Sky & Telescope - USA (2019-09)

Event Horizon Telescope

20 SEPTEMBER 2019 • SKY & TELESCOPE

Instead, the EHT’s worldwide team

of researchers, comprising 200

people working in 20 countries

and regions, constructed the black

hole’s image using a technique

called very long baseline interfer-

ometry (VLBI). VLBI combines the

data from multiple radio telescopes

scattered across the globe into a

single image.

The basic principle of interferom-

etry is this: Take two telescopes, sepa-

rated by some distance, and observe an

object simultaneously with both of them. Light

comes from the object as a wavefront, like ripples

in a pond, explains imaging team leader Michael Johnson

(Center for Astrophysics, Harvard & Smithsonian). The two

telescopes will catch a slightly different part of each ripple.

Account for that delay, then carefully add the data together,

and you can measure the object’s structure with the resolu-

tion you’d have from a telescope that is the size of the distance

between the two dishes.

But when observing something with structure on a variety

of scales, things get complicated — it’s like having a fl ock

of ducks cavorting in the pond, their waves interacting and

changing the pattern in complex ways. In order to recon-

struct the image, you need a detailed understanding of how

the radio waves are augmenting or canceling one another out

as they travel to the dishes. The solution is a bunch of tele-

scopes with different separations, which enable you to mix

and match the pairs and detect structures of various sizes

and orientations.

The approach is thus akin to the old joke about a bunch of

blindfolded scientists studying different parts of an elephant.

With VLBI, each blindfolded scientist represents the separa-

tion, or baseline, between two telescopes. But instead of sam-

pling different body parts, each baseline observes a different

scale of the elephant: One says it’s 10 feet high, another says

there’s a foot-long ear, still another explores the fi ne texture

of the elephant’s skin. As Earth turns, different baselines

see the target, detecting different scales of the elephant. The

scientists then piece these bits of information together into a

coherent image.

SMT,

KP

SMA,

JCMT

APEX,

ALMA

SPT

IRAM

NOEMA

GLT

LMT

Correlation

Fringe

fi tting

Te a m 1

Calibration

Validation

Te a m 3

Te a m 2

Te a m 4

Telescopes scattered

across Earth simultane-

ously record emission

from near the black hole.

Correlators

combine the raw

data, sifting out

signals from noise.

Te a m me mbe r s

align all the

signals to within

picoseconds.

Data are converted

to radio brightness

measurements.

Blind imaging stage: Teams use

independent algorithm approaches

to create images.

If there were an elephant in the center

of M87, they wanted to see it.

pA GLOBAL TELESCOPE Eight stations participated in the 2017 EHT

observing campaign. Seven of them (connected by solid lines) observed

M87*; dotted lines mark sites that observed the calibration source, the

quasar 3C 279. In 2018 the Greenland Telescope joined the array, and

two more stations (red) will sync up in 2020.

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OBSERVATORIES

Atacama Large

Millimeter/

submillimeter Array

(ALMA)

Atacama

Pathfi nder

Experiment (APEX)

IRAM 30-meter

Tele scope (IR A M)

James Clerk

Maxwell Telescope

(JCMT)

Large Millimeter

Tele scope (LMT )

Submillimeter

Array (SMA)

Submillimeter

Tele scope (S MT )

South Pole

Tele scope (SP T )

Greenland

Telescope (GLT )

Kitt Peak 12-meter

Tele scope (K P)

NOEMA

Observatory

(NOEMA)