The James Webb Space Telescope

The James Webb Space Telescope is one of those instruments that will help humanity spread across the universe. This telescope is the most powerful telescope ever launched into space. As Webb continues to make groundbreaking discoveries, I wanted to know how exactly it worked. I have read many articles regarding this telescope already, some were very technical. I want to share my information in a simple manner so a wide range of audiences could understand.

Physical construction:

The James Webb Space Telescope is divided into three parts: (1) the Integrated Science Instrument Module (ISIM), (2) the Optical Telescope Element (OTE), including the mirrors, backplane, and (3) the Spacecraft Element. The spacecraft element includes the Spacecraft Bus and Sunshield.


The Integrated Science Instrument Module (ISIM) includes Webb’s science instruments.

The Integrated Science Instrument Module (ISIM), a framework that provides electrical power, computing resources, cooling capability, and structural stability to the Webb telescope. The ISIM holds the four science instruments and a guide camera.

NIRCam (Near InfraRed Camera) is an infrared imager that has a spectral coverage from visible light (0.6 μm) to near-infrared (5 μm). NIRCam will also help align and focus the main mirror segments.

NIRSpec (Near InfraRed Spectrograph) will also perform spectroscopy over the same wavelength range. The NIRSpec has three observing modes that simultaneously allow observations of hundreds of individual objects anywhere in NIRSpec’s field of view. 

MIRI (Mid-InfraRed Instrument) will measure the mid-to-long-infrared wavelength ranging from 5 to 27 μm. It contains both a mid-infrared camera and an imaging spectrometer. Although,  the temperature of the MIRI must not exceed 6 K (−267 °C). For this, helium gas mechanical coolers are situated on the warm side of the environmental shield which provides this cooling.

NIRCam and MIRI feature starlight-blocking coronagraphs for observation of faint targets such as extrasolar planets and circumstellar disks very close to bright stars.

FGS/NIRISS (Fine Guidance Sensor and Near-Infrared Imager and Slitless Spectrograph), is used to stabilize the line-of-sight of the observatory during science observations. Measurements by the FGS are used both to control the overall orientation of the spacecraft. The Near-Infrared Imager and Slitless Spectrograph (NIRISS) module is used for astronomical imaging and spectroscopy in the 0.8 to 5 μm wavelength range.


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 James Webb Space Telescope has a 6.5-meter gold-coated beryllium primary mirror made up of 18 separate hexagonal mirrors. The gold coating helps reflect infrared wavelengths. The mirror by itself covers an area of 25.4 m2 (273 sq ft).

The OTE also consists of:

The backplane supports the mirrors.


The instruments have to maintain a low temperature. Webb has a built-in sunshield that will protect the telescope from the sun.

The sunshield separates the warm sun-facing side (spacecraft bus) and the 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 the Observatory can be kept cold. I talk more about the detail later in the article. 

The Spacecraft Bus is the primary support component of the James Webb Space Telescope  The bus consists of 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.  It weighs 350 kg and must support the 6,200 kg space telescope.

Finally, Webb should accurately position itself while taking pictures. It has two pairs of rocket engines for station keeping—maintaining the correct position in the halo orbit. Eight smaller thrusters are used for attitude control—the correct pointing of the spacecraft.