# The Naked Scientists Forum

### Author Topic: How powerful can an interferometer/telescope array be?  (Read 2523 times)

#### Supercryptid

• Hero Member
• Posts: 606
##### How powerful can an interferometer/telescope array be?
« on: 21/09/2013 21:31:53 »
I thought it was pretty neat to learn that several relatively small telescopes/antennae can be linked together over a distance to simulate a much larger device. The Very Large Array in New Mexico is one such example.

I ended up thinking in terms of extremes and began wondering just how powerful an array linking telescopes on Mercury and telescopes on Pluto might be. I ended up calculating that such an array could theoretically directly image an Earth-sized planet anywhere in the visible Universe if the measured wavelengths were 10 nanometers (the lower end of the UV spectrum). However, that seems awfully extreme and I can't help but wonder if I am making some faulty assumptions in the telescope's design. Are there limits on how far away or how detailed an array can image objects? Is the limit based on quantum physics? I imagine there should be some limit to how many photons are reflected by an astronomical object and that should represent some limit.

For those wondering, I used the following equation in my calculations: Angular resolution = wavelength / telescope separation (baseline), where angular resolution is in radians and wavelength and baseline in meters.

I got the equation here: http://en.wikipedia.org/wiki/Angular_resolution#Telescope_array

#### CliffordK

• Neilep Level Member
• Posts: 6321
• Thanked: 3 times
• Site Moderator
##### Re: How powerful can an interferometer/telescope array be?
« Reply #1 on: 22/09/2013 05:12:19 »
Perhaps the Voyager Probes can be re-purposed for use as a radio telescope, except that I think they only each have a single large antenna, and limited computer memory.  And, if you managed to "loose Earth", it would be a tragedy.  Or...  did they have a backup antenna (wasn't one of the probes using the backup antenna as the primary?)

If you were building your space based interferometer, there would be no need to only use planets and moons, unless you needed to use them as a shield (like using mercury to shield from the sun, except that Mercury slowly rotates).  You'd get better 3D focusing and relaying without the probes being planet based.  As well as getting away from radiation and magnetic fields of the large planets.

It appears as if you should be able to make an optical interferometer telescope as easily as a radio interferometer, although timing would still be an issue with the receivers spaced hours apart.  Distance between receivers could be maximized by only putting receivers in the outer solar system.

Resolution in Pixels would be based on the number of receivers, I think.  But, given an arbitrarily large budget, and assuming one could plan for long lifespan of the systems (a century?)  Then there would be no reason why you couldn't slowly put up hundreds or thousands of probes.  The probes would be best if imaging perpendicular to their plane, but assuming one could do the necessary gravitational assists, there would be no need to keep the probes orbiting in the solar system plane (and some could be sent out of the solar system too).

I'm not familiar with all of the space systems, but I don't see why one couldn't have better multipurpose probes and systems.  So, send a satellite to Mars, and also plan on it being a leg of a large space based interferometer telescope.

It would be a pain to have everything always moving, as well as having your "telescope" being spaced out at about 10+ light hours side to side (5+, side to middle).  Say you wished to give 1 hr for each image, then would you be re-positioning parts of the array before other parts of the array had received their data?

Anyway, I think your calculations are correct.  Wikipedia does seem to indicate a need to have multiple telescopes, but doesn't fully quantify it.  Another issue.  Consider that Earth is moving at about 30 km/s (108,000 km/h), or displacing through its diameter in about 7 minutes.  Movement is always nice when imaging stuff, but it would also add a significant complication when imaging stuff using an array that was spaced light hours apart.  Unless one picked the planets up at the tangent from their star (giving only moderate reflected light, but the most optical distance from the star, and the speed would seem to be the slowest).

#### evan_au

• Neilep Level Member
• Posts: 4116
• Thanked: 245 times
##### Re: How powerful can an interferometer/telescope array be?
« Reply #2 on: 22/09/2013 05:18:47 »
To get a high-resolution image via interferometry, you need more than 2 small telescopes - you need a large number of telescopes, and it is better if they are not small. There are quite a number of challenges:
• If the telescopes cover the entire region, that equation holds. But it is very expensive to scatter many telescopes throughout space.
• If the telescopes are well-separated, they can resolve fine details. But sometimes you want to image large-scale structures, and closely-packed telescopes are better for this. That is why some interferomerters allow moving the telescopes between closely-packed and widely-spread configurations.
• Even if many telescopes are scattered in a line across the Solar System, that only provides increased resolution in one dimension. To be able to measure fine details in any direction, you need a three-dimensional spread of telescopes.
• Small telescopes are sufficient to image bright/close objects, but if the source is dim or distant, you can't pick up enough signal, and you need a large collecting area (ie larger telescopes, or more telescopes).
• You also need very accurate time coordination between the different telescopes, high-bandwidth and low error-rate communication from the telescopes back to the data-processing site, and massive computing power to integrate the results.
I am sure that astronomers would love a large interferometer such as you describe, but the challenge to current technology is trying to integrate one satellite-mounted telescope into an Earth-based interferometer.

#### CliffordK

• Neilep Level Member
• Posts: 6321
• Thanked: 3 times
• Site Moderator
##### Re: How powerful can an interferometer/telescope array be?
« Reply #3 on: 23/09/2013 01:17:16 »
I think I need a bit more in depth study of what these interferometer arrays provide one with.

However...

Are your eyes considered optical interferometers?  And your brain does excellent optical depth perception using just two eyes.
So, by inserting just two satellites into Neptune's orbit, one should be able to do pretty good optical interferometry.  The orbit around the sun would be about 164 years.  So, if one wished, it could be designed to view in a single equidistant plane including vertical and horizontal, completing a solar system orbit (full circle survey) in just under two centuries.  Using Earth as a third point for signal compilation, would generally simplify the design as the delay calculations would be minimized, or potentially it could be focused so that Earth would be the exact center, and there would be no signal delay necessary.  And, your distance between the two eyes would remain more or less fixed at say 8 light-hours.

The problem, of course, as Evan mentioned is that while one may get better depth discrimination, one wouldn't be amplifying the signal strength.

As far as I can tell, the brightness of the Sun vs the Earth is about a 100,000 fold difference which would remain even with improved depth perception from the two eyeball interferometry.

Could you build the system to be 100% optical from the mirrors and lenses in the Neptune orbit to the receiver at Earth.  And, thus bandwidth would not be an issue as the signal would be uncompressed optical, and upgrades of the receiver and processing equipment would be possible near Earth, and one would just need to maintain the focusing ability in the Neptune orbit, hopefully significantly reducing the maintenance and upgrade time and costs.  One might even be able to focus the images at Neptune using extremely cheap FLAT mirrors.
« Last Edit: 23/09/2013 01:31:50 by CliffordK »

#### evan_au

• Neilep Level Member
• Posts: 4116
• Thanked: 245 times
##### Re: How powerful can an interferometer/telescope array be?
« Reply #4 on: 23/09/2013 10:42:04 »
The existance of human Binocular vision does not imply that humans have a 2-element optical interferometer. Our brain correlates the shape and colour of objects in our vision to extract depth information.

However, interferometers correlate the phase of photons received at widely separated locations. The time resolution of the human optic nerve (about 1ms) is not sufficient to achieve this feat at optical frequencies.

Wide-Baseline Interferometry across the width of the Earth is possible at microwave frequencies using the timing accuracy of GPS and atomic clocks to achieve the necessary time/phase synchronisation.

However, the communication bandwidths and timing requirements are far more challenging to achieve the necesary phase alignment at the much higher frequencies of visible light. With today's technology, multiple telescopes forming an optical interferometer need to be mounted adjacent to each other, with a direct optical connection - either in adjacent telescope domes or mounted on a single mechanical structure.

As I recall, the technical challenges of optical interferometry come up in this podcast:
http://omegataupodcast.net/2012/12/111-optical-astronomy-and-the-large-binocular-telescope/

#### The Naked Scientists Forum

##### Re: How powerful can an interferometer/telescope array be?
« Reply #4 on: 23/09/2013 10:42:04 »