CHARM: a room-temperature 345GHz receiver for the Large Millimeter Telescope

A 345 GHz room-temperature single-pixel heterodyne receiver using sub-harmonic Schottky barrier diode mixers has been installed at the Large Millimeter Telescope (LMT) on the Sierra Negra in Mexico. The receiver was developed at the Science and Technology Facilities Council (STFC) Rutherford Appleton Laboratory (RAL) in the UK in 2013 to perform ground-based atmospheric studies between 312 GHz and 360 GHz. With support from the STFC Global Challenge Research Fund (GCRF) project “Astronomical System Training, Engineering and Collaboration (ASTEC)” the instrument has been reconfigured to support astronomical research and installed on the 50-meter LMT to be used as a pathfinder for sub-millimeter wavelength observations. This new receiver, CHARM (Collaborative Heterodyne Astronomical Receiver for Mexico), has exchanged an originally implemented single-sideband mixer design for a double-sideband device. In addition, a broader bandwidth intermediate frequency (IF) chain, additional digital sampling spectrometers and appropriate interface quasi-optics have been installed. The modifications have resulted in a turnkey receiver system with a double sideband (DSB) receiver noise temperature (Trec) of ~1200 K as measured in the laboratory. The inclusion of a wider IF and a total of four digital spectrometers the instrument encompasses a 12 GHz IF bandwidth with 1.46 MHz resolution. Use of Schottky mixers allows room temperature operation and whilst both of these attributes sacrifice noise performance, and thus detection sensitivity, when compared with cryogenic superconducting systems, they allow a relatively simple system architecture to be implemented that has objective of establishing the LMT sub-millimeter wave performance potential and related quality local atmospheric ‘seeing’ conditions. The detailed design of the instrument, description of the optical system for the LMT adaptation and first-light results are presented.