An MIT-led team of scientists has developed a new algorithm – named the CHIRP (Continuous High-resolution Image Reconstruction using Patch priors) — that could help astrophysicists take the first-ever image of a black hole.
An artist’s impression of the disc of material around Sagittarius A*. Image credit: L. Calçada / ESO.
CHIRP would stitch together data collected from the Event Horizon Telescope (EHT), an array of telescopes spread out across the globe and linked using a technique known as Very Long Baseline Interferometry.
“The EHT uses a technique called interferometry, which combines the signals detected by pairs of telescopes, so that the signals interfere with each other,” explained MIT student Katherine Bouman, who led the development of CHIRP, and co-authors.
“Usually, an astronomical signal will reach any two telescopes at slightly different times. Accounting for that difference is essential to extracting visual information from the signal, but the Earth’s atmosphere can also slow radio waves down, exaggerating differences in arrival time and throwing off the calculation on which interferometric imaging depends.”
The team adopted a clever algebraic solution to this problem: “if the measurements from three telescopes are multiplied, the extra delays caused by atmospheric noise cancel each other out. This does mean that each new measurement requires data from three telescopes, not just two, but the increase in precision makes up for the loss of information.”
The prime EHT target is Sagittarius A*, the black hole at the center of our Milky Way Galaxy. Even though it is 4 million times more massive than the Sun, it is tiny to the eyes of astronomers.
Because Sagittarius A* is smaller than Mercury’s orbit around the Sun, yet almost 26,000 light years away, studying its event horizon in detail is equivalent to standing in California and reading the date on a penny in New York.
“A black hole is very, very far away and very compact. Taking a picture of the black hole in the center of the Milky Way Galaxy is equivalent to taking an image of a grapefruit on the Moon, but with a radio telescope,” Bouman said.
“To image something this small means that we would need a telescope with a 10,000-km diameter, which is not practical, because the diameter of the Earth is not even 13,000 km.”
Currently, six observatories have signed up to join the EHT project, with more likely to follow.
But even twice that many telescopes would leave large gaps in the data as they approximate a 10,000-km-wide antenna. Filling in those gaps is the purpose of algorithms like CHIRP.
Bouman and her colleagues will present CHIRP at the Computer Vision and Pattern Recognition Conference on June 27, 2016.
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Katherine L. Bouman et al. Computational Imaging for VLBI Image Reconstruction. CVPR 2016, 10 pp.
