After years of budget woes and concerns about cost, the James Webb Space Telescope appears to be on track after NASA announced that a dozen of its eighteen mirror segments that make up the primary mirror have been installed.

Each hexagonal-shaped segment measures just over 4.2 feet across and weighs approximately 88 pounds. After being pieced together, the 18 primary mirror segments will work together as one large 21.3-foot mirror. Made of ultra-lightweight beryllium, the mirrors are placed on the telescope's backplane using a robotic arm, guided by engineers. The 12 were installed Jan. 2 and the full installation is expected to be completed in a few months.

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"This milestone signifies that all of the hexagonal shaped mirrors on the fixed central section of the telescope structure are installed and only the 3 mirrors on each wing are left for installation," Lee Feinberg, NASA's Optical Telescope Element Manager at NASA Goddard, said in a statement. "The incredibly skilled and dedicated team assembling the telescope continues to find ways to do things faster and more efficiently."

While completing the mirror assembly is significant, there is plenty more to do before the Webb telescope is ready to go ahead of its planned launch in 2018.

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Along with the primary mirror and the tennis-court-sized sunshield, there are four smaller components that are also critical. The instruments that will fly aboard Webb - cameras and spectrographs with detectors able to record extremely faint signals — are part of the Integrated Science Instrument Module, which is currently undergoing its final test and will be integrated with the mirror later this year.

There is also something called the Near InfraRed Spectrograph, which has programmable microshutters which enable observation up to 100 objects simultaneously. The Near Infrared Camera, meanwhile, is equipped with coronagraphs, instruments that allow astronomers to take pictures of very faint objects around a central bright object, like stellar systems.

Then, there is the cryocooler for cooling the Mid-Infrared Instrument (MIRI) to a very cold 7 Kelvin so they can work. MIRI has both a camera and a spectrograph that sees light in the mid-infrared region of the electromagnetic spectrum, with wavelengths that are longer than our eyes see.

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The James Webb Space Telescope is the scientific successor to NASA's Hubble Space Telescope. It will be the most powerful space telescope ever built and most likely the costliest.

The soaring cost – fast approaching $ 9 billion - put the project at risk several times, with some Republicans in the past raising concerns about its budget and what they perceived as NASA’s mismanagement of the project.