For a wavelength of 1.3 mm, we'd want the time tagging to be better than a quarter of the wavelength at least - say 0.3 mm. The speed of light is 300 mm/ns (a foot per nanosecond is the shorthand beloved of circuit and chip designers). So, for 0.3 mm, we're going to have to get down to a wavefront tagging accuracy of 0.001 ns.
No clock is going to get there, but if we can get ~close enough, we can use a procedure called fringe fitting to determine the clock corrections by looking at the wavefronts. (Does it line up this way? How about this way? How about now? Yes, it's as laborious as it sounds, but computers, eh.)
This is all in the calibration of data, before we do the Fourier inversion to create images - the magic of radio interferometry is that we can record the signal to disk while preserving phase. Optical photons can not be recorded and played back with phase preserved - optical interferometry has to split up the photon streams and send different parts to be correlated against streams from other telescopes, so you run out of signal quickly. Meanwhile, we can record radio waves at the 27 VLA dishes, say, and play them back for correlation on all 27*26/2 = 354 baselines, no problem. That's why radio VLBI is a thing, but not optical VLBI.
Even as a professional radio astronomer, the underlying physics is deep and almost magical.
Hi, long baseline optical interferometrist here who specializes in modeling and image reconstruction.
To set the record straight, long baseline optical interferometry really is a thing. At present there are two optical interferometers operating in the USA and one under construction: Georgia State University's Center for High Angular Resolution Astronomy (CHARA), and the Navy Precision Optical Interferometer (NPOI), and New Mexico Tech's Magdalena Ridge Optical Interferometer (MROI, under construction). Europe operates the Very Large Telescope Interferometer (VLTI) in Chile. Australia has the Sydney University Stellar Interferometer (SUSI). Optical interferometers have been around for a really long time. Michelson famously measured the diameter of Betelgeuse in December 1920. The first image from an optical interferometer was of Capella produced by the University of Cambridge's COAST telescope in September 1995.
The key difference between VLBI and optical interferometry is that we must combine the light from each telescope in real time, rather than recording the RF data to disk and forming the interference patterns later using correlation. Our interference patterns are recorded on high speed cameras, extracted, calibrated, and then stored as OIFITS files. These files are then later reconstructed using a variety of methods, including Markov chain processes and regularized maximum entropy.
Except for the CLEAN deconvolution process, the methods used to reconstruct images from the EHT data are identical to what optical interferometry has been doing for decades (see https://iopscience.iop.org/article/10.3847/2041-8213/ab0e85, Section 2.2.2 for references to literature). The maximum entropy process used for optical interferometric image reconstruction was, in turn, developed for MRI image reconstruction.
Don't get me wrong, I am not attempting to trivialize the result of the EHT team. The effort involved is monumental and the result is astonishing. In fact, I suspect my facial expression was very similar to Katie Bouman's now famous photo when I first saw the image. Then my jaw hit the floor when I found that some of my work (Baron, Monnier, Kloppenborg 2010) was cited in their imaging paper! However, my first inspection of the "eht-imaging" and "SMILI" repositories has yet to reveal anything new or novel that is not regularly employed by optical interferometrists.
> the magic of radio interferometry is that we can record the signal to disk while preserving phase. Optical photons can not be recorded and played back with phase preserved
Why is that for the optical photons, when it’s “deeper” than just a higher frequency as you answered elsewhere?
Black holes are small things. They're just very heavy.
The moon is about 350,000 to 400,000 km away. By my estimate (of other people's estimate), we're looking at a spot in the sky roughly the size of a dime (US 10-cent coin) on the moon.
I think my reptilian brain understands the distance of the moon and the size of a dime.
The moon is very far away, but it DOES grow and shrink as it comes closer and further away from us. Using the size differential, you can get an innate feel to the size of the moon. Technically speaking, just driving towards (or away from) the Moon will have you traveling closer / further away from it, and give you a sense of scale.
https://en.wikipedia.org/wiki/Lunar_distance_(astronomy)#/me...
#/media/File:Lunar_perigee_apogee.png){kind=link}
The next time you drive to a major city or large, recognizable landscape, keep an eye on how big and small mountains (or buildings) are and how quickly they move parallax to the foreground. It really does give an instinctive sense of scale. Train this instinct well enough, and you can use it on the moon.
“You need to be able to tell the difference between one wave front and the next, and if the next wave front is 1.3 mm behind and traveling at the speed of light, then you need to reliably distinguishing between events 4 picoseconds apart (4 trillionths of a second). So, every telescope needs a shiny new atomic clock and a really fast camera. You begin to get a sense of why the data consolidation looks more like a cargo shipment than an email attachment; trillions of snapshots every second of not just the waves you’re looking for, but also the waves that will cancel out once all the data is processed. Add to that the challenge of figuring out (generally after the fact) where every detector is to within a fraction of a millimeter even as their orientation changes and the Earth rotates, and you’ve got a problem.”
Just stupefyingly complex and amazing!