[SatNews] This July 11, 2015 photograph captures one of the final, if not the final, James Webb Space Telescope flight primary mirror segments to be processed through NASA Goddard Space Flight Center's Calibration, Integration and Alignment Facility (CIAF).

Image Credit: NASA/Chris Gunn.

The mirror is seen here on the Configuration Measurement Machine (CMM), which is used for precision measurements of the backs of the mirrors. These precision measurements must be accurate to 0.1 microns or 1/400th the thickness of a human hair.

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. Webb is an international project led by NASA with its partners, the European Space Agency and the Canadian Space Agency.

The Observatory of the James Webb Space Telescope is comprised of three elements: the Integrated Science Instrument Module (ISIM), the Optical Telescope Element (OTE), which includes the mirrors and backplane, and the Spacecraft Element, which includes the Spacecraft Bus and Sunshield.

The Integrated Science Instrument Module (ISIM) includes Webb's science instruments. The OTE is the eye of the Observatory and it gathers the light coming from space and provides it to the science instruments located in the ISIM. The backplane is like the Webb's "spine". It supports the mirrors.

The sunshield subsystem separates the observatory into a warm sun-facing side (spacecraft bus) and a cold anti-sun side (OTE and ISIM). The sunshield keeps the heat of the Sun, Earth, and spacecraft bus electronics away from the OTE and ISIM so that these pieces of the Observatory can be kept very cold (The operating temperature has to be kept under 50 kelvins or -370 deg F).

The Spacecraft Bus provides the support functions for the operation of the Observatory. The bus houses the six major subsystems needed to operate the spacecraft: the Electrical Power Subsystem, the Attitude Control Subsystem, the Communication Subsystem, the Command and Data Handling Subsystem, the Propulsion Subsystem, and the Thermal Control Subsystem.

The momentum flap balances the solar pressure on the sunshield, like a trim flap in sailing. It's not adjustable on orbit, but it is while it's on the ground.