Deep sub-arcsecond wide-field imaging of the Lockman Hole field at 144 MHz

Recent observations of the radio sky show that the vast majority of sources detected at 144 MHz are unresolved at the typical resolution of a few arcseconds1, demonstrating the need for sub-arcsecond-resolution surveys to make detailed studies. At low radio frequencies, high spatial resolution is challenged by the ionosphere and by the propagation delay of radio waves that it induces2. If not adequately corrected for, this blurs the images to arcsecond or even arcminute scales. In addition, the required image size to map the degree-scale field of view of low-frequency radio telescopes at sub-arcsecond resolution is far greater than what typical software and hardware are currently capable of handling. Here we present deep degree-scale sub-arcsecond imaging at low radio frequencies. We derive ionospheric corrections in 44 directions on individual sources with compact sub-arcsecond structures. This has yielded a sensitive 6.6 deg2 144 MHz map with a resolution of 0.38″ × 0.30″ and a sensitivity of 25 μJy per beam, near the phase centre. This will allow mapping of the entire northern low-frequency sky at sub-arcsecond resolution.