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How big a VLBI baseline would they need to see much more detail? Are there any plans for a space based VLBI; not easy when you consider the utterly huge amounts

How big a VLBI baseline would they need to see much more detail? Are there any plans for a space based VLBI; not easy when you consider the utterly huge amounts of information they have to transfer, a radio telescope in a L5 would be a start. Also I'd be interested to read how they got around scintallation of the Interstellar Medium.

thatcherc 8y ago
Space VLBI is a thing already! Seems to have been around for a while [0][1]. I imagine that the challenge with during EHT-style VLBI in space is downlink bandwidth... there's just no way to get the petabytes of data they talk about down from a satellite. It's already so much data that it's easier to fly hard drives around the world than to send it over the internet, so the transfer would probably take ages over a microwave link. Maybe with laser communication in the near future but even then... it's a lot of data. Resolution-wise, the angular resolution is inversely proportional to the farthest baseline, as given by the Rayleigh criterion. [2] To get twice the resolution, you "just" need to double the size of your baseline (and do that in both dimensions, otherwise the angular resolution will be different in x & y). We've maxed out the Earth's baseline, so it seems like orbital radio telescopes are the only way to better resolution. Pretty exciting! [0] - https://www.jpl.nasa.gov/missions/space-very-long-baseline-interferometry-space-vlbi/ https://www.jpl.nasa.gov/missions/space-very-long-baseline-i... [1] - https://asd.gsfc.nasa.gov/blueshift/index.php/2016/07/25/thirty-years-of-space-vlbi/ https://asd.gsfc.nasa.gov/blueshift/index.php/2016/07/25/thi... [2] - https://en.wikipedia.org/wiki/Angular_resolution https://en.wikipedia.org/wiki/Angular_resolution
- privong 8y ago
  > We've maxed out the Earth's baseline, so it seems like orbital radio telescopes are the only way to better resolution. Going to higher frequency gives you higher resolution for a given physical baseline length.
  - JoeAltmaier 8y ago
    For some observations, they've used the earth's moving position in space to create a synthetic-radar style image. That gives an 'aperture' of 180M miles.
    - privong 8y ago
      > For some observations, they've used the earth's moving position in space to create a synthetic-radar style image. That gives an 'aperture' of 180M miles. That may not work for imaging the immediate surroundings of black holes – one can only combine the data for that SAR-style imaging if the source you're observing doesn't vary significantly (e.g., in brightness or flux distribution) between observations.
- richk449 8y ago
  > I imagine that the challenge with during EHT-style VLBI in space is downlink bandwidth... there's just no way to get the petabytes of data they talk about down from a satellite. Why do you say that? The latest High Throughput commercial satellites do 500Gpbs downlink throughput, so shouldn't the transfer time for a petabyte of data be reasonable? Of course those satellites are configured to transmit data all over the earth, so the technology would have to be used differently for this application.
privong 8y ago
> Are there any plans for a space based VLBI; not easy when you consider the utterly huge amounts of information they have to transfer, a radio telescope in a L5 would be a start. There is space-based VLBI at lower radio frequencies, with the "Radio Astron" project[0]. That effort works at frequencies which are roughly a factor of ~10 lower than that of the EHT. I'm not aware of any plans for millimeter Space VLBI, but the higher frequency would require higher data rates. [0] http://www.asc.rssi.ru/radioastron/ http://www.asc.rssi.ru/radioastron/
stargazer-3 8y ago
>> Also I'd be interested to read how they got around scintallation of the Interstellar Medium. One of the reasons they've imaged M87* rather than a black hole in our own galaxy was to avoid dealing with ISM scattering - we don't have to look through our edge-on disk, so it's easier to image another galaxy's center somehow. But the Sgr A* image might be in the works already, it was mentioned in both the press-conference and one of the paper's future work section.
- zeristor 8y ago
  How does one even deal with ISM scattering? The optical equivalent can be dealt with by active optics, or picking frames with best seeing, but for ISM I imagine it is a fair bit different.
pps 8y ago
You can ask them directly: https://www.reddit.com/r/askscience/comments/bbknik/askscience_ama_series_we_are_scientists_here_to/ https://www.reddit.com/r/askscience/comments/bbknik/askscien...

thatcherc 8y ago

Space VLBI is a thing already! Seems to have been around for a while [0][1]. I imagine that the challenge with during EHT-style VLBI in space is downlink bandwidth... there's just no way to get the petabytes of data they talk about down from a satellite. It's already so much data that it's easier to fly hard drives around the world than to send it over the internet, so the transfer would probably take ages over a microwave link. Maybe with laser communication in the near future but even then... it's a lot of data. Resolution-wise, the angular resolution is inversely proportional to the farthest baseline, as given by the Rayleigh criterion. [2] To get twice the resolution, you "just" need to double the size of your baseline (and do that in both dimensions, otherwise the angular resolution will be different in x & y). We've maxed out the Earth's baseline, so it seems like orbital radio telescopes are the only way to better resolution. Pretty exciting! [0] - https://www.jpl.nasa.gov/missions/space-very-long-baseline-interferometry-space-vlbi/ https://www.jpl.nasa.gov/missions/space-very-long-baseline-i... [1] - https://asd.gsfc.nasa.gov/blueshift/index.php/2016/07/25/thirty-years-of-space-vlbi/ https://asd.gsfc.nasa.gov/blueshift/index.php/2016/07/25/thi... [2] - https://en.wikipedia.org/wiki/Angular_resolution https://en.wikipedia.org/wiki/Angular_resolution

privong 8y ago
> We've maxed out the Earth's baseline, so it seems like orbital radio telescopes are the only way to better resolution. Going to higher frequency gives you higher resolution for a given physical baseline length.
- JoeAltmaier 8y ago
  For some observations, they've used the earth's moving position in space to create a synthetic-radar style image. That gives an 'aperture' of 180M miles.
  - privong 8y ago
    > For some observations, they've used the earth's moving position in space to create a synthetic-radar style image. That gives an 'aperture' of 180M miles. That may not work for imaging the immediate surroundings of black holes – one can only combine the data for that SAR-style imaging if the source you're observing doesn't vary significantly (e.g., in brightness or flux distribution) between observations.
richk449 8y ago
> I imagine that the challenge with during EHT-style VLBI in space is downlink bandwidth... there's just no way to get the petabytes of data they talk about down from a satellite. Why do you say that? The latest High Throughput commercial satellites do 500Gpbs downlink throughput, so shouldn't the transfer time for a petabyte of data be reasonable? Of course those satellites are configured to transmit data all over the earth, so the technology would have to be used differently for this application.

privong 8y ago

> Are there any plans for a space based VLBI; not easy when you consider the utterly huge amounts of information they have to transfer, a radio telescope in a L5 would be a start. There is space-based VLBI at lower radio frequencies, with the "Radio Astron" project[0]. That effort works at frequencies which are roughly a factor of ~10 lower than that of the EHT. I'm not aware of any plans for millimeter Space VLBI, but the higher frequency would require higher data rates. [0] http://www.asc.rssi.ru/radioastron/ http://www.asc.rssi.ru/radioastron/

stargazer-3 8y ago

>> Also I'd be interested to read how they got around scintallation of the Interstellar Medium. One of the reasons they've imaged M87* rather than a black hole in our own galaxy was to avoid dealing with ISM scattering - we don't have to look through our edge-on disk, so it's easier to image another galaxy's center somehow. But the Sgr A* image might be in the works already, it was mentioned in both the press-conference and one of the paper's future work section.

zeristor 8y ago
How does one even deal with ISM scattering? The optical equivalent can be dealt with by active optics, or picking frames with best seeing, but for ISM I imagine it is a fair bit different.

pps 8y ago

You can ask them directly: https://www.reddit.com/r/askscience/comments/bbknik/askscience_ama_series_we_are_scientists_here_to/ https://www.reddit.com/r/askscience/comments/bbknik/askscien...