Highest frequency that's been imaged by a radio telescope?

Question

In this answer to Should we update definitions and remove the 100 GHz hard limits on radio astronomy related tags? I wrote the following partial answer:

Yes, there are plenty of dishes that focus into waveguides or horns that feed antennas connected to electronic amplifiers using transistor amplifiers connected to heterodyne down-conversion with balanced mixers and then analog-to-digital conversion, where intensity images are produced by interference implemented mathematically in a computer (rather than in wavefronts incident on material producing intensity signals in the form of electrons (CCDs) or phonons (bolometers)) which work up to circa 1,000 GHz, so the 100 GHz limit is obviously wrong!

With the idea in mind that a radio telescope might be defined as using conversion to electrical signals before conversion from amplitude to intensity (image formation), what's the highest frequency where this has been done?

One number I know is 230 GHz, which was used by the Event Horizon Telescope

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The question How does ALMA produce stable, mutually coherent ~THz local oscillators for all of their dishes? suggests this is at least 950 GHz, but I don't know if that's a record or not. Narrow band optical emission could be mixed with a laser in a nonlinear crystal producing a microwave signal that could be detected via radio, so potentially the answer might be visible or near infrared light, but I don't know.

update: So I've gone ahead and asked this separately: Has optical interferometry been done at radio frequency using heterodyning with a laser in a nonlinear material?

Answer 1

Some extensive search e.g. with scholar.google.com led me to a manuscript from October 2020 entitled Design and Characterization of 275-500 GHz Corrugated Horns and Optics for a Wideband Radio Astronomy Receiver which already has all the information in the title: It looks like there is current development towards another 500 GHz receiver.

The question mentioned a 0.95 THz receiver which is even above that.

I also found an online course at nrao.edu Essential Radio Astronomy where the Introduction argues that 1 THz is the absolute upper boundary to far infrared in astronomy.

Radio astronomy is the study of natural radio emission from celestial sources. The range of radio frequencies or wavelengths is loosely defined by atmospheric opacity and by quantum noise in coherent amplifiers. Together they place the boundary between radio and far-infrared astronomy at frequency ν∼1 THz (1 THz ≡1012 Hz) or wavelength λ=c/ν∼0.3 mm, where c≈3×10¹⁰cms−1 is the vacuum speed of light. The Earth’s ionosphere sets a low-frequency limit to ground-based radio astronomy by reflecting extraterrestrial radio waves with frequencies below ν∼10 MHz (λ∼30 m), and the ionized interstellar medium of our own Galaxy absorbs extragalactic radio signals below ν∼2 MHz.

The radio band is very broad logarithmically: it spans the five decades between 10 MHz and 1 THz at the low-frequency end of the electromagnetic spectrum. Nearly everything emits radio waves at some level, via a wide variety of emission mechanisms. Few astronomical radio sources are obscured because radio waves can penetrate interstellar dust clouds and Compton-thick layers of neutral gas. Because only optical and radio observations can be made from the ground, pioneering radio astronomers had the first opportunity to explore a “parallel universe” containing unexpected new objects such as radio galaxies, quasars, and pulsars, plus very cold sources such as interstellar molecular clouds and the cosmic microwave background radiation from the big bang itself.

Telescopes observing from above the atmosphere have since opened the entire electromagnetic spectrum to astronomers, but radio astronomy retains a unique observational advantage. Coherent amplifiers, which preserve phase information, allow the construction of sensitive multielement aperture-synthesis interferometers that can image complex sources with angular resolution and absolute astrometric accuracies approaching 10−4 arcsec. Quantum noise forever restricts sensitive coherent amplification to the low photon energies E=hν (where h= Planck’s constant ≈6.626×10⁻²⁷ erg s) of the radio band. Also, coherent signals can be shifted to lower frequencies and digitized, permitting the construction of radio spectrometers with extremely high spectral resolution and frequency accuracy.