  Interferometric imaging now achieves angular resolutions as fine as 10
microarcsec, probing scales that are inaccessible to single telescopes.
Traditional synthesis imaging methods require calibrated visibilities; however,
interferometric calibration is challenging, especially at high frequencies.
Nevertheless, most studies present only a single image of their data after a
process of "self-calibration," an iterative procedure where the initial image
and calibration assumptions can significantly influence the final image. We
present a method for efficient interferometric imaging directly using only
closure amplitudes and closure phases, which are immune to station-based
calibration errors. Closure-only imaging provides results that are as
non-committal as possible and allows for reconstructing an image independently
from separate amplitude and phase self-calibration. While closure-only imaging
eliminates some image information (e.g., the total image flux density and the
image centroid), this information can be recovered through a small number of
additional constraints. We demonstrate that closure-only imaging can produce
high fidelity results, even for sparse arrays such as the Event Horizon
Telescope, and that the resulting images are independent of the level of
systematic amplitude error. We apply closure imaging to VLBA and ALMA data and
show that it is capable of matching or exceeding the performance of traditional
self-calibration and CLEAN for these data sets.
