James Webb Space Telescope

The James Webb Space Telescope (JWST or "Webb") is a space telescope that is planned to succeed the Hubble Space Telescope as NASA's flagship astrophysics mission. The JWST will provide improved infrared resolution and sensitivity over Hubble.

The primary mirror of the JWST, the Optical Telescope Element, is composed of eighteen 1.32 meters (4 ft 4 in) hexagonal mirror segments made of gold-plated beryllium which combine to create a 6.5 meters (21 ft) diameter mirror that is considerably larger than the Hubble's 2.4 meters (7 ft 10 in) mirror. Unlike the Hubble, which observes in the near ultraviolet, visible, and near-infrared (0.1 to 1 μm) spectra, the JWST will observe in a lower frequency range, from long-wavelength visible light through mid-infrared (0.6 to 28.3 μm), which will allow it to observe high redshift objects that are too old and too distant for the Hubble to observe.[8][9] The telescope must be kept very cold in order to observe in the infrared without interference, so it will be deployed in space near the Earth-Sun L2 Lagrangian point, and a large sunshield made of silicon- and aluminum-coated Kapton will keep its mirror and instruments below 50 K (−220 °C; −370 °F).

Kapton is known as polyamide film, polyamide is made of Amide and carboxylic link. 

Sun Shield protection:

 It uses a large sunshield to block light and heat from the Sun, Earth, and Moon, and its position near the Earth-Sun L2 point keeps all three bodies on the same side of the spacecraft at all times. Its halo orbit around L2 avoids the shadow of the Earth and Moon, maintaining a constant environment for the sunshield and solar arrays.

The five-layer sunshield, each layer as thin as a human hair,[is constructed from Kapton E, a commercially available polyimide film from DuPont, with membranes specially coated with aluminum on both sides and doped silicon on the Sun-facing side of the two hottest layers to reflect the Sun's heat back into space.

The sunshield is designed to be folded twelve times so it will fit within the Ariane 5 rocket's 4.57 m (5 yards) × 16.19 m (17.7 yards) payload fairing.

Optics:

JWST's primary mirror is a 6.5-meter-diameter gold-coated beryllium reflector with a collecting area of 25.4 m2. This is too large for existing launch vehicles, so the mirror is composed of 18 hexagonal segments, which will unfold after the telescope is launched. JWST's optical design is a three-mirror anastigmat, which makes use of curved secondary and tertiary mirrors to deliver images that are free of optical aberrations over a wide field. In addition, there is a fast steering mirror, which can adjust its position many times per second to provide image stabilization.

What is a three-mirror anastigmat?

A three-mirror anastigmat is an anastigmat telescope built with three curved mirrors, enabling it to minimize all three main optical aberrations – a spherical aberration, coma, and astigmatism. This is primarily used to enable wide fields of view, much larger than possible with telescopes with just one or two curved surfaces.

Spherical aberration is a type of aberration found in optical systems that use elements with spherical surfaces. Lenses and curved mirrors are most often made with surfaces that are spherical because this shape is easier to form than non-spherical curved surfaces. Light rays that strike a spherical surface off-center are refracted or reflected more or less than those that strike close to the center. This deviation reduces the quality of images produced by optical systems.

In optics (especially telescopes), the coma (/ˈkoʊmə/), or comatic aberration, in an optical system refers to aberration inherent to certain optical designs or due to imperfection in the lens or other components that results in off-axis point sources such as stars appearing distorted, appearing to have a tail (coma) like a comet. Specifically, a coma is defined as a variation in magnification over the entrance pupil. In refractive or diffractive optical systems, especially those imaging a wide spectral range, coma can be a function of wavelength, in which case it is a form of chromatic aberration.

An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp focus at two different distances.

Equipment inside the telescope: 

NIRCam is an imager from 0.6 to 5-micron wavelength, and as a wavefront sensor to keep the 18-section mirrors functioning as one. In other words, it is a camera and is also used to provide information to align the 18 segments of the primary mirror. It is an infrared camera with ten mercury-cadmium-telluride (HgCdTe) detector arrays, and each array has an array of 2048x2048 pixels.

The NIRSpec design provides three observing modes: a low-resolution mode using a prism, an R~1000 multi-object mode, and an R~2700 integral field unit or long-slit spectroscopy mode. Switching of the modes is done by operating a wavelength preselection mechanism called the Filter Wheel Assembly and selecting a corresponding dispersive element (prism or grating) using the Grating Wheel Assembly mechanism. Both mechanisms are based on the successful ISOPHOT wheel mechanisms of the Infrared Space Observatory. The multi-object mode relies on a complex micro-shutter mechanism to allow for simultaneous observations of hundreds of individual objects anywhere in NIRSpec's field of view.

MIRI is a camera and a spectrograph that observes mid to long infrared radiation from 5 microns to 28 microns. It also has coronagraphs, especially for observing exoplanets.

Note: A coronagraph is a telescopic attachment designed to block out the direct light from a star so that nearby objects – which otherwise would be hidden in the star's bright glare – can be resolved.

Fine Guidance Sensor and Near-Infrared Imager and Slitless Spectrograph (FGS-NIRISS) is an instrument for the planned James Webb Space Telescope that combines a Fine Guidance Sensor and a science instrument, a near-infrared imager, and a spectrograph.

NIRISS is designed for performing:

1.Near-infrared imaging

2.Wide-field slitless spectroscopy

3.Single object slitless spectroscopy

4.Aperture masking interferometry

Note: Interferometry is a family of techniques in which waves, usually electromagnetic waves, are superimposed, causing the phenomenon of interference, which is used to extract information.

Interferometers are widely used in science and industry for the measurement of small displacements, refractive index changes, and surface irregularities.

First Telescopes by our ancestors:

It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610). A Galilean telescope, because the design has no intermediary focus, results in a non-inverted and upright image.

Parallel rays of light from a distant object (y) would be brought to a focus in the focal plane of the objective lens (F′ L1 / y′). The (diverging) eyepiece (L2) lens intercepts these rays and renders them parallel once more. Non-parallel rays of light from the object traveling at an angle α1 to the optical axis travel at a larger angle (α2 > α1) after they passed through the eyepiece. This leads to an increase in the apparent angular size and is responsible for the perceived magnification.

The final image (y″) is a virtual image, located at infinity and is the same way up as the object.

Telescopes of the future:

• Lynx

It is expected that Lynx is designed to identify the massive black holes which exist in the distant galaxies. 

• Luvoir 

It is expected that this telescope is designed to identify and point out the first galaxies.

• Habex

It is expected that this telescope of the future is designed to identify and collect the data about exo-planets.

• Origin Space Telescope 

This telescope is designed to capture and provide more information about the origin of the universe.