It's actually one of the sharpest images ever made - in terms of angular resolution. The Sag.A* ring is about 51 microarcseconds in diameter. The EHT has a theoretical resolution of ~25 micro arcseconds. For comparison NirCam on Webb has a pixel resolution of 70 miliarcseconds (about 2800 times worse resolution than EHT).
The reason it looks blurry is that the black hole features are close to the resolution of EHT so it's only a dozen or so pixels worth of information enlarged to typical image size.
EHT is essentially a radio telescope with a dish the size of the Earth. The only way to get higher angular resolution is to use higher frequencies (which they are working on) or use radio telescopes in space to get longer baselines than the diameter of the Earth.
I also want to point out that the mass around Sagittarius A* is rapidly shifting which makes getting a sharp image still harder. From the NYT:
Sagittarius A*, the black hole in the Milky Way galaxy, is a harder target. It is less than one-thousandth the mass and size of the M87 hole and, therefore, evolves a thousand times faster. The M87 black hole barely budges during a weeklong observing run, but Sagittarius A* changes its appearance as often as every five minutes.
> all while compiling an unprecedented library of simulated black holes to compare with the observations
It also sounds like this is the combination of an image-generating model hypothesis as well as the raw data itself. Ie, this is the image that the model produces which best-fits the sparse interferometry data.
> The only way to get higher angular resolution is to use higher frequencies (which they are working on) or use radio telescopes in space to get longer baselines than the diameter of the Earth.
Would it be possible to use the same trick in space? IE: get a baseline the diameter of Earth's orbit (roughly)?
For Interferometry to work the data from different baselines has to be collected at the same time so waiting 6 months does not help. Spacecraft could theoretically be used to extend the baselines but the volume of data to be transferred is prohibitive with current spacecraft comm tech.
Even on Earth they resort to shipping cases of hard drives instead of transferring over the Internet.
One technique where simply waiting 6 months works very well is in measuring parallax. The Gaia spacecraft takes advantage of this.
So I work at one of the participating institutes, and you're very much dead-on about transfer speeds/logistics limiting options here. I'm not an astronomer, so grain of salt, but the data is generated at a bit of an unwieldy pace: there are four collector nodes at each site, each of which generates ~16Gbps of raw data (though read speeds from disk after observation is more like 8Gbps). This, as you say, forces most locations to ship physical drives, and also makes centrally planning these observations rather tricky, as there's very little visibility into each station's observations until some time afterwards.
But I'm optimistic that some institutions (hopefully including mine!) will be able to transfer these data over the network after the next run of EHT observations. Exciting stuff for sure.
Where can I read more about how that volume of data is generated? I'm looking at the description of ALMA right now. So is each "node" a cluster of antennas? And each antenna is collecting a high-resolution snapshot across a wide frequency band. And each snapshot has to be timestamped. What's the sampling rate? It says the frequency range is 31GHz to 950GHz but how wide can a single snapshot be? Then to move it around are you using InfiniBand or something even faster?
So ALMA, being an array, is a bit of a different beast than single-dish telescopes like the one where I'm employed; they do indeed have an array of antennae and correlate all the collected data at a central correlator (at least, for normal observations). My institute has a single, much larger primary reflector (30 meter diameter) and does not require such a process during normal observation. However, during VLBI observations, which EHT is, the receiver is dumping data to four collector computers, which is what I was referring to as "nodes" generating ~16Gbps of data apiece.
I wish I could shed some more light on the ins-and-outs of exactly how these observations work, but I just run the computers, man. :) What I can tell you is that in order to move data off of the collector machines, they typically use m5copy, which is a part of the JIVE project (they have a Github repo: https://github.com/jive-vlbi/jive5ab). All communication between the control computer and the collectors happens within a private, physically-distinct network, but it's just standard commodity networking equipment between the control computer and the collectors. The folks in charge of the node's design are in the process of removing some of the bottlenecks to make electronic transfers more viable (the current spec doesn't even include a 10Gbps uplink!).
For the entire image? I'm not sure, but a lot. Usually an EHT observing session is days in length, and the responsible astronomer(s) will reside at the telescope for a week or so...but I don't believe that 100% of that time is spent feeding data into the collectors. I can ask my coworker tomorrow for clarification on the actual observing time and post an update.
> Spacecraft could theoretically be used to extend the baselines but the volume of data to be transferred is prohibitive with current spacecraft comm tech.
Space-based very long baseline interferometry (VLBI) has been done at lower frequencies, most recently with the RadioAstron satellite[0]. There hasn't yet been a VLBI satellite observing at the same frequencies that the EHT uses, but there are mission concepts being discussed. [1] discusses some of the technical challenges.
I wonder what sort of range/throughput you could get with "Heavy" versions of the inter-satellite communications lasers which SpaceX is putting on their Starlink satellites...
The reason it looks blurry is that the black hole features are close to the resolution of EHT so it's only a dozen or so pixels worth of information enlarged to typical image size.
EHT is essentially a radio telescope with a dish the size of the Earth. The only way to get higher angular resolution is to use higher frequencies (which they are working on) or use radio telescopes in space to get longer baselines than the diameter of the Earth.