Twinkle – Twinkle little Staror, how “untwinkling” the stars led to better astronomy

Stars twinkle, they always have Why do they twinkle? Why do Astronomers not like it?

Can we do something about it? Select high altitude observatory sites Go out into Space? Go to Antarctica? Adaptive Optics

Untwinkling the Stars Active Optics

The path to extra large telescopes

Serendipity Laser Cannons

Twinkle, little Star

Twinkle, twinkle, little star, How I wonder what you are! Up above the world so high, Like a diamond in the sky. Twinkle, twinkle, little star, How I wonder what you are!

Ah! vous dirai-je, Maman

Ah! I shall tell you, mum,what causes my torment.Papa wants me to reasonLike an adult.I say that candyIs better than reason.

1761 French nursery rhyme where the ‘Twinkle’ melody originated

OK, so why do stars twinkle?The light of a star on its way to us passes through numerous layers in the Earth atmosphere. Minute movements or density changes in the air above us caused by temperature fluctuations and pressure waves makes the image of a pin-point star wiggle or “twinkle” in shape and brightness.

It has always been so, and this is actually an aspect of the night-sky that makes it seem alive. In contrast to a Planetarium show where the stars are shown clear and bright, but appear dead in comparison to the real sky.

Can we “untwinkle” the Night Sky?

.

Richard Dawkins, the renowned biologist / writer and father of the expression the “God Delusion” once wrote a book called:

Un-weaving the Rainbow.

In it he tells of the public’s shock reaction to Newton’s discovery of the laws of refraction and the nature of the rainbow. “Interfering in God’s handiwork”, they shouted, “destroying our faith in His biblical promise” and “taking the wonder out of one of Nature’s most beautiful mysteries”.

Will the twenty-first century’s ‘Untwinkling of the Stars” cause a similar public outcry?

Will we no longer be able to sing one of the most popular English nursery rhymes in faithful innocence, or claim that chocolate is better than reason?

Unweaving the Rainbow

Dawkins and his Rainbow

But Astronomers don’t like it

It is clear that what appears to the naked eye as a lovely “twinkle” plays havoc with high resolution images.

Grouped under the generic term astronomical “seeing” it changes with the angle of elevation and the altitude of the observatory.

The higher, the better! Mountain tops and deserts are the preferred locations.

With small telescopes it does not really matter

But when, with the manufacture of larger telescopes, the measurement resolution reached the sub-arc-seconds range, the consistency of air became a major hurdle.

Turbulence in the atmosphere causes spatial and temporal anomalies in the atmosphere's refractive index. It appeared to be an insurmountable barrier to going any further with larger telescopes.

Unless you could someway-how compensate for air turbulence

Objective Area ~ 500,000cm2

80% efficient95% efficient

Reflection and Diffraction Reflection is the response of light (or of other kinds

of wave) from a surface, in which light from a single incoming direction (a ray) is reflected into a single outgoing direction. Such behavior is described by the law of reflection, which states that the direction of incoming light (the incident ray), and the direction of outgoing light reflected (the reflected ray) make the same angle with respect to the surface normal.

"The angle of incidence is equal to the angle of reflection.” This behavior was already discovered through careful observation and measurement by Hero of Alexandria (10–70 AD)

It works the same way for curved surfaces. Simply draw the tangent line to the point of the curve and reflect the light according to the tangent line.

A short digression to ResolutionAn imaging system's resolution can be limited either by aberration or by diffraction causing blurring of the image. These two phenomena have different origins and are unrelated. Aberrations can in principle be solved by increasing the optical quality of the system. But diffraction comes from the wave nature of light and is determined by the finite aperture of the optical elements.

Light from an objective interferes with itself creating a ring-shape diffraction pattern, known as the Airy pattern, limiting resolution. The angular resolution R of a telescope at visible light can be approximated by:

R= 11.6/D where D the diameter of the lens in centimeter. For a resolution of ~ 0.1 arc second we need (at visible light) D = 1.2 m.

Airy diffraction pattern generated by a plane wave falling on a circular aperture, such as the pupil of the eye.

The human eye has a theoretical diffraction limit of 23 arcseconds but an average resolution of 1 arcminute

Overview Chart of

Arcsecond Diffraction Limits versus telescope size and radiation frequency of some of the better-known detectors.Visible light

Hubble SpacetelescopeGalileo’s telescope

Human eye

On Mountain Tops The Anglo-Australian Telescope (AAT) is a 3.9 m

equatorially mounted telescope operated by the Australian Astronomical Observatory and situated at the Siding Spring Observatory, Australia at an altitude of a little over 1100 m.

Commissioned in 1974 with a view to allowing high quality observations of the sky from the southern hemisphere.

In 2001-2003 it was considered the most scientifically productive 4 meter-class optical telescope in the world, based on scientific publications using data from the telescope.

In 2009, the telescope was ranked as the 5th highest impact of the world's optical telescopes.

It has also been fitted with adaptive optics

Official Opening of the AAT in 1974 with Prince Charles and Fred Hoyle (on the left)

In Space Space Telescopes provide beautiful

images. Not affected by atmospheric movement or absorption, their sensitivity and frequency range are only limited by their design

But they are very expensive to install and upgrade, and have a limited life-time (with attitude propellant etc.)

And the diffraction limit still applies to them, At visible light for instance:

R = 11.6/D, were D is in centimeters and R in arcseconds

And there is (at present at least) still a practical limit to their physical size

But they can detect all frequencies.

The Hubble Space Telescope was carried into orbit by a Space Shuttle in 1990 and is still in operation. Above the atmosphere its 2.4 meter aperture allows it to take extremely sharp images to diffraction limit of the universe's most distant objects. It has aRitchey / Chretien reflector - focal length 57.6 m - mass11,110 kg – orbital period 96 –97 minutes - Diffraction Limit ~ 0.05arcsec

The “Pillars of Creation” - a famous Hubble image of dust clouds in the Eagle Nebula, where new star formation is taking place. Taken in 1995 it is still classed as one of the 10 best photos taken by the Hubble. About 7,000 light years from Earth.

What to doAs the technology of telescope making improved, and the diffraction limit was pushed back with larger and larger diameter telescopes, atmospheric absorption and instability became a serious stumbling block limiting resolution.

Placing telescopes on high mountains and in a dry climate provides some relief. The mountains in Hawaii and in Chile became popular observatory locations.

But putting the telescope beyond Earth’s atmospheric envelope seemed to be the only answer to overcome this hurdle of the twinkling atmosphere. The Hubble Space Telescope provided the world with stunning pictures that earthbound telescopes could never hope to equal. Or could they?

This Hubble Deep Field image covers an area 2.5 arcminutes across, about one 24-millionth of the whole sky, which is equivalent in angular size to a 65 mm tennis ball at a distance of 100 metres.

Why Size Matters As seen before, telescope size does of course

matter for greater light gathering power and the ability to see farther.

But just as important is the fact that the Diffraction Limit of a curved reflecting / refracting surface decreases in line with the curvature (the diameter) of the lens / mirror.

How does a perfect 10m telescope compare with a perfect 30m telescope when it comes to resolving visible light (5.50 x 10-7m) detail? The 10m diffraction limit is

2.1x105 x 5.50x10-7 / 10 = 0.012 arcseconds

And the 30m telescope is 2.1x105 x 5.50x10-7 / 30 = 0.0039 arcseconds In line with diameter, about three times better.

Defining Diffraction Italian scientist Francesco Maria Grimaldi coined

the word "diffraction" and was the first to record accurate observations of the phenomenon in 1665. In classical physics, the diffraction phenomenon is described as the apparent bending of waves around small obstacles and the spreading out of waves past small openings. Thomas Young’s sketch of 1803

Diffraction occurs with all waves, including sound waves, water waves, and electromagnetic waves such as visible light, x-rays and radio waves.

Richard Feynman once said that "no-one has ever been able to define the difference between interference and diffraction satisfactorily. It seems when there are only a few sources (as in Young's slits) we call it diffraction, but with a large number of sources the process is labeled interference”.

The Airy disk around each of the stars from the 2.56 m telescope aperture can be seen in this lucky image of the binary star zeta Boötis

The European Southern Observatory

is an intergovernmental research organisation for astronomy, supported by fifteen countries. Created in 1962, ESO has provided astronomers with state-of-the-art research facilities and access to the southern sky. The organisation employs around 730 staff members and receives annual member state contributions of approximately 143 million Euros.

The Very Large Telescope of the European Southern Observatory on Cerro Paranal in the Atacama Desert of northern Chile consists of four individual telescopes, each with a primary mirror 8.2m across, which can be used together to achieve very high spatial resolution of about 0.001 arc-second.

This is equivalent to separate the headlights of a car on the Moon Each individual telescope can detect objects roughly four billion times fainter

than what can be detected with the naked eye. The light beams are combined, using a complex system of mirrors in

underground tunnels, where the light paths must be kept equal with precision of less than 1/1000 mm over a hundred metres

Most modern telescopes are reflectors, with the primary element being a very large mirror. At 8.2m the VLT are amongst the largest single mirrors built.

Historically, the mirrors had to be very thick to hold its shape to the required accuracy as the telescope travelled across the sky.

A new generation of telescopes built since the 1980s uses instead very thin mirrors, which are too thin to keep themselves rigidly in the correct shape.

Instead, an array of actuators behind the mirror keeps it in an optimal shape. The telescope may also be segmented into many small mirrors, preventing most of

the gravitational distortion that occurs in large, thick mirrors.

The adaptive optics images of the VLT are up to three times sharper than those of the Hubble Space Telescope, and the spectroscopic resolution is many times better than

Hubble. The VLTs are housed in compact, thermally controlled buildings, which rotate

synchronously with the telescopes. This design minimises any adverse effects on the observing conditions, such as air turbulence in the telescope tube due to variations in the temperature and wind flow.

The European Southern Observatory with the New Technology Telescope (NTT) pioneered active optics technology.

It is fitted to most large telescopes built in the last decade.

Active Optics Active optics is a technology used with reflecting telescopes developed in

the 1980s, which actively shapes a telescope's mirrors to prevent deformation due to external influences such as wind, temperature, mechanical stress. Without active optics, the construction of 8 meter class telescopes would not be possible, nor would telescopes with segmented mirrors be feasible.

The method is used by, among others, the Nordic Optical Telescope, the New Technology Telescope, the Telescopio Nazionale Galileo and the Keck telescopes, as well as all of the largest telescopes built in the last decade.

Active optics is not to be confused with adaptive optics, which operates at a shorter timescale and corrects different distortions

What else can we do?

How about going to Antarctica?

The driest atmosphere on Earth

Constant cold Temperature

Reasonable elevation Low Background

noise

Astronomy in Antarcticaa talk by Professor Jeremy Mould at the ASV in June 2011

A feasibility study on a remote test site for a large telescope

PLATO (Plateau Observatory is a current international collaboration between America, Australia and China (in alphabetical order) of a feasibility study project in Antarctica

Instruments are contributed from Australia, China, New Zealand, the United Kingdom, and the United States of America

PLATO is a fully-robotic observatory designed for operation in Antarctica. It generates its own electricity (about 1kW), heat (sufficient to keep two 10-foot

shipping containers comfortably above 0C when the outside temperature is at -70C), and connects to the internet using the Iridium satellite system (providing 30MB/day of data transfer).

At the elevation of ‟Dome A” (4,000m) it is feasible to greatly simplify or even eliminate the adaptive optics normally needed to remove the effects of air turbulence.

This is probably the only place on Earth that can routinely observe at the terahertz frequencies crucial to the understanding of the interstellar medium, and the life cycle of stars.

The birth of Adaptive OpticsIt started with a simple Tip / Tilt

computer controlled mirror (TT) arrangement in the Telescope’s light-path. The idea was to refocus the image of a point-like star and thereby eliminate the atmosphere’s twinkling.

Simple enough in principle. But the number-crunching to cope with multiple of corrective data every second had to wait for modern computer speed

Nowadays Adaptive Optics allows earth-bound telescope resolution to their optic’s Diffraction Limit, taking full advantage of the larger collecting area of new, huge mirror sizes

Schematic illustration of an adaptive optics system. Light is shown with dotted lines, control connections with dashed lines. The wavefront enters the system at the top. The light first hits a tip–tilt mirror and is then directed to a deformable mirror. The wavefront is corrected and part of the light is tapped off by a beamsplitter . The control hardware then sends updated signals to the mirrors.

Laser Beam to the Rescue For Adaptive Optics to work, the system needs a bright

reference star in the field of view of the telescope, close to the observed object.

As this is not every time the case (very seldom in fact for the images astronomers are interested in) a Laser is used to project a beam of light into the field of view of the telescope.

The Laser makes molecules of sodium in the upper atmosphere glow, forming the point source needed.

The Laser is usually pulsed to allow skipping interference from the lower atmosphere

Sometimes several laser beams are used for telescopes with a wider field of view.

Here is a short Presentation on Adaptive Optics by John O’Byrne, School of Physics University of Sydney

The expanding use of Adaptive Optics

High Power Laser Welding and Cutting Tools for Precision Manufacturing Medical Physics: Optamology Laser Surgery and SUPERVISION? Medical Physics: Color Perception and SUPERVISION Medical Physics: Non intrusive Surgery

Of course, as happens with any new invention, the military soon find a use for it: the so-called Laser Cannon (or Death Ray) depends on accurate focussing of the Laser Beam. With Adaptive Optics the weapon of the future on the drawing board is a 100KW cannon with unlimited range.

A directed-energy weapon (DEW) emits energy in an aimed direction without the means of a projectile. It transfers energy to a target for a desired effect. Intended effects may be non-lethal or lethal. Some such weapons are real, or are under active research and development

Adaptive Optics in action

From Untwinkling the Stars to non-intrusive Surgery and to Weapons of the future, the FUTURE of Adaptive Optics is virtually unlimited

THE END