Detailed Analysis of Space Telescopes: WMAP, JWST, Hubble, and Chandra

This report provides a detailed comparative analysis of four significant space telescopes: the Wilkinson Microwave Anisotropy Probe (WMAP), the James Webb Space Telescope (JWST), the Hubble Space Telescope, and the Chandra X-ray Observatory. The report begins with an overview of WMAP, detailing its mission to measure temperature differences in the Cosmic Microwave Background (CMB), its operational frequency range, and its role in establishing the standard model of cosmology. Next, the report examines JWST, highlighting its infrared capabilities, hexagonal mirror design, and its mission to observe objects obscured by dust and study the formation of planets, stars, and galaxies. The report then discusses the Hubble Space Telescope, focusing on its contributions to astronomical discoveries, its operational wavelengths, and its history, including the initial imaging problems and subsequent servicing missions. Finally, the report analyzes the Chandra X-ray Observatory, describing its elliptical orbit, its ability to detect X-rays, its mirror design, and its role in studying black holes, supernovae, and galaxies. The report concludes by summarizing the key features, functionalities, and contributions of each telescope to our understanding of the universe.

Wilkinson Microwave Anisotropy Probe
The Microwave Anisotropy Probe, later named as Wilkinson Microwave Anisotropy Probe in
honour of cosmologist David Wilkinson (1935-2002) was a spacecraft operating from 2001-
2010. WMAP’s main application is to determine the differences in temperature across the sky
in Cosmic Microwave Background (CMB) ("Wayback Machine", 2019). The anisotropies
measured are used to test the space’s geometry, Big-Bang model and the cosmic inflation
(2019).
As the name suggests, it works in the microwave electromagnetic spectrum. It played an
integral role in establishing the ‘CDM-λ’ model which is currently the standard model of
cosmology. The complete project was spearheaded by Professor Charles L. Bennet of John
Hopkins University and was joint venture between NASA, Goddard Space Flight Centre and
Princeton University ("Wayback Machine", 2019).
It is a middle-class spacecraft (MIDEX) in the NASA Explorer program. The primary lenses
of the telescope are two Gregorian dishes (dimensions: 1.4m*1.6m). They are placed in
opposite direction to cover maximum area for microwave absorption. The light falling on
these primary lenses get focussed on the secondary reflecting mirrors (dimensions:
0.9m*1.0m) shown in Figure 1. These mirrors further transmit the signal to a corrugated
feedhorn, placed on an array box beneath the primary mirrors as illustrated in Figure 2. The
mirrors are coated with aluminium and silicon dioxide coatings to increase the absorption of
photons on its surface (2019).
WMAP operates in central frequency range of 23-94 GHZ and wavelength range of 3.2mm -
13mm. Polarization-sensitive differential radiometer receivers are used to measure the
difference between the two telescope beams. This signal is amplified with HEMT low-noise
amplifiers. The data collected is relayed daily via a GHz transponder into the deep space
network telescope (2019). WMAP operated for nine years from the L2 orbit (Sun-Earth
Lagrangian point) measuring the sky every six months.
Figure 1: WMAP Spacecraft diagram
Source: nasa.gov

Figure 2 Ray Diagram of WMAP capturing light from the sky
Source: researchgate.net
James Webb Space Telescope
JWST planned to be the successor of the Hubble telescope ("About Webb/NASA", 2019) is
another telescope operating in L2 Lagrangian point, approximately 1.5 million kms away
from the earth’s orbit. JWST works in the infrared region. Infrared waves are highly piercing
and so this telescope will be able to see objects blocked by small particles of dust. This will
help us in better understanding of how the planets were formed, discovery of nova, stars and
other colder objects which would not normally glow with visible EM waves.
Figure 3: James Webb Space Telescope
Source: nasa.gov

Although the mass of JWST is half of Hubble, its collecting area is five times as large
("JWST Telescope", 2019). Its primary mirror is 6.5m beryllium reflector having a peculiar
hexagonal design as shown in Figure 3 by the yellow reflective panels. It was designed so, to
allow it to fold 12 times and be fitted into the spacecraft during the launch. Its focal length is
131 m and collecting area of 25 m2. Intended to orbit in the Halo orbit regime. The central
wavelength ranges from 28.5 micro m (mid-infrared) to 0.6 micro m (orange).
In space, the EM waves travel for millions of years as a result, they get stretched and convert
into infrared radiations. Also, atmosphere of earth prohibits the entry of IR waves which
makes it difficult to analyse the celestial bodies.
Owing to the low temperatures of IR radiations, the telescope has to maintain a temperature
below 50K to capture the radiations. In order to do so, it is provided with large sunshields
that will refrain light rays from Earth, Sun and Moon to interfere with its IR absorption
spectrum.
The primary mirror of the telescope is 7 times larger than Hubble. This gives it the access to
larger radiation absorption. The stretched waves will then have allowed us to see back in time
from the Big Bang about 800 million years. The coating used is of Beryllium as it is more
resistant to cold temperature, compared to glass. There is also a thin coating of gold (24
karat) which can reflect near, mid-IR and visible red-light waves ("Telescope: Webb Space
Telescope", 2019).
JWST will use a three-mirror system, where light hitting the primary mirror gets reflected
onto the secondary and further to the tertiary mirrors as shown in figure 4. This system helps
correct optical aberrations like spherical aberrations, cone and astigmatism, also providing
better image stabilization. It will also use phase retrieval technique to align mirrors in the
image plane wave front using micro motors. Webb telescope's IR imaging will help us
understand the formation of galaxies, look back in time and understand how the universe and
its bodies came into existence.
Figure 4: Three mirror Anastigmat ray diagram for JWST
Source: sciencebuddies.org

Hubble Space Telescope
Hubble telescope launched in 1990 is low earth orbit space telescope. Named after
astronomer Edwin Hubble, this observatory can identify astronomical objects without the
earth’s atmosphere blocking its view. It works in near infrared, visible light and ultraviolet
regimes (400nm – 1050nm) of electromagnetic spectrum.
Figure 5: Hubble Space Telescope
Source: nasa.gov
The Hubble Telescope has a long focal length of 57.6 m. As shown in Figure 6, the parallel
light rays hit the collector surface made of aluminium-coated glass surface. These rays strike
the primary (concave) mirrors, which have a hole in between. The rays get reflected onto the
secondary (convex) mirrors and further directed through the hole onto a focal point in the
image processing instrument ("Telescope: Hubble Space Telescope", 2019).
Figure 6: Hubble Working principle- Ray Diagram
Source: nasa.gov

With a 2.4m wide diameter of the mirror and 4.5m2 collecting area, this telescope was
essential in many discoveries; such as Age of Universe, Expansion of Universe, Black Holes,
solar system discoveries, reappearance of supernova, mass and size of milky way, etc.
Using the geosynchronous tracking and Data Relay Satellite System (TDRSS), Hubble
transmits the data to microwave antennas on earth ("ESA/Hubble Space Telescope", 2019).
In 1990, when Hubble arrived in space, astronomers to astonished to see that the images were
blur due to spherical type aberrations. The primary mirror was wrongly prescribed and out of
focus. It was termed as the $1.5 billion fiasco. This required an urgent emergency service
which was catered by Astronaut Story Musgrave in 1993. His team installed corrective
mirrors in front of the primary mirror to fix the focal error. Later there were five upgrades in
the coming years: 1993, 1997, 1999, 2002 and 2009. There are no further plans for servicing
of the telescope and is expected to be taken down by atmospheric drag by 2030-40 (Tate,
2019).
Chandra Telescope
Chandra X-ray telescope uses X-rays to image astronomical objects in space. It is named
after the Noble-prize winner and astrophysicist Subrahmanyam Chandrashekar. The telescope
is an advanced version of X-ray telescope sent into space in 1970. It orbits around the earth in
an elliptical path, roughly 200 times higher than the Hubble Telescope. It has to keep itself
clear of the Van Allen belts which are small fragments of rocks emitting radiations and
moving at high velocities.
It is powerful enough to detect 20 times weaker signals and thus, works in the central
wavelength range of 0.12 - 12nm. Figure 7 shows all the essential parts of the telescope
(labelled). It is Reflector type telescope with iridium coated glass mirrors. The convex
mirrors are 0.8m in diameter. The mirrors are placed perpendicular to the incoming EM
waves so that the photons glide through the surface and the X-rays get absorbed.
Intensity of x-rays is so high that it can penetrate any mirror while also reflecting from their
surface. To avoid this, there are four pairs of mirrors carefully aligned so that they are parallel
to the incoming waves. The waves after multiple reflections from the mirrors, passes through
an 8m long tube and finally reaches the imaging instrument.
The instruments serve two purposes. First, to provide the information of the position, energy
and time of the entering waves. Second, to determine the intensity and wavelength of the
incoming waves. To predict this, the device uses another instrument called as 'gratings'.
Gratings have multiple narrow slits which are placed between the mirrors and the instrument.
By the process of diffraction through these openings the waves get separated according to
their wavelengths. (Benningfield, 2019)
The telescope is powered by Solar panels. The highlight discoveries of the Chandra X-ray
would be the study of black holes, supernovae, galaxies as these are high entropy and energy
releasing objects. Its low energy detection instruments were also able to discover new planets
with disks and different stars in the neighbouring solar systems. ("Telescope: Chandra X-ray
Observatory", 2019)

Figure 7: Labelled diagram of CXO
Source: Chandra.harvard.edu
Figure 8: Chandra Telescope Ray Diagram

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