The High Cadence CAL: Enabling Commensal Surveys with Radio Telescopes

One of the most significant challenges in radio astronomy has been the inability to do fully commensal observations, where multiple science objectives can be achieved simultaneously. This has been in large part due to incompatibility between the calibration requirements of different types of projects, most notably pulsar observations and spectral line imaging. Imaging of spectral lines, especially the 21cm HI line, requires strict calibration of a telescope’s electronics gain using the periodic injection of a white noise signal with known intensity (CAL). Due to its periodic nature, such a signal completely overwhelms a pulsar’s power-spectrum in the time domain. We developed a new method, namely the High-Cadence CAL (HCC), which injects the standard electronic noise at twice the sampling rate. Such cadence allows both Pulsar Search and Spectral Line projects to be observed and calibrated simultaneously. The HCC system has been implemented at the Five-hundred-meter Aperture Spherical radio Telescope (FAST) and facilitated its commensal survey of Galactic HI (narrow band), HI galaxies (wide band), and pulsars, the first of its kind. The HCC has greatly enhanced the survey efficiency of FAST. Widespread implementation of HCC could greatly expand scientific output in radio astronomy. During this talk we will introduce this new calibration method as well show some of the recent results from FAST.