  High quality low-frequency radio surveys have the promise of advancing our
understanding of many important topics in astrophysics, including the life
cycle of active galactic nuclei (AGN), particle acceleration processes in jets,
the history of star formation, and exoplanet magnetospheres. Currently leading
low-frequency surveys reach an angular resolution of a few arcseconds. However,
this resolution is not yet sufficient to study the more compact and distant
sources in detail. Sub-arcsecond resolution is therefore the next milestone in
advancing these fields. The biggest challenge at low radio frequencies is the
ionosphere. If not adequately corrected for, ionospheric seeing blurs the
images to arcsecond or even arcminute scales. Additionally, the required image
size to map the degree-scale field of view of low-frequency radio telescopes at
this resolution is far greater than what typical soft- and hardware is
currently capable of handling. Here we present for the first time (to the best
of our knowledge) widefield sub-arcsecond imaging at low radio frequencies. We
derive ionospheric corrections in a few dozen individual directions and apply
those during imaging efficiently using a recently developed imaging algorithm
(arXiv:1407.1943, arXiv:1909.07226). We demonstrate our method by applying it
to an eight hour observation of the International LOw Frequency ARray (LOFAR)
Telescope (ILT) (arXiv:1305.3550). Doing so we have made a sensitive $7.4\
\mathrm{deg}^2$ $144\ \mathrm{MHz}$ map at a resolution of $0.3''$ reaching
$25\ \mu\mathrm{Jy\ beam}^{-1}$ near the phase centre. The estimated $250,000$
core hours used to produce this image, fit comfortably in the budget of
available computing facilities. This result will enable future mapping of the
entire northern low-frequency sky at sub-arcsecond resolution.
