Reading this question I was struck by the fact that the "pointing" of an interferometer is effectively done in the correlator, at least within the common field of view of all the telescopes, which is many thousands of times wider than the imaged area. Could the same data be reprocessed to reveal images of other "tiles" from this common field of view? Could it image the accretion disk, for example, or the base of the jets? Are there any targets that are close enough and bright enough to be worth doing this?
The question asks about "targets" but doesn't specify black holes. However the other "targets" are related to infall of black holes.
Nonetheless, I think the answer is a solid "yes" either way.
Data is recorded separately at each site. Hours and hours of digitization of one or more 2 GHz wide data channels around 230 GHz, down converted to baseband and synched using a local atomic clock. See this answer and this answer
Any signal reaching the feed horn of each radio telescope is going to be recorded in its data stream. So if there is another source nearby that is covered by the radiation pattern of all of the dishes in the EHT collection, then it certainly could be imaged to some (indeterminate) level of resolution, with (at least) the following caveats:
- You have to run the numerical correlations again, with a new range of time-offsets for each telescope. If the source is much brighter, you wouldn't have to use the whole dataset. But if it's just as dim or dimmer, you will have to correlate lots of data.
- If the source is far away from the imaged event horizon, there may be several different types of calibrations for each dish and site that have been optimized, and may not work as well for a substantially removed object. The existing data may result in slightly lower resolution far from the target, depending on how calibrations were done. In addition to signals of the black hole, signals from other sources were recorded to assist in calibration of the whole array.
I need to read further on that, I'll update the part about calibration tomorrow.