The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ∼ 2,000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz. However, it is technically and logistically challenging to process LOFAR data at this resolution. To date only a handful of publications have exploited this capability. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. It is implemented in a pipeline using mostly dedicated LOFAR software tools and the same processing framework as the LOFAR Two-metre Sky Survey (LoTSS). We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LoTSS pointing with an 8 hour observation and 13 international stations. We perform in-field delay calibration, solution referencing to other calibrators in the field, self-calibration of these calibrators, and imaging of example directions of interest in the field. We find that for this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ∼1.5 degrees away, while phase solution transferral works well over ∼1 degree. We also demonstrate a check of the astrometry and flux density scale with the in-field delay calibrator source. Imaging in 17 directions, we find the restoring beam is typically ∼0.3 ×0.2 although this varies slightly over the entire 5 square degree field of view. We find we can achieve ∼80 to 300 µJy bm −1 image rms noise, which is dependent on the distance from the phase centre; typical values are ∼ 90 µJy bm −1 for the 8 hour observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image roughly 900 sources per LoTSS pointing. This equates to ∼3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution (LoTSS-HR) makes this estimate a lower limit.

We present Low-Frequency Array (LOFAR) telescope observations of the radio-loud gravitational lens systems MG 0751+2716 and CLASS B1600+434. These observations produce images at 300 milliarcseconds (mas) resolution at 150 MHz. In the case of MG 0751+2716, lens modelling is used to derive a size estimate of around 2 kpc for the low-frequency source, which is consistent with a previous 27.4 GHz study in the radio continuum with Karl G. Jansky Very Large Array (VLA). This consistency implies that the low-frequency radio source is cospatial with the core-jet structure that forms the radio structure at higher frequencies, and no significant lobe emission or further components associated with star formation are detected within the magnified region of the lens. CLASS B1600+434 is a two-image lens where one of the images passes through the edge-on spiral lensing galaxy, and the low radio frequency allows us to derive limits on propagation effects, namely scattering, in the lensing galaxy. The observed flux density ratio of the two lensed images is 1.19±0.04 at an observed frequency of 150 MHz. The widths of the two images give an upper limit of 0.035 kpc m −20/3 on the integrated scattering column through the galaxy at a distance approximately 1 kpc above its plane, under the assumption that image A is not affected by scattering. This is relatively small compared to limits derived through very long baseline interferometry (VLBI) studies of differential scattering in lens systems. These observations demonstrate that LOFAR is an excellent instrument for studying gravitational lenses. We also report on the inability to calibrate three further lens observations: two from early observations that have less well determined station calibration, and a third observation impacted by phase transfer problems.