In this video Katie Bouman mentions that an earth sized telescope needs to be used for getting images like the M87 Black Hole:

Stellar distances are estimated using parallax measurements at different sides of the earth's orbit. Why can't we send telescopes to diametrically opposite ends of the earth's orbit and take images and later merge them using the same algorithms? Its seems that only a few telescopes were required and we could probably send a few across the solar system or larger distances. This would help us resolve more details.

  • $\begingroup$ Interesting question about spacecraft, space telescopes and various solar system orbits, and is definitely on-topic here. $\endgroup$
    – uhoh
    Jul 25, 2019 at 5:49
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    $\begingroup$ geez I miss black boards, so much easier than spending the night before battling with PowerPoint. $\endgroup$
    – uhoh
    Jul 25, 2019 at 6:55

1 Answer 1


This is a great question!

Parallax and stellar positions

To measure an object's parallax assuming no proper motion, we can get by with as few as two images of a foreground (moving) star against a background of several "fixed" stars. We only need enough resolution to make out one star's diffraction (Airy) disk from the other stars with enough precision to resolve the movement due to parallax.

You can measure the center location of a "blob" with much more precision than the FWHM of the blob, as long as you have plenty of photons and you can get a good handle on systematic errors and pixel-size-related issues. As long as these are separate and individual stars, you don't need to resolve each star's disk to 1 mas to get a relative distance between them with precision of 1 mas.

The Gaia spacecraft is an example of a space telescope with a modest (rectangular) aperture of about 0.5 x 1.4 meters, yet it produces huge volumes of extremely precise parallax measurements.

Over the period of a half year, Gaia does move to "diametrically opposite ends of the earth's orbit" and it records images continuously. The idea is to get five to seven images of most of the sky in order to separate out the parallax from the proper motion from the systematic and random sources of noise.


However, these are images and contain no phase information. As I explained in this answer to Is Digital Adaptive Optics Possible? the techniques we use to produce and record optical images generally lose all phase information, what you are left with is only intensity, not complex amplitude.

Interferometry requires the interference of amplitudes and the phase of each signal is key. As pointed out in that answer, we can do this for microwaves and lower frequencies using high speed (GHz) ADCs and often some amount of down-conversion, but we don't generally do this for visible light.

For the EHT's image of the black hole, they used atomic clocks at each telescope's site and synchronized them using things like GPS time signals and nearby calibration objects in the sky.

Not that this hasn't been demonstrated for visible or near IR light, but it's not something you can easily put in a satellite. One way you could do this is share a laser beam between two spacecraft and mix it with a narrow range of optical frequencies from each of the telescope and record the resulting heterodyne signal with a GHz or hopefully much higher bandwidth. You'd also have to reconstruct the distance between the two satellites to the order of a wavelength of light to get any kind of meaningful data.

This is not impossible, but it is really hard and would be quite a technological and budgetary challenge.

  • $\begingroup$ You're asking for two things in this answer: optical or IR VLBI interferometry, and a baseline at least 2 AU long. They are independent. It seems entirely feasible (if probably very expensive) to launch a number of mm-wave telescopes comparable to the EHT instruments and do something very like the EHT but with an AU-scale baseline. Switching to much shorter wavelengths is something you'd want to explore on Earth first. $\endgroup$ Jul 25, 2019 at 8:26
  • $\begingroup$ @SteveLinton I'm not asking for anything in this answer. The question asks about optical interferometry with a 1 AU baseline (opposite ends of the Earth's orbit). I'm describing what pieces have been developed (i.e. current state of the art in interferometry) and what would have to be done to achieve what the OP is asking for. (e.g. "Not that this hasn't been demonstrated for visible or near IR light..." means that down-conversion of light to microwaves followed by interferometry has already been demonstrated on Earth, a while ago but you'd need something else for 1 AU) $\endgroup$
    – uhoh
    Jul 25, 2019 at 8:34
  • $\begingroup$ No matter your observation wavelength is visible light or microwaves, if you want a 1 AU baseline you'll have to either down convert to of order GHz and digitize, or use an optical reference beam over the 1 AU baseline. Either way "would be quite a technological and budgetary challenge." $\endgroup$
    – uhoh
    Jul 25, 2019 at 8:36
  • $\begingroup$ Downconverting to GHz and digitizing is exactly what the EHT does now. $\endgroup$ Jul 25, 2019 at 11:05
  • $\begingroup$ @SteveLinton yes it is, as do ALMA and several other arrays. It's a totally standard technique called a superheterodyne receiver and its 120 years old (also here). Why not post an answer to the OPs question from your own perspective? I don't understand what it is that troubles you about my answer; perhaps the quickest way to find out is for you to write a better answer to show me how it's done? $\endgroup$
    – uhoh
    Jul 25, 2019 at 11:15

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