene (LDPE), we have developed a diplexer that separates frequency bands centered around 220 GHz and 350 GHz. This diplexer design has applications in the wideband Submillimeter Array (wSMA). Future optical diplexer designs utilizing dielectric stacks will enable dual polarization multi-band observations with the next generation Event Horizon Telescope (ngEHT) and the Black Hole Explorer (BHEX).
After having adapted the Smithsonian Astrophysical Observatory (SAO) atmospheric phase monitoring system that was developed for the submillimeter array (SMA), IRAM is currently refining the concept for a future permanent implementation on the northern extended millimeter array (NOEMA) site. This system is based on commercial low noise block attached to a satellite dish that receive signal from geostationary satellites. The baselines (one so far) amplitude and phase are processed to provide real-time statistical data in the direction of the satellite and give information about the atmospheric phase distortion experienced by the NOEMA interferometer antennas.
New experiments that target the B-mode polarization signals in the Cosmic Microwave Background require more sensitivity, more detectors, and thus larger-aperture millimeter-wavelength telescopes, than previous experiments. These larger apertures require ever larger vacuum windows to house cryogenic optics. Scaling up conventional vacuum windows, such as those made of High Density Polyethylene (HDPE), require a corresponding increase in the thickness of the window material to handle the extra force from the atmospheric pressure. Thicker windows cause more transmission loss at ambient temperatures, increasing optical loading and decreasing sensitivity. We have developed the use of woven High Modulus Polyethylene (HMPE), a material 100 times stronger than HDPE, to manufacture stronger, thinner windows using a pressurized hot lamination process. We discuss the development of a specialty autoclave for generating thin laminate vacuum windows and the optical and mechanical characterization of full scale science grade windows, with the goal of developing a new window suitable for BICEP Array cryostats and for future CMB applications.
We report our studies of the effect of microwave radiation, with a frequency much lower than that corresponding to the energy gap of the superconductor, on the performance of the NbN hot-electron bolometer (HEB) mixer incorporated into a THz heterodyne receiver. It is shown that exposing the HEB mixer to microwave radiation does not result in a significant rise of the receiver noise temperature and degradation of the mixer conversion gain so long as the level of microwave power is small compared to the local oscillator drive. Hence the injection of a small, but controlled amount of microwave radiation enables active compensation of local oscillator power and coupling fluctuations which can significantly degrade the stability of HEB mixer receivers.
A millimeter wave source derived from a phase modulated optical signal has been developed. The spectral purity and phase control
of the phase of the source allowed it to be used as a local oscillator with an astronomical millimeter wave interferometric array. The
phase and amplitude stability of the correlated signals of the array are comparable to that produced by Gunn based local oscillators.
The system is explained in the following article first as a simple open loop system and then as a more complex closed loop device
where the phase is controlled. A mathematical description is given which predicts system behavior. The telescope correlator output
graphs show phase and amplitude stability.
Efficient operation of a submillimeter interferometer requires remote (preferably automated) control of mechanically tuned local oscillators, phase-lock loops, mixers, optics, calibration vanes and cryostats. The present control system for these aspects of the Submillimeter Array (SMA) will be described. Distributed processing forms the underlying architecture. In each antenna cabin, a serial network of up to ten independent 80C196 microcontroller boards attaches to the real-time PowerPC computer (running LynxOS). A multi-threaded, gcc-compiled program on the PowerPC accepts top-level requests via remote procedure calls (RPC), subsequently dispatches tuning commands to the relevant microcontrollers, and regularly reports the system status to optical-fiber-based reflective memory for common access by the telescope monitor and error reporting system. All serial communication occurs asynchronously via encoded, variable-length packets. The microcontrollers respond to the requested commands and queries by accessing non-volatile, rewriteable lookup-tables (when appropriate) and executing embedded software that operates additional electronic devices (DACs, ADCs, etc.). Since various receiver hardware components require linear or rotary motion, each microcontroller also implements a position servo via a one-millisecond interrupt service routine which drives a DC-motor/encoder combination that remains standard across each subsystem.
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