What you have to stitch together is effectively the Fourier transforms of the images. This is quite a complicated process. When I was more active in this field, this would have been thought quite impossible at such short wavelengths, because of the effect of the atmosphere on submillimetre wave radiation. Clever techniques, known as closure phase and self-calibration are the key to this observation.

I was wrong about one major thing, they showed today the image of the centre of M87, an elliptical galaxy in the Virgo cluster of galaxies, not of our own galaxy. They are doing our own galaxy but havn't finished analysing the data yet.