Active optics

Big telescopes use mirrors to see stars.

These mirrors can be very thin. They are light so they can be large. But wind or heat can bend them.

Small tools push on the back of the mirror. These tools keep the mirror in the right shape. A computer tells the tools what to do. This helps the telescope take great pictures of space.

Most big telescopes use mirrors to see space.

In the past, mirrors had to be very thick. This helped them stay in the right shape. Thick mirrors can only be a certain size. They cannot be much larger than 5 or 6 metres.

New telescopes use thin, light mirrors instead. These mirrors can be much bigger. But thin mirrors can bend easily. Wind, heat, or gravity can change their shape.

To fix this, scientists use active optics. This is a way to keep the mirror in its best shape.

Small tools called actuators are on the back of the mirror. These tools push or pull on the mirror. A computer and a detector help the tools work. The computer tells the actuators how to move. This keeps the mirror smooth even when the weather changes.

Active optics works slowly, over a few seconds. It is not the same as adaptive optics. Adaptive optics works much faster to fix blurry air.

Big telescopes use large mirrors to see the stars.

These mirrors reflect light to create images. In the past, mirrors had to be very thick. Thick mirrors stay in the right shape on their own. This thickness limits how large a mirror can be. Most old mirrors were only 5 or 6 metres wide. One famous example is the Hale Telescope at Palomar Observatory. Today, we want much larger mirrors to see further.

Modern telescopes use thin and light mirrors instead. These thin mirrors allow for much bigger sizes. However, thin mirrors bend quite easily. Wind and temperature can change their shape. Gravity also makes large mirrors sag under their own weight. Active optics is the way we fix these problems.

This system keeps the mirror in its best shape.

How does active optics work? It uses a group of small tools called actuators. These actuators sit on the back side of the mirror. They apply different amounts of force to the mirror body. A computer and an image quality detector work together too. The detector sees the image quality. Then, the computer tells the actuators how to move. This process happens over a few seconds. It keeps the mirror steady against slow changes.

Scientists developed this technology in the 1980s. It changed how we build huge telescopes. Without it, 8 metre class telescopes would not be possible. It also makes segmented mirrors work well. Segmented mirrors use many small pieces instead of one huge piece. Many famous telescopes use this method today. The Keck telescopes and the New Technology Telescope use it. The Nordic Optical Telescope and the Telescopio Nazionale Galileo also use it.

Do not confuse active optics with adaptive optics. They both help mirrors, but they work differently. Active optics fixes slow changes like heat or gravity. Adaptive optics works much faster to fix blurry air. It corrects for the atmosphere at a very high speed. Active optics handles changes that happen about once per second. Adaptive optics handles changes hundreds of times every second. One keeps the mirror shape, while the other fixes the light path.

Active optics is a vital technology used in modern reflecting telescopes.

This system actively shapes a telescope's mirrors to prevent deformation. External influences like wind, temperature, and mechanical stress can change a mirror's shape. If a mirror loses its perfect curve, the images of stars become blurry. Active optics ensures that the reflecting surface remains in its optimal shape. Without this technology, building 8 metre class telescopes would be impossible. It also makes telescopes with segmented mirrors feasible for astronomers.

To understand the mechanism, we must look at how the system functions. Modern telescopes often use thin, lightweight mirrors instead of very thick ones. An array of actuators is attached to the rear side of the mirror. Actuators are small devices that apply variable forces to the mirror body. These forces help keep the reflecting surface in the correct shape as the telescope moves. The process involves three main parts: the actuators, an image quality detector, and a control computer. The detector monitors the image quality and sends data to the computer. The computer then calculates the necessary adjustments and tells the actuators how to move. This entire cycle happens over timescales of roughly seconds.

There are different ways to design these large mirrors. Some telescopes use a single, large, monolithic mirror. However, large monolithic mirrors can suffer from sagging due to their own weight. To solve this, scientists can use segmented mirrors. A segmented mirror is made of multiple smaller mirror pieces. This design reduces the sagging that occurs in very large, single pieces. Regardless of the mirror type, the active optics system works to compensate for distorting forces. These forces include gravity, thermal expansion, and telescope axis deformation.

Active optics was developed in the 1980s. This innovation changed the history of astronomical observation. Before this era, primary mirrors had to be very thick to stay rigid. This thickness limited the maximum diameter of a mirror to about 5 or 6 metres. An example of this older style is the Hale Telescope at Palomar Observatory. Since the mid-1990s, all the largest telescopes built have used active optics. This technology is used in the Nordic Optical Telescope and the New Technology Telescope. It is also used in the Telescopio Nazionale Galileo and the Keck telescopes.

The significance of active optics is seen in the scale of modern instruments. It allows for much larger collecting areas than the 5 or 6 metre limit of the past. The system handles environmental influences that are intrinsically slow, often around 1 Hz. This means the changes occur roughly once per second. Because these forces have a large amplitude in aberration, they require the strength of the actuators to correct. By maintaining the mirror's shape, the telescope remains "actively still" in its optimal state.

It is important not to confuse active optics with adaptive optics.

While they sound similar, they operate on very different timescales and solve different problems. Active optics corrects for slow mirror deformations caused by temperature or gravity. Adaptive optics corrects for atmospheric distortions that affect light as it travels through the air. Atmospheric distortions are much faster, occurring at the Greenwood frequency of 100 to 1000 Hz. Because adaptive optics must work so quickly, it uses much smaller corrective mirrors. These smaller mirrors might be the second, third, or fourth mirror in a telescope's light path.

Beyond standard telescopes, active optics can be used in other complex scientific setups. It can be used to actively stabilize complicated laser setups and interferometers. In these systems, a small part of a laser beam leaks through beam steering mirrors. A four-quadrant-diode is used to measure the position and direction of the beam. Scientists can also use a PID controller to speed up the system or make it more immune to noise. For pulsed lasers, the controller can be locked to the repetition rate. Researchers are even investigating X-ray active optics that use actively deformable grazing incidence mirrors.