Eight radio telescopes were used to image to capture the first image of a black hole but there are more radio telescopes that will be added to array to improve future images.
As more telescopes are added to the Event Horizon Telescope (EHT), they will be able to produce images of the emission around black holes. In general, the fidelity of images produced by an interferometric array increases as additional telescopes are added to the array.
Improved images will resolve the structure of black holes and image the emissions from black holes.
They have obtained ave obtained detections on baselines between Hawaii (SMA + JCMT), and the continental United States (LMT + SMT), with an angular resolution corresponding to 6 Schwarzschild radii for Sgr A*. ALMA doubled this angular resolution.
Future EHT observations may be able to obtain a resolution as fine as 1.5 Schwarzschild radii.
The table below shows the resolution (both in Microarcseconds and Sgr A* Schwarzschild Radii) achievable on Sgr A* with current and future EHT baselines.
The Schwarzschild radius for Sgr A* is 10 microarcseconds, an exceedingly small size even by astronomical standards. EHT observations to date have achieved a resolution of better than 60 microarcseconds— about the angular size of an orange on the moon. They are working to include other millimeter telescopes into the EHT array in order to improve the resolution of the EHT and eventually be able to produce images of the black holes in Sgr A*, M87, and other sources.
Enhancing the Sensitivity of the EHT – Data Collection at Wide Bandwidths
One way to increase the sensitivity of the EHT is to capture more energy from the black hole targets at each EHT site. Black holes emit radiation at many frequencies. Sensitivity is improved by increasing the range of frequencies that are recorded during EHT observations. Electronic systems and recording systems need to operate at higher speeds.
Computer and electronics upgrades enable this improvement.
Increase In Telescope Aperture
The most straightforward way to boost the sensitivity of the EHT is to increase the net collecting area of the dishes in the array. New telescopes can be added— the 12 meter diameter Greenland Telescope is due to come on line in 2018— but larger dishes are especially valuable since collecting area scales as the square of the dish diameter. A larger collecting area means more photons emitted by hot gas near the black hole event horizon can be captured on the Earth. The Large Millimeter Telescope (LMT), an EHT station in Mexico, is 50 meters in diameter, making it the largest fully steerable millimeter/submillimeter wave telescope in the array.
But building large reflectors is an expensive and sometimes impractical proposition, especially at these short wavelengths, because the mechanical precision and rigidity of the dish has extremely tight tolerances, which are hard to meet.
Some of the EHT sites, such as ALMA, the SMA, and the planned IRAM NOEMA are themselves collections of smaller antennas. Like the EHT they are interferometers, but they operate on local short baselines up to hundreds of meters, rather than thousands of kilometers as for the EHT, and their dishes are connected by cables. To use these sites as EHT stations, the small dishes must be electronically phased together, which allows their collecting area to be combined. Phased ALMA, for example, combines up to 64 dishes, each with a 12m diameter, for a total collecting area of 7200 square meters, which is about three times the 2000m collecting area of LMT.
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