I wonder why only 4 days of data were used in the M87 Event Horizon Telescope (EHT) observation. What is preventing you from obtaining a more detailed and clearer image over a longer period of time?
This is a really good question in that there are good answers to it.
Alas I'm no expert, but I will get the ball rolling.
About the "only four days", this is almost certainly only the period for the data that ended up in the final image. There is probably a much larger body of observing time spent in technology development. The EHT really pushed VLBI radio Astronomy to new limits, one of which is processing the recorded data offline to discover the exact distances and orientations of each telescope dish with respect to the other to sub-millimeter accuracy. For more on that see Physics SE's Precise Event Horizon Telescope locations?
The "light" (radio waves) in the image comes from swirling, changing hot gas in the accretion disk
Now for the data contributing to the image, I can't state with certainty that this is the exact and complete answer, but it is certainly a limiting factor for exposure time.
The goal is to produce a static image, not a movie. To use interferometry to build up a clear "snapshot" requires the exposure time to be fast enough that the object doesn't blur.
The algorithms they used at the time to generate that image from that pretty sparsely populated array of telescopes, for which at any given moment a significant fraction can't see the target, assumed that the the target was static, that it didn't change. See
- How can the Event Horizon Telescope image Sgr A* when it's not visible from all sites at one time?
- How does the Event Horizon Telescope implement the interferometry?
above: (click for larger) "The eventual EHT array will have 12 widely spaced participating radio facilities". From The BBC's February 16, 2017 Event Horizon Telescope ready to image black hole.
It's the same way we assume, or at least hope that our targets don't move much during our photograph exposure times of a fraction of a second, but for the EHT's image of M87* that shutter time was four days.
The problem of the motion of the material in the accretion disk is one reason that M87* was chosen instead of our own black hole SgrA* in the center of our own Milky Way galaxy. M87's black hole is much much larger than ours and the accretion disk is much brighter and more active at radio wavelengths, offsetting to some extent its larger distance, but because its event horizon and accretion disk are so much larger than ours, it changes much more slowly.
To my understanding that's why it was chosen as the EHT's first target, rather than SrgA*. For more on that see answers to:
- How will they know when to start taking the picture of the black hole at the center of the Milky Way?
- Will the first Event Horizon Telescope image of the Milky Way's black hole Sgr A* just be another lumpy orange donut?
- Why didn't the Event Horizon Telescope team mention Sagittarius A*?
- Why not take a picture of a closer black hole?
- Why doesn't the black hole in the center of the Milky Way glow similarly to the famous M87 image?
- Why does the Event Horizon Telescope (EHT) not include telescopes from Africa, Asia or Australia?
Illustration of what photographing a spinning object might look like where the exposure time (shutter speed) is too long and the motion too fast: