Episode 23
View from the space
Dr T V Venkateswaran
Main points
- A large part of electromagnetic spectrum including X-rays, gamma rays, ultraviolet ray
of certain wavelengths cut off by Earth’s atmosphere
- Astronomy from space allows observation in these wavelengths
- X-ray stars, Chandra X-ray telescope and the discoveries made by it
- The infrared sky, IRAS
- Ultraviolet sky, IUE
- Gamma rays from space, Compton GRO
- Chandrayaan, Astrosat
- Chandrayaan II
Points to be emphasised
- Limitations of ground-based observation
- Going beyond the atmosphere
- First X-ray image of the Sun taken by V2 rocket-borne X-ray camera in 1947
- Discovery of other X-ray objects
- Discoveries made by the Chandra X-ray telescope
- Brief accounts of IRAS, IUE and Compton GRO
- The cosmos revealed in its most violent form
- Description of the objectives and achievements of Hubble, Chandra
- How Hubble was upgraded /
repaired in space
- Experiments Astrosat would carry
Why go above earth’s atmosphere?
Until Galileo, largely astronomers made observations only with naked eye. Only those
bright objects or stellar objects near enough to earth could be observed. With the advent
of telescope, humans could see far reaches and also details such as surface features of
planets, distant galaxies and so on. However not all stellar objects emit visible light and
hence was hidden from our view until humans developed capacity to detect these ‘unseen
light’- x rays, gamma rays and radio waves.
Visible light that comes from a lamp in your house and radio waves that come from a
radio station are two types of electromagnetic radiation. Other examples of EM radiation
are microwaves, infrared and ultraviolet light, X-rays and gamma-rays. Hotter, more
energetic objects and events create higher energy radiation than cool objects. Only
extremely hot objects or particles moving at very high velocities can create high-energy
radiation like X-rays and gamma-rays.
As universe is filled with cool clouds, hot stars and hotter star cores and even hotter
objects such as pulsars and so on, radiation of various types emanate from the space. In
fact even sun emits energy in all through the spectrum. Sun too emits x rays, gama raya
and so on. Study of these rays provide us clue as to the process that takes place, which are
not normally visible in the light spectrum.
Nevertheless, electromagnetic radiation from space is unable to reach the surface of the
Earth except at a very few wavelengths, such as the visible spectrum, radio frequencies,
and some ultraviolet wavelengths. Earth’s atmosphere acts like a shade and prevents
certain types of radiation from reaching the ground. In fact if the Earth’s atmosphere was
permeable to harmful X rays, gama rays and ultra violet rays then perhaps life would not
have been possible on earth.
Astronomers has to go above enough of the Earth's atmosphere to observe some infrared
wavelengths. They had to build telescopes on mountain tops for this reason. They were
also able to fly their telescopes in an aircraft. Experiments can also be taken up to
altitudes as high as 35 km by balloons which can operate for months. Rocket flights can
take instruments all the way above the Earth's atmosphere for just a few minutes before
they fall back to Earth, but a great many important first results in astronomy and
astrophysics came from just those few minutes of observations. For long-term
observations, however, it is best to have your detector on an orbiting satellite ... and get
above it all!
The upper layers of atmosphere completely blocks x rays and gamma rays and hence
ground based detection of these rays from stellar objects are rather impossible. Therefore
scientists build special satellites- space telescopes to go above the earth’s atmosphere to
low earth orbit to make observation in the x ray, gamma ray range of EM spectrum.
How can one detect x rays
X Ray telescope and Lobster’s eyes- sound as if chalk and cheese? Indeed it is by musing
on the structure of the eye of Lobsters, scientist got the idea in the first place to catch,
focus and build an efficient wide-angle vision X-ray telescopes. Roger
Angel, an astronomer realised that the arrangement of lobster-eye is ideal for making an
X ray telescope, when he fortuitously read a paper on the lobster eye by Mike Land and
Kalus Vogt in 1978. It was only in 1996 a team of British scientists finally cracked the
main technological problem in manufacturing an ‘X ray lens’ based upon the lobster-eye
principle.
Fantasising ‘X Ray vision’
Recall Hollywood movie ‘SpyKids; FBI provides to the spy-kids in their hair raising
mission a number of incredible, astounding and amazing gadgets, palpably futuristic and
imaginary. Have you noticed the special goggle that allows one to acquire x-ray vision
that enable perceiving unclothed- naked body. To identify hidden weapons is the ruse;
adolescent kids being what they are, actually the gadget is exploited to many a naughty
intents. From Hollywood movie Spy kids to Bollyhood of Mumbai and Golyhood of
Chennai, world over films have fantasised ‘X-ray’ goggles that would allow one to ‘see
through’ and acquire ‘naked vision’! While what are fancied in movies is predictably
hilarious, Natalya Demkina a sixteen-year-old girl from Saransk was deadly serious. She
stunned the world with her claim of ‘X-ray’ vision- that she is capable of discerning a
person’s internal organs without using X-ray or ultrasound. While controlled studies of
her claims have clearly contradicted her assertions, however, the question remains: Is Xray vision possible?
For Hollywood and Golyhood, X Ray vision is a matter of comedy. X-ray vision for
adolescent’s flight of the imagination may be just erotic, but the keen interest shown by
scientist to perfect X-ray vision is actually to unravel the mystery of our universe. Stellar
objects emit X-rays just as they emit light. While normal ageing process, a star may give
off energy in the form of visible light, whenever galaxies collide, stars explode, or chunks
of matter plunge into massive black holes, they emit a torrent of X rays, which are 100 to
1,000 times more energetic than the light that is observed. Imaging the X ray sources
provides a view hitherto unknown, perhaps even unforeseen. With X Ray telescope at
their service, astronomers can now see sharply and deeply enough in the X-ray spectrum
to solve some long-standing mysteries and uncover some new ones.
Early X ray telescopes
Telescopes have been around for about 400 years and since Galilio it has been used to
study distant objects in the stellar world. X ray, as we know are just like light, another
form of Electromagnetic waves. In optical telescope, light is collected, refracted and
focused to produce sharp and enhanced images of distant stellar objects. X-rays too travel
in straight line and could in principle be,
refracted and reflected. But usual arrangement
Fig 1: flanged at certain angle pebble
is inadequate for gathering and focusing x-rays;
skip on the surface of water , while in
normal lenses are woefully inadequate with
ordinary circumstances it would sink.
regard to X rays.
Why can’t we use a “normal” lens, of the sort
found in cameras and some optical telescopes?
X-ray photons posses hundreds to thousands of
times more energy than photons of visible light.
Therefore, when an x-ray photon strikes the
surface of an ordinary, concave-shaped optical
mirror, it passes through it like a bullet fired
through tissue paper - consequently such a lens
would be of no use. In ordinary telescope we try to detect the visible wavelength- light.
The light emitted by stellar objects are gathered by a telescope and the same is focused on
to a screen or a detector (such as CCD camera). However, as we know X ray being low
wavelength electromagnetic radiation, they either pass through most objects or absorbed
by the object on its path. The lenses made with glass, obviously cannot focus the X rays;
in fact it is this penetrating power that is exploited in medical and other imaging
applications of X rays. Therefore the question remains; how to focus X rays, without it
being absorbed in the first place?
Science may not be just a plaything; but at times games that children play may edify
scientist with one or two insights. It is evident that pebble thrown in to water would
immediately sink. However, sure as children we have all tried bouncing pebble off water.
Indeed we have had had match - who can made the pebble skim farthest on the surface of
the water?
Drop a pebble in water- Mirror shells are arranged in parabolic or hyperbolic shapes
it will sink without fail. that allow the incident rays to graze and get focused. The
But then how and why
same principle is used to focus X rays in traditional design of
does pebble bounce and
skim on the surface of the X ray telescopes.
water? The answer is
quite simple: you make
sure that the pebble hits
the surface of the water
at a very shallow angle
called“grazing
incidence”. Given the
right shape of pebble
and with little practice,
one can get the pebble
to “skip” across the
water even a few times,
bouncing on and off. Little experience would inform us; flat pebbles are better than round
ones; pebbles, flanged at certain angles are propelled gliding on the surface of the water
to maximum distance.
X-ray telescopes essentially use this principal to make X-rays reflect off the mirror
material- made from lead glass. Shape a mirror with parabolic and hyperbolic contour
curves, X-rays grazes along the surface of the mirror; and image is produced in sharp
focus. Early X Ray telescopes were made with this design. Such crude designs, nascent
x-ray telescopes, were able to provide glimpse of X ray emitting objects of the universe.
This technique works well, although it’s rather inefficient, because the mirrors are used
almost edgeways-on. Thus the amount of X Rays collected are usually inadequate to get
high resolution. This minor hurdle is overcome by having a ‘nest’ of X-ray telescopes many such mirrors – so as to collect sufficient x-rays from faint, distant astronomical
objects. For illustration, each module in XMM (X Ray Milti Mirror) -Newton X-ray
telescope consists of 58 mirror shells. The mirrors are made of gold-plated nickel. There
are three such mirror modules on XMM-Newton to increase its ability to collect faint Xrays from distant astronomical objects.
X Ray telescopes
Quite obviously as Earth’s atmosphere absorbs x-rays observations of the high-energy
objects that emit x-ray cannot be made with ground-based telescopes. If mountain cannot
come to Mohammad, sure Mohammad can to mountain; if X-rays cannot reach ground,
place the X Ray telescope above the Earth’s atmosphere. The post World War II era gave
these physicists an opportunity to place their budding detectors on leftover sounding
rockets and launch them a 160 kilometres above Earth.
Using those nascent X ray detectors, the first celestial x-ray source was discovered in
1962. It was an unusual bright blue star located in the direction of the constellation
Scorpius, called scorpius X -1. The observation lasted all of 350 seconds. Scientists
persisted in sending their instruments up on rockets and balloons whenever they could. In
1970 the first Earth-orbiting satellite dedicated to x-ray astronomy—named Uhuru—was
launched and operated for three years. A couple dozen missions have flown since, and
right now there are a number of orbiting x-ray observatories—NASA’s Rossi X-ray
Timing Explorer and the European Space Agency’s XMM-Newton Observatory, Chandra
observatory and so on. Since 1960s, fledging field of X-ray astronomy has giving us
spectacular vision of an explosive and turbulent universe, the significance of revealing
the x-ray sky came in October 2002 when Riccardo Giacconi, a stalwart x-ray
astronomer, was recognized and awarded the Nobel Prize in physics.
From hesitant wobbly steps of early days, X Ray telescope have been making steady
strides. Two of the most sophisticated space observatories currently in operation are
Chandra and XMM-Newton - both capable of imaging the sky at X-ray wavelengths.
Chandra, named after Indian born noble laureate, for example, sports the smoothest x-ray
mirrors ever constructed, so it can resolve x-ray sources with unprecedented clarity, with
less than 10angle of grazing incidence. The European Space Agency’s XMM-Newton Xray Observatory, on the other hand, has larger but rougher mirrors. XMM-Newton can
capture more x rays for spectroscopic analysis, but its imaging resolution is poor
compared to Chandra’s. X-ray astronomers are currently designing x-ray interferometers,
which would use networks of x-ray telescopes to resolve, among other things, accretion
disks around black holes.
X Ray picture of universe
Just as the X ray view of the body is way apart from the normal visual view, the x ray
universe is distinctive. Of course one should not imagine that we will be able to see
‘inside’ of stars and other objects under x rays. As x rays are packed with high energy,
only the most violent, hot, and exotic objects in the universe can produce them in copious
quantities.. The science goals for the new X Ray telescope are many and varied, ranging
from the study of comets to quasars. Some of the most luminous x-ray sources include
supernova remnants, black holes and neutron stars that are gobbling matter from their
surroundings, high-speed particle jets shooting out of the cores of active galaxies, and
vast intergalactic clouds of multi-million-degree gas. Number of novel X-ray phenomena
are also observed. Many of these events, it is anticipated will originate from the centres
of distant galaxies containing black holes- called active galactic nuclei, AGN. Further
nearby stars, X-ray binaries, nearby galaxies and cataclysmic variables such as a low
mass normal star in a close orbit with a compact white dwarf are expected to emit X
Rays.
The expectations were not belied; Chandra revealed a unique X Ray source called, Deep
Field South. When the X Ray telescopes were directed at a tiny patch of sky in the
southern hemisphere that appeared blank optical telescope, faintest x-rays from this
remote spot were soaked up, resulting in a image called a Deep Field South, with great
variety of x-ray sources, including luminous quasars and active galactic nuclei powered
by supermassive black holes. Moreover, first ever images of the remnants of the
supernova Cassiopeia A, that exploded in 1054 CE and observed then by the Chinese
observers were obtained by Chandra observatory. Chandra images of this explosion,
today called as Crab Nebula which lies about 6,000 light-years from Earth, show a
brilliant ring encircling the spinning neutron star at the heart of the nebula. The pictures
also show material blasted out from the explosion crashing into surrounding matter at 10
million miles an hour, causing violent shock waves that generate the x-rays visible to
Chandra. Resolution is of such high quality with the telescope, that when trained on what
was thought to be a pin-point target quasar, it revealed the object to have an x-ray
emitting jet 200,000 light years long. X ray telescopes have fundamentally changed our
view of universe as peaceful, staid place to one that is violent, turbulent and exciting
However traditional designs of X Ray telescopes suffered from one infirmity; although
the technology worked well, it cannot be used to take wide-angle images of the entire
sky. Lobster-ISS, a modern X Ray telescope, designed to overcome this deficiency, still
uses the “grazing incidence” method to reflect X-rays, but the design and size of the
device are unparalleled and unique; Enter lobster.
Lobsters Eyes
The eye of a lobster, viewed with a microscope.
Right: close-up of a small area of the eye. The eye
consists of millions of square "channels"; each
channel measures approximately 20 microns (or
two hundredths of a millimetre) across.
The eyes of creatures that inhabit the
deep dark ocean are, often, specially
adapted to the conditions, which
prevail there- darkness and low light.
The eyes of certain crustaceans, such
as lobsters, prawns and crayfish, are
radically different; geared to
function in low light condition.
Rather than employing a refractive
lens, they use a system of mirrors to
focus light onto the retina. That is
they use reflection rather than
refraction to collect and focus light
that falls on their eyes. Refraction
usually ‘wastes’ some amount of
incident light, while reflection is more economical with hardly any ‘wastage’.
The eye of a crustaceans like lobster, shrimps and prawns show a astonishing eye
arrangement- it has tiny facets that are perfectly square. Seen through the microscope it
appears like a perfect graph paper. Actually, the graph paper appearance is caused by the
ends of many tiny square tubes on a spherical surface. The sides of the tubes are very flat
and shiny mirrors, and their precise geometrical arrangement means that parallel light
rays are all reflected to a focus. The square arrangement is crucial, because only with the
reflectors at right angles can it form an image from light rays from any direction. Also,
the tubes are about twice as long as they are wide; and thus they reflect most light rays
off exactly ‘two mirrors’. These ‘mirrors’, are in fact, materials that utilise alternating
high and low refractive index multilayers to render their surfaces reflective.
Rays from a distant object enter the microscopic pores of the eye, and are reflected at
shallow angels such that the light graze downs the channel. The refracted light arrives at a
point on the focal surface. This surface in lobster is analogous to the retina of our eyes.
The point at which the light is focused depends on the location of the object. Two sources
are focused to distinct two points on the focal surface. Say for example two parts of a
single object such as the nose and tail of a fish, and all the points in between, are brought
to focus at distinct points
on the retina of lobster.
This, then, is an imaging
system - just like our own
eyes, albeit not with usual
lens but with special
arrangement. Concentrating
light from a relatively wide
area is useful when it’s
quite dark. How does the
lobster cope with increase
Left: photograph of an MCP measuring approximately 35mm
in brightness? In bright
across, and only three quarters of a millimetre thick. Right:
light the lobster’s eye
seen under the microscope, the tiny pores (each about 2
moves opaque pigment to
hundredths of a millimetre across) become visible.
block all light rays to the
retina other than those
parallel to the tubes. Thus the amount of light that falls on the retina is controlled.
Learning from Lobsters
Biologist had studied and had unravelled the astounding structure of the lobster’s eye.
Astronomers who were searching for a way to overcome the shortcomings of traditional
X rays telescope, came across this study and exclaimed that that this problem ‘might be
overcome by copying the design of crustacean eyes’.
Lobster eyes work by reflecting visible light at these small angles. In order to focus Xrays, we have to make mirrors which work by reflecting X-rays at shallow angles that is
“grazing incidence”. We can therefore use the same arrangement of lobster’s eyes to
build a telescope to work at X-ray wavelengths, without having to use the bulky,
relatively inefficient mirrors, which have been used up until now in the traditional X-ray
telescopes.
This diagram shows how an array of microscopic
holes, or "channels" can be used to focus light. The
glass microchannel plates used in these
instrument work on precisely the same principle
as that of Lobster’s eyes.
It is clear that to use this method of
focusing X-rays, we must somehow
recreate the eye of a lobster,
providing millions of tiny square
holes or channels down which the
light –or rather in this case X-rayscan be reflected. In fact, this sort of
technology has been around for quite
some time and has been used, but for
a different purpose: to detect the light,
rather than refract and focus it. In
other words, analogous to do the
work of the photographic film rather
than the camera lens. These devices
are called microchannel plates or
MCPs. Rooted in military technology,
MCPs were being developed to
provide a solution to the problem of
high resolution imaging in low light
conditions. This technology were
being used in products such as night
vision goggles.
Gaining insight from the Lobster-eye optics and taking the cue of MCP technology,
astronomers are developing a new x-ray telescope, called Lobster- ISS. The MCP based
telescope is an elaborate a 5 by 5 cm array of tiny (10–200 microns (µm) across) square,
hollow tubes made of X-ray-reflecting lead glass. A hundred of these would be grouped
into modules, and 20 modules fitted to the telescope. These micro tubes are about 0.5–1.0
mm deep. The array are heated and curved into part of a sphere to a radius of curvature of
75 centimetres, just like the lobster eye. Lobsters X Ray telescope are without lenses, and
work by reflecting light from the inside of large numbers of square tubes, arrayed on the
surface of a sphere. The telescope consists of six identical modules, each fabricated from
a large number of microchannel plates and each module has a large-area imaging counter
in its focal plane to detect X-rays. Such a telescope could image faint objects in the 0.52.4keV
band
with
a
signal
to
noise
ratio
of
5.
These
devices are expected to allow very wide field (more than 1000 square degrees)
monitoring of the sky in X-rays (up to 10 keV and perhaps even more) with faint limits.
The Lobster telescope would, therefore, provide unprecedented opportunities for
monitoring time-varying X-ray sources.
Gamma ray telescopes
Gamma-ray astronomy is a late bloomer. The techniques needed to detect the highest
energy photons have only become available since the late 1960's - a blink of the eye in
terms of our involvement in astronomical research. Gamma-rays simply pass through
most materials and thus cannot be reflected by a mirror like optical or even X-ray
photons. Therefore detectors are much more sophisticated and intricate.
Gamma rays are produced in the processes that includes cosmic ray interactions with
interstellar gas, supernova explosions, and interactions of energetic electrons with
magnetic fields. Gamma rays unravel a distinct face of universe- 'violent' universe,
because the kinds of events in space that produce gamma-rays tend to be explosions,
high-speed collisions, and such.