US space agency NASA has completed the construction of the world's largest space telescope, after almost two decades.

The James Webb Space Telescope is considered to be the successor to NASA's 26-year-old Hubble Space Telescope, the agency announced on Wednesday.

NASA said an Ariane 5 rocket will launch it from French Guiana in October 2018, Xinhua news agency reported.

"Today, we're celebrating the fact that our telescope is finished and we're about to prove that it works," Nobel laureate and Webb's senior project scientist John Mather said during a news conference posted on Youtube, adding that the new telescope will open up "a whole new territory of astronomy".

A time-lapse video of the construction:

"We will see things we have not seen before because this telescope is much more powerful than even the great Hubble telescope," Mather said, adding "To give you some perspective about what we can do with it. If you were a bumblebee at a distance of the moon, we will be able to see you, both by your reflective sunlight and by thermal radiation and heat you emitted."

According to NASA, its engineers and technicians on Wednesday successfully completed the first important optical measurement of Webb's fully assembled primary mirror, known as a Centre of Curvature test, to measure the mirror's shape.

Next, the 6.5-metre primary mirror consisting of 18 hexagonal mirrors will go through a series of rigorous tests that will simulate the violent sound and vibration environments the telescope will experience inside its rocket on its way out into space.

The Centre of Curvature test will be repeated after the launch environment testing and the results compared to find if there are any changes or damages to the optical system.

NASA said the Webb telescope will be used to observe distant objects in the universe, provide images of the first galaxies formed and see unexplored planets around distant stars.

The $8.7 billion project is led by NASA but also supported by the European Space Agency and the Canadian Space Agency. 

(With IANS inputs)