  Relatively little information is available about the Universe at ultra-low
radio frequencies, i.e. below 50 MHz (ULF), although the ULF spectral window
contains a wealth of unique diagnostics for studying galactic and extragalactic
phenomena. Sub-arcsecond resolution imaging at these frequencies is extremely
difficult, due to the long baselines (>1000 km) required and large ionospheric
perturbations. We have conducted a pilot project to investigate the ULF
performance and potential of the International LOFAR Telescope (ILT), a
trans-European interferometric array with baselines up to ~2000 km and
observing frequencies down to 10 MHz. We have successfully produced images with
sub-arcsecond resolution for 6 radio sources at frequencies down to 30 MHz.
This is more than an order of magnitude better resolution than pre-ILT
observations at similar frequencies. The six targets that we have imaged (3C
196, 3C 225, 3C 273, 3C 295, 3C 298 and 3C 380) are bright radio sources with
compact structures. By comparing our data of 3C 196 and 3C 273 with
observations at higher frequencies, we investigate their spatially resolved
radio spectral properties. Our success shows that at frequencies down to 30
MHz, sub-arcsecond imaging with the ILT is possible. Further analysis is needed
to determine the feasibility of observations of fainter sources or sources with
less compact emission.
