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Spektr-R is a 10 meter diameter radio telescope in Earth's orbit with an apogee of 338,000 km, nearly a Lunar distance. It does interferometry together with the Very-Long-Baseline Interferometry telescopes (VLBI on Earth at wavelengths 92, 18, 6 and 1.3 cm.

How hard would it be to build and operate multiple formation flying infrared space telescopes that do interferometry together? Infrared has many astrophysical applications, from small bodies in the Solar system to the earliest universe. JWST will observe in 0.6 to 28.5 µm, about a factor of 1,000 shorter than Spektr-R, while sub-millimeter starts only a factor of 13 shorter. Is there any rule of thumb for how much shorter a baseline would have to be when the wavelength decreases? One kilometer would still be much better than JWST's 6.5 meters.

Aside from making the interferometry work, it should be cheaper to build multiple smaller identical telescopes, with the same total mirror area as the huge LUVOIR and HDST envisioned. And it could be extended without having to touch existing hardware. Why is this not already on the to-do-list?

enter image description here

Spekr-R/Radioastron - the worlds largest space telescope.

enter image description here Credit: C. Godfrey (STScI). HDST, a proposed High definition Space Telescope. An 11 meter class aperture could be made from 54 peaces of 1.3 meters segments. In infrared, space interferometry seems to be an alternative to huge mirrors like this.

LocalFluff
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  • You should add some links. Spektr-R doing interferometry with other VLB arrays for example. I see a lot of conditional tense and future tense articles, but can you link to some actual interferometric results? And especially a link to that "to do" list - I really wanna read that! – uhoh Mar 26 '17 at 15:24
  • There is a handy list of papers here: http://www.asc.rssi.ru/radioastron/publications/publ.html also some news: http://www.russianspaceweb.com/spektr-r-2017.html and this about Спектр-7 https://i.stack.imgur.com/Q02zA.jpg – uhoh Mar 26 '17 at 15:49
  • VLBI gets more difficult when the wavelength gets shorter, because you have to record the electromagnetic waves themselves for interferometry to work. Optical interferometry has been done only with telescopes located next to each other (so you can do the interferometry with optical equipment instead of having to digitize the data) for this reason. – Hobbes Mar 27 '17 at 08:29
  • @uhoh I read a paper a year or two ago about Spektr-R actually resolving the surface of a neutron star! That is of course impossible and was AFAIU interpreted as a limit to resolution, much larger than a neutron star, set by the diffraction of light through the interstellar medium. If it is true that space content itself limits resolution, then bigger telescopes and longer baselines might not help. Surprisingly few papers seem to have been published on results from Spektr-R, when I search on it anyway. It's a Lunar distance interferometer, I'd thought it would be hot news. – LocalFluff Apr 03 '17 at 11:31
  • @LocalFluff That sounds precisely like the third paper in the list I liked above; Substructure in the scatter-broadened image discovered with RadioAstron on baselines up to 330 000 km. – uhoh Apr 03 '17 at 11:50
  • @uhoh Yeah. And that doesn't sound very helpful. But such scatter should be a lesser problem for higher energy photon radiation, right? And infrared is also easier for interferometry than what visual light or NIR is. Why isn't a multiple spacecraft IR interferometric space telescope loud and clear already on the top of astronomer's wish list? – LocalFluff Apr 03 '17 at 12:03
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    @LocalFluff the scatter may turn out to be absolutely fascinating in itself. Its not just diffuse haze or fuzziness, the few percent amplitude fringes they are seeing "...reflect(s) a substructure from scattering in the interstellar medium and offer a new probe of ionized interstellar material." It might turn out to have useful science when giant arrays of radio telescopes are in space in the not-so-distant future. – uhoh Apr 03 '17 at 12:19
  • I'm assuming that the Earth's atmosphere as well as galactic dust and gases impose some constraints (or offer favorable regions of analysis) on the choice of wavelengths. The comment above regarding need for wavelengths to be longer for digital processing to be feasible was helpful. Side-note: The Spectr-R satellite is apparently no longer functioning: bbc.com/news/world-europe-46849347 – 42- Apr 18 '19 at 17:57

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