[mapt]

Optical band sensors are not fast and low-noise enough, by several orders of magnitude, to do the sort of digital interferometry available with radio telescopes. Optical interferometers so far require optical correlators. This geometric rather than data-analytical approach mandates knowing and controlling the delay distances between the beams of light down to the nanometer. Combined with adaptive optics, this is some of the highest resolution imagery we can produce; But it is extremely limited because of the practical difficulties of these hardware concerns, even in a situation where there is a massive underground lab connecting the telescopes like at the VLT. Satellites floating in free space on opposite sites of the Earth affected by differential atmospheric drag, geomagnetic and heliomagnetic effects, and even relativistic differences would present what appears to be an intractable problem for this technology.

[omegant]

What about some kind of periodical signal with known timing and fase that could be used to calibrate and syncronise the signals at post processing? Something like a film clapping board, but in space.

[mapt]

Several orders of magnitude too slow sensors to discern that signal, again; to say nothing of actually controlling the spacecraft's distance. The periodical signals you mention would be just fine for discerning the position of radio interferometry nodes because it's a lot less of a technical challenge and because we can afford to do a few days of computational work for a given millisecond of data, but not at the nanometer & femtosecond scale of precision in real time at low latency.

The difference between radio and optical-infrared regions of the EM spectrum is not just about what the human eye can see, it's a fundamental difference in how we measure - the former allows us to directly sense the shape of the EM waves as they come in, while the latter is much higher resolution, but forces us to indirectly make conjectures based upon how much energy was deposited onto some specialized sensor in a macro-scale time period.

Here's what Wikipedia says about the boundary region between the two bands, which is poorly studied because neither paradigm works well: "Terahertz radiation occupies a middle ground between microwaves and infrared light waves, and technology for generating and manipulating it is in its infancy, and is a subject of active research. It represents the region in the electromagnetic spectrum that the frequency of electromagnetic radiation becomes too high to be measured by directly counting cycles using electronic counters, and must be measured by the proxy properties of wavelength and energy. Similarly, in this frequency range the generation and modulation of coherent electromagnetic signals ceases to be possible by the conventional electronic devices used to generate radio waves and microwaves, and requires new devices and techniques."

The type of precision required here is well out of the reach of normal engineering, it requires specialized tools like diffraction interference wavefront sensors just to make sure the shape of the mirror is precise enough. That we've gotten it working at all with multi-focal optical systems (for very bright stars, at least) is a bit of a miracle, and some of the techniques required certainly involve the type of signal you're talking about just to get it operating in the lab. Measuring distance with this kind of resolution is just not something we have to do often, especially for very large distances.

The next step in optical interferometry is certainly not this kind of solution, it's building a space interferometer with a normal real-time optical correlator, as a single structural satellite, 2 mirrors on opposite ends of an enclosed truss. A project modeled on that principle with 2x 50cm mirrors has been a bridge too far for us, delayed and cancelled with NASA budget cuts: https://en.wikipedia.org/wiki/Space_Interferometry_Mission

[omegant]

Amazing reply, thank you very much! This is an awesome physics unexplored field (maybe not unexplored, but yet with lots of stuff to learn) How cool would be a kind of antenna that is able perform the same function as a mirror but in an electronic way and not in a visual one.