Eimear Gallagher

CHARM (The Collaborative Heterodyne Receiver for Mexico) is an astronomical heterodyne receiver developed for the Large Millimetre Telescope (LMT) in Puebla Mexico as part of the ASTEC project (Astronomical Systems, Technology and Engineering Collaboration). The LMT is a huge Cassegrain radio telescope system with a 50m diameter primary dish, designed to operate at wavelengths from 4mm to 0.85mm (an equivalent frequency range of 75GHz to 350GHz). Located at an elevation of 4,600m on the Sierra Negra mountain within the Pico de Orizaba National Park, the LMT became operational in 2018, and its 50m diameter primary dish was yet to be characterised for wavelengths shorter than 1mm. The CHARM instrument allows this important performance step to be achieved and the telescope aperture efficiency to be determined at approximately 0.86mm, thus paving the way for future use of the telescope at sub-millimetre wavelengths.

The project is supported through a Global Challenges Research Fund (GCRF) award with the objective of developing links with Mexican researchers and training them in high-frequency heterodyne receiver development and use. It also demonstrates the LMT at a higher operational frequency then previously used. The project is led by the University of Manchester with the STFC Rutherford Appleton Laboratory Space Department (RAL Space) providing technical development. Additionally, Mexican scientists and engineers have been trained in the design, construction and operation of astronomical heterodyne receivers that employ new technologies.

CHARM was based on a previous instrument built by the MMT group at RAL Space, this was the Sub Harmonic Image Rejection receiver (SHIRM). This was a compact instrument that demonstrated technical advancements in signal down-conversion and data processing. It was subsequently recognised that the basic system could also be used to observe a primary spectral feature that is relatively common within the ISM, the carbon monoxide rotational transition line at ~ 345.8GHz.

CHARM relies on heterodyne mixer technology along with a local oscillator signal from the instrument to make the incoming signal a frequency that is more manageable to process using off the shelf components, allowing us also to use 4 wide band spectrometers to obtain spectra. CHARM uses a double sideband system and although this can complicate spectral identification, it provides an overall improvement in the system sensitivity and allows increased tuning and wider spectral coverage. The improvements made have increased the instantaneous bandwidth of CHARM to 8GHz per sideband.

CHARM is being used to observe astronomical spectral lines originating from young star forming regions of the interstellar medium (ISM) thus supporting and encouraging interaction between Mexican and UK astronomers. The instrument has two settings for astronomical observing - On the Fly (OTF) and Position switching (PSW). OTF is commonly used for mapping large extended sources, and includes rapid scanning of the area of interest, and PSW is used for smaller sources, and requires ON and OFF source integrations.

CHARM was tested using the Allan Deviation method, where the square root of Allan variance against observation time was plotted for the best bin number in a given spectra. This implied that a system integration time of ~10 seconds was achievable (which is more stable than the fluctuation of the atmosphere,) and was likely to improve in the more controlled environment of the LMT receiver cabin

Following successful installation in Sept 2019, CHARM first light observations showed the first demonstration of LMT capability at 0.85mm/345GHz. This could result in increase to LMT VLBI capabilities by demonstrating performance at this wavelength. Observations will continue into 2021. Results are to be presented at the SPIE Astronomical telescopes + instrumentation conference by Edgar Colin-Beltran of INAOE on behalf of the group.