# Tag Archive

## Atmosphere

### Forecasting of optical turbulence

Optical turbulence (OT) is a generic term referring to the impacts of atmospheric turbulence on the propagation of optical waves in general. It is often used in both fields of astronomy and optical communication, especially when describing different quantities, tools or approaches to characterize atmospheric effects on optical waves. In this post, a general approach for forecasting OT is presented, that has many applications at astronomical and optical communication sites.

### Meet the refractive index structure parameter

Put a straw in a glass of water and take a close look. It appears to be bent or, even worse, broken in two. This is because of refraction, the physical phenomenon behind the redirection of waves (such as light) when there is a change of propagation medium (i.e. water to air in this case). Similarly, the Earth’s atmosphere modifies the trajectory of light. In fact, our atmosphere is not a uniform medium since each layer of air has its own properties (pressure, temperature, density, …). Hence, refraction occurs between the different layers, forcing the light from a star to zigzag before reaching our eyes for example. Why a zigzag and not just a curved path? Because of turbulence in the atmosphere that randomly modifies local atmospheric properties, thus randomly changing the redirections of the light. This leads to the twinkling of stars, also named scintillation.

### Atmospheric challenges for astronomy

In their fascinating quest of observing the universe, astronomers are facing gargantuan challenges such as the faintness of distant objects, the blindness arising from absorption in nebulae (i.e. gas clouds) or even light pollution from human activities. And one of the biggest of these challenges is also the closest to us: the Earth’s atmosphere and its associated blurriness.

### Twinkling of stars

Stars are twinkling. Any of us can observe this phenomenon by looking at a clear night sky: stars seem to quickly change of apparent brightness, colour or even position. This is what is described as twinkling. In fact, the origin of this phenomenon is not related to the stars themselves but rather to the medium in which their light propagates at the end of their long journey through space: our atmosphere.

## Scintillation

### Meet the refractive index structure parameter

Put a straw in a glass of water and take a close look. It appears to be bent or, even worse, broken in two. This is because of refraction, the physical phenomenon behind the redirection of waves (such as light) when there is a change of propagation medium (i.e. water to air in this case). Similarly, the Earth’s atmosphere modifies the trajectory of light. In fact, our atmosphere is not a uniform medium since each layer of air has its own properties (pressure, temperature, density, …). Hence, refraction occurs between the different layers, forcing the light from a star to zigzag before reaching our eyes for example. Why a zigzag and not just a curved path? Because of turbulence in the atmosphere that randomly modifies local atmospheric properties, thus randomly changing the redirections of the light. This leads to the twinkling of stars, also named scintillation.

### Atmospheric challenges for astronomy

In their fascinating quest of observing the universe, astronomers are facing gargantuan challenges such as the faintness of distant objects, the blindness arising from absorption in nebulae (i.e. gas clouds) or even light pollution from human activities. And one of the biggest of these challenges is also the closest to us: the Earth’s atmosphere and its associated blurriness.

### Twinkling of stars

Stars are twinkling. Any of us can observe this phenomenon by looking at a clear night sky: stars seem to quickly change of apparent brightness, colour or even position. This is what is described as twinkling. In fact, the origin of this phenomenon is not related to the stars themselves but rather to the medium in which their light propagates at the end of their long journey through space: our atmosphere.

## Light propagation

### Forecasting of optical turbulence

Optical turbulence (OT) is a generic term referring to the impacts of atmospheric turbulence on the propagation of optical waves in general. It is often used in both fields of astronomy and optical communication, especially when describing different quantities, tools or approaches to characterize atmospheric effects on optical waves. In this post, a general approach for forecasting OT is presented, that has many applications at astronomical and optical communication sites.

### Meet the refractive index structure parameter

Put a straw in a glass of water and take a close look. It appears to be bent or, even worse, broken in two. This is because of refraction, the physical phenomenon behind the redirection of waves (such as light) when there is a change of propagation medium (i.e. water to air in this case). Similarly, the Earth’s atmosphere modifies the trajectory of light. In fact, our atmosphere is not a uniform medium since each layer of air has its own properties (pressure, temperature, density, …). Hence, refraction occurs between the different layers, forcing the light from a star to zigzag before reaching our eyes for example. Why a zigzag and not just a curved path? Because of turbulence in the atmosphere that randomly modifies local atmospheric properties, thus randomly changing the redirections of the light. This leads to the twinkling of stars, also named scintillation.

### Introduction to optical communication for space applications

$\cdot \cdot - \cdot \text{ } \cdot \cdot \cdot \text{ } - - -$? Do you remember once using a flashlight to send Morse code to a friend? Then both of you were using a form of optical wireless communication. Now imagine doing it so fast that your eyes cannot see the flashlight blinking anymore, transmitting more data that your brain would never be able to process, over distances that cannot be travelled in a lifetime. THIS is real optical wireless communication.

### Atmospheric challenges for astronomy

In their fascinating quest of observing the universe, astronomers are facing gargantuan challenges such as the faintness of distant objects, the blindness arising from absorption in nebulae (i.e. gas clouds) or even light pollution from human activities. And one of the biggest of these challenges is also the closest to us: the Earth’s atmosphere and its associated blurriness.

## Space telescope

### Atmospheric challenges for astronomy

In their fascinating quest of observing the universe, astronomers are facing gargantuan challenges such as the faintness of distant objects, the blindness arising from absorption in nebulae (i.e. gas clouds) or even light pollution from human activities. And one of the biggest of these challenges is also the closest to us: the Earth’s atmosphere and its associated blurriness.

## Free space optical communications

### Introduction to optical communication for space applications

$\cdot \cdot - \cdot \text{ } \cdot \cdot \cdot \text{ } - - -$? Do you remember once using a flashlight to send Morse code to a friend? Then both of you were using a form of optical wireless communication. Now imagine doing it so fast that your eyes cannot see the flashlight blinking anymore, transmitting more data that your brain would never be able to process, over distances that cannot be travelled in a lifetime. THIS is real optical wireless communication.

## Space optics

### International Conference on Space Optics (ICSO), 2022

For this new edition of the International Conference on Space Optics (ICSO), organized by European Space Agency (ESA), nothing has been left to chance. Plenary, technical and poster sessions were perfectly intertwined with several networking events, much appreciated by the space optics community after more than two years of online events.