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I usually think of SOFIA, the Stratospheric Observatory for Infrared Astronomy as an infrared optical telescope:

SOFIA uses a 2.5 m (8.2 ft) reflector telescope, which has an oversized, 2.7 m (8.9 ft) diameter primary mirror, as is common with most large infrared telescopes. The optical system uses a Cassegrain reflector design with a parabolic primary mirror and a remotely configurable hyperbolic secondary. In order to fit the telescope into the fuselage, the primary is shaped to an f-number as low as 1.3, while the resulting optical layout has an f-number of 19.7. A flat, tertiary, dichroic mirror is used to deflect the infrared part of the beam to the Nasmyth focus where it can be analyzed. An optical mirror located behind the tertiary mirror is used for a camera guidance system.

and one thing that makes that happen is that it looks just like a "normal" telescope with shiny polished and metal coated mirrors.

I've just seen Phys.org's Stellar feedback and an airborne observatory: Team determines a nebula to be much younger than previously believed (which links to Stellar feedback and triggered star formation in the prototypical bubble RCW 120) which talks about a "1.9-THz fine-structure line".

As far as I can tell this is done with little antennas as pixels, a heterodyne down-converter and radio receivers just like the Atacama Pathfinder Experiment data at 345-GHz it compares to.

In other words, it sounds just like a low granularity focal-plane array on a dish radio telescope.

I am not asking about the difference between the overlapping labels of radio and infrared as they apply to the electromagnetic spectrum, though that could well be part of an answer. These days radio astronomers call what they receive "light" as often as optical astronomers do. However the equipment used is different. Radio offers the possibility of recording phase for off-line interferometry, whereas optical detector arrays are generally intensity based only. So instead, am simply asking:

Question: Is Sofia a radio telescope proper?

SOFIA

Source

Figure 1 from "Stellar feedback and triggered star formation in the prototypical bubble RCW 120"

A) SOFIA [CII] integrated intensity, scaled from 0 to 260 K km/s. The red circles indicate the approximate inner and outer PDR boundaries defined from Spitzer GLIMPSE 8-μm emission (55), and the red star shows the location of the ionizing source, CD −38°11636. The yellow “+” indicates “Position 1” (see the Supplementary Materials). (B) Spitzer GLIMPSE 8-μm emission. The contours are of [CII] integrated intensity, scaled from 40 to 160 K km/s in 40 K km/s increments. (C and D) APEX 12CO(3-2) and 13CO(3-2) integrated intensity, scaled from 0 to 260 K and 0 to 90 K km/s, respectively. The contours are the same as in (B). The areas enclosed by the dashed yellow lines in (A), (C), and (D) were used to extract the position-velocity diagrams shown in Fig. 2.

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    $\begingroup$ Side remark: The offical SOFIA announcements stated on April 4th, 2021 about the current measurement campaign: "Now, the observatory [=SOFIA] will take advantage of its proximity to science teams at the Max Planck Institute of Radio Astronomy in Bonn and the University of Cologne, which operate the instrument called German Receiver at Terahertz Frequencies, or GREAT, to conduct research flights from the Cologne Bonn Airport." $\endgroup$
    – B--rian
    Apr 14, 2021 at 9:42
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    $\begingroup$ Note that the rest of SOFIA's instruments operate at optical to mid-IR wavelengths, so it's primarily an infrared-optical telescope. I'm not sure it makes a lot of sense to call something capable of taking near-UV CCD images (with the FPI+ instrument) a "radio telescope proper". $\endgroup$
    – Peter Erwin
    Apr 14, 2021 at 12:34
  • $\begingroup$ Given the fact that SOFIA flies around while taking observations, I don't think it would work as an element in an interferometric array (where you need to know the positions of the elements very precisely), even if you could record phase information. (I don't see any evidence that the GREAT instrument can record phase information.) $\endgroup$
    – Peter Erwin
    Apr 14, 2021 at 13:24
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    $\begingroup$ @PeterErwin which in turn offers the possibility of calling a telescope using that technology a radio telescope. The purpose of the paragraph is to head off comments like "You have to tell us what your definition of a radio telescope is." or "What is your definition of a radio telescope proper?" After asking over 3,000 Stack Exchange questions I've found that there are a few classes of recurring comments, so I try to always explain enough so that they are addressed pre-emptively, rather than defensively after the comment is dropped forever under the question. $\endgroup$
    – uhoh
    Apr 14, 2021 at 13:38
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    $\begingroup$ @uhoh OK, I really don't understand what your question is. (I guess the answer would be, "Sure, every optical-IR telescope ever built 'offers the possibility' in some nebulous sense, so they're all 'proper' radio telescopes.") $\endgroup$
    – Peter Erwin
    Apr 14, 2021 at 13:58

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The GREAT receiver attached to SOFIA to detect far-infrared radiation incorporates elements reminiscent of both radio and optical detectors, but since it works by mixing a signal from a tunable oscillator with the oscillating field from the incoming radiation rather than by the photoelectric effect as in most optical detectors, you might consider it more akin to a radio telescope receiver.

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  • $\begingroup$ Thanks! I'm just curious why stop at "might... more akin to" and not just "is"? When operating with a small array of receivers at the focal plane isn't it the same as these dishes? My thinking is that it is 100% a radio telescope when operated like this, as there's no functional difference between it and other things that are called radio telescopes except a higher frequency. Compare to the single-dish Parkes multi-beam receiver for example. $\endgroup$
    – uhoh
    Apr 14, 2021 at 21:50

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