# Do we have the technology to create a satellite telescope constellation that uses interferometry?

Apologies for putting the bulk of the question in the description. I'm honestly not sure how to ask this properly. The idea for this question came from a Vsauce video from several year ago I can't even find again.

As I understand it we can combine the image from multiple telescopes into one, increasing the resolution, but not the brightness. The further away these two telescopes are from each other the better resolution, and as I understand it we've done this with Earth based telescopes.

To me the next logical step would be to put a couple of these telescopes into space. The distances would give incredible resolution, no?

Do we have the technology to do this kind of thing, if so what kind resolution would it get? Also how precisely do we need to know the satellite's position? The Earth based telescopes like this I know of are mechanically connected.

• Combining light (including radio waves) from two or more telescopes does increase the brightness as well as the resolution: you're receiving and measuring more photons. Commented May 22 at 8:19
• Possibly you're confused by the fact that an array of small telescopes spanning some total diameter can provide resolution equivalent to a monolithic mirror/antenna of the same diameter, but won't collect as much total light (because the total collecting area of the array is smaller). Commented May 22 at 8:22
• LISA plans to use interferometry to carefully track the distances between a constellation of satellites. I suppose it might be possible in principle to build a LISA-like mission with a telescope strapped to each satellite in the constellation. Commented May 22 at 11:34
• As far as precision of positioning, we don't need to know the satellites' positions as such, but we do need to know the precise distance between them, with accuracy determined by the frequencies we're looking at. For meter- to centimeter-long radio waves, it's not that hard, but for visible light we have to be precise down to nanometers, so it's a lot harder to arrange and maintain. Earth based radio telescopes are often set up for interferometry without being physically linked -- see the VLA for example. Commented May 22 at 16:17

If we look at both existing, Earthbound optical and radio interferometers together, we see a general trend. When all the "collectors" (telescopes or dishes) are within of order one hundred meters or less from each other, optical beam lines or radio transmission lines have been successfully used to combine the signals.

For longer baseline interferometry, the signals are usually down-converted and digitized locally (say a few Gbit/sec bandwidth max) before they are transmitted to a central location. ALMA and the future SKA use fiber optic cables.

For longer baselines like Event Horizons Telescope the data is locally recorded on old-fashioned hard drives, then flown to a central location where interferometry is implemented off-line in a big computer. This required very careful determination and recording of the relative positions of all the dishes around the planet.

An exception is Spekt-R which down-converted, digitized, then live-transmitted the 2 x 72 Mb/sec data to Earth over the sometimes very long baseline, only while it was visible to the receiving ground station.

As I understand it we can combine the image from multiple telescopes into one, increasing the resolution, but not the brightness. The further away these two telescopes are from each other the better resolution, and as I understand it we've done this with Earth based telescopes.

Yes that's right. I don't yet know how optical imaging data is combined interferometrically...

but in optical interferometer "beam lines" without digitzation, I believe you need periodic placement of relay lenses because light diverges whether you want it to or not. In space over long distances, conservation of etendue prevents us from optically trasporting an image over a very long distance.

That means for optical as well as radio, you need to down-convert the eletromagnetic radiation to a manageable frequency where it can then modulate (either digitally or via analog techniques) a stable laser beam (analagous to the fiber optic lasers used in ALMA), then shoot that free-space optical connection between your satellites.

You might get a THz of bandwidth, which would correspond to say a 1 nm wide band at 1 micron, so you won't be doing broadband imaging.

In my personal opinion if you had several billion dollars in cash handy, you could put a few satellites in orbit based on existing technology. It would take time to get it to work well (the devil is always in the details) but there is no fundamental show-stoppers here beyond sufficient funding and brainpower.

• I suspect it would be much more difficult for optical/near-IR than you're assuming. For example, the "intensity interferometry" ("Brown-Twiss-Townes") method you refer to in your last link appears to requires interfering the incoming light with a laser of the same frequency, and this laser is used for both/all individual telescopes, with the phase shift between different telescopes carefully controlled (which is relatively easy to do when the telescopes aren't constantly moving relative to each other). Commented May 22 at 10:44
• @PeterErwin the link does not refer to intensity interferometry. It's the same heterodyning that AM and short wave radios have done for a century, and everyone's local oscillator is different than everyone else's. As long as each local oscillator is stable, or its drift is checked against a local atomic clock (the way the Event Horizon Telescope does it) there is no need for real-time phase stabilization. Read my answer to physics.stackexchange.com/q/473320/83380 A drift in physical position and in local oscillator frequency can both be repaired off-line.
– uhoh
Commented May 22 at 11:07
• I was referring to this: "These may be what I was thinking of, an optical intensity interferometer based on the Hanbury Brown and Twiss effect effect", and also the Johansson & Letokhov arXiv link in the same answer, which describes using "a tunable semiconductor laser diode transporting its radiation via an optical fiber". (I may have misunderstood that last bit.) Commented May 22 at 16:43