Astronomical Telescopes and Adaptive Optics

Astronomical optics enables next generation, extremely large telescopes on Earth. At their core, adaptive optics technology plays an essential role to fully utilize the very large telescope aperture sizes.

Let’s imagine a Hubble space telescope in space looking at a star. The star is basically a point source located at infinity. The point source emits a spherical wavefront. It becomes a parallel collimated beam when it arrives the primary aperture of the Hubble Space Telescope.

The telescope optics can be simplified as a lens focusing the beam onto the detector plane. This is how a space telescope delivers an excellent quality image of the universe. However, a rocket launching the telescope cannot fit extremely large telescopes. In order to image very far away faint objects, astronomers have to build very large telescopes, which doesn’t fit in a rocket at all. The telescope needs to be built on ground.

On ground, the collimated light is not parallel anymore, but disturbed due to the atmospheric air turbulence. A fixed imaging system cannot focus the beam, and your image blurs. Unless you can solve this challenge an extremely large telescope on the ground will never produce beautiful photos of the universe.

The enabling technology is adaptive optics, utilizing a deformable mirror. A deformable mirror is a thin membrane mirror formed by hundreds of precision actuators behind the mirror surface. It can change the shape of the mirror down to nanometers of precision at the speed of 1 to 2,000 times per second. This enables compensating the disturbed light pass back to its original beam pass, focusing to a tight spot on the detector.

As shown in the video, once you turn on the adaptive optics, a blurred image becomes sharp. In order to compensate the disturbed light pass, one must know the perturbed light pass. The aberrated light pass can be measured using wavefront sensors, such as Shack-Hartmann wavefront sensor or pyramid wavefront sensor.

The measured wavefront data quickly controls the actuators in the deformable mirror to close the adaptive optics loop, canceling the light pass deviation.

Large binocular telescope launches high power, laser guide star using its ARGOS system. It creates a known light source in the upper atmospheric layer by looking at the reference laser star using the wavefront sensor. The wavefront aberration is precisely measured and corrected. The powerful adaptive optics concept becomes reality every night at the observatory.

Two giant LBT primary mirrors collect lots of photon energy from the astronomical object. These secondary mirrors are adapted mirrors with actuators to compensate the turbulence. Finally, the two beams are combined coherently at the center of the telescope in order to achieve diffraction limited point spread function shown on the right side.

The spatial resolution of the LBT interferometric measurement is defined by the baseline length of 23 meters between the two mirrors. This is the most powerful benchmark representing the advanced telescope technology as of today.

The next generation of extremely large telescopes are approaching quickly. The giant Magellan telescope is the future of astronomy, utilizing seven 8.4 meter primary mirrors paired with seven adaptive secondary mirrors. Its enormous 24.5 meter full aperture size will provide about 100 times light collecting power and 10 times spatial resolution of the Hubble Space Telescope.