Taking the twinkle out of stars

Earth's atmosphere has long plagued ground-based astronomical observations because turbulence blurs incoming light as it passes through the atmosphere. The Adaptive Optics Facility (AOF) at the European Southern Observatory's Very Large Telescope (VLT) in Paranal, Chile offers the world's most sophisticated optical systems for removing these distortions from ground-based observations of deep space objects.

In August 2017, the telescope, coupled with the AOF, produced wide-field images of distant galaxies having twice the contrast previously possible. Now, new modules are paving the way for astronomers to clearly zoom in on single stars and more distant deep space objects over a larger swath of the sky.

Four-star design
Earlier, astronomers corrected for blurring caused by atmospheric turbulence by observing a bright natural reference star. But as telescopes looked deeper in space, finding a neighboring bright star became impossible. This led astronomers to use high-powered lasers to create a well-defined point of light in the sky that could be treated as an artificial ‘guide star' for adaptive correction.

To meet the VLT's requirements of a bright guide star system that operates reliably every night, physicist Domenico Bonaccini Calia and the laser group at ESO conceived a system that uses not one, but four lasers. The project team built the Four Laser Guide Star Facility (4LGSF), which became operational at the end of 2016.

Each laser excites sodium atoms at the edge of the mesophere, 90 km above Earth's surface, producing four artificial stars. The team patented and demonstrated a new amplification principle, based on stimulated Raman scattering of photons, to generate a narrow band, high-power fiber amplifier, which has been converted into a free-space 589nm source at 22W.

"When we started, we pre-paid twenty days' laser support from the contractor," says Bonaccini Calia. "[The laser] works so well that we've not pre-paid any manpower in our renewed laser support contract!"


Schematic view of the Four Laser Guide Star Facility on Unit Telescope 4 of the VLT. Credit: ESO/L. Calcada

The VLT has four individual unit telescopes. In each telescope stellar light collected by an 8.2m-diameter primary mirror is concentrated, by the combination of the primary and a secondary mirror, to focal points where imaging instruments sit. The shape of the new deformable secondary mirror is directly responsible for the image quality. The ‘magic' of adaptive optics lies in changing the mirror's shape to compensate for aberrations caused by the atmosphere.

Sensors in the telescope analyze light from the guide stars by measuring aberrations of the incoming wavefront. Because reflected light from the guide stars travels approximately the same path through the atmosphere as did light from real stars, those wavefront aberrations provide a direct measurement of atmospheric blurring. A computer calculates the correction that must be applied to the telescope's deformable mirror to compensate.

In most adaptive optics systems, a separate deformable mirror is attached to the telescope to give an intermediate focus. But the VLT's 1120mm-diameter deformable mirror takes the place of the telescope's secondary mirror. It's the largest adaptive optics mirror ever produced, and is only 1.95mm thick.

Key to its design is a rigid mount that holds 1170 actuators. These change the thin shell mirror's shape based on measured distortions to the returning wavefront of the guide star. A current applied to the actuators generates a magnetic field that interacts with each of 1170 magnets on the back of the flexible mirror. Guide stars and sensors work together to continuously adjust the mirror by sending signals to the actuators.

"The goal is to make the use of this very complex facility straightforward. The astronomer should not have to set numerous parameters to optimize the system," says Robin Arsenault, AOF Project Manager.

Testing the thin shell mirror for the deformable secondary mirror. The flexible shell is 1120 mm across and less than 2 mm thick (SAGEM/ESO). B)1170 actuators mounted on the deformable secondary mirror. Credit: ESO

Ground floor
Sensors in the adaptive optics module GALACSI (Ground Atmospheric Layer Adaptive Corrector for Spectroscopic Imaging) are designed to correct for the high level of turbulence found close to Earth's surface. Turbulence in the atmospheric boundary layer is one of the most challenging problems for wide-field observations, where crisp images require long periods of time to produce.

The Multi Unit Spectroscopic Explorer (MUSE) instrument operates in the visible wavelength range and, aided by GALACSI, produces sharp images from faint objects over a wide section (1 x 1 arcmin) of the sky. Observations released in August showed dramatic improvements in image sharpness, revealing previously unseen structures in planetary nebulae in the constellations Lupus and Ophiuchus.

"That was not possible before. Before we could get the rare good image. Now with ground layer correction we can get that maximum quality over the wide field on a regular basis," says Joël Vernet, MUSE instrument scientist.

MUSE was designed to accommodate adaptive optics. The instrument generates a 3D data set of a target object, where each pixel of the image corresponds to a specific wavelength of the light from the object. Thousands of images of the object are produced simultaneously, each at a different wavelength of light resulting in a wealth of information. The design couples high spatial resolution with high spectral resolution.

"Everything has worked as planned, it's been quite a long journey," says Vernet. "It's reactive and efficient with a short acquisition time."


Planetary nebula NGC 6369 in the constellation Ophiuchus, before and after the AOF ground layer correction. Credit: ESO/P. Weilbacher

Cutting edge
But "fishing for lots of objects in the wide field mode" is only part of the telescope's mission, adds Vernet. The team is now commissioning a narrow-field mode. With this, astronomers will be able to zoom in to observe distant planets and study the kinematics of stars near black holes at the centers of galaxies.

Turbulence occurs not only in the boundary layer, but also higher in the atmosphere. The narrow-field mode corrects for turbulence at different altitudes of 6 and 13 km in addition to the ground layer, allowing observations of smaller fields of view (7.5x7.5 arcseconds) at resolution reaching the telescope diffraction limit.

Simply averaging the wavefront sensor signals, as for the boundary layer correction, does not work for turbulence higher in the atmosphere. "In a normal guide star correction, you're probing turbulence in a cylinder above the telescope. With a laser guide star you're probing up to a single point, so you're measuring a cone. You don't see all the turbulence," explains Arsenault.

Instead, he and his colleagues developed a new technique called laser tomography. Refractive index fluctuations in each layer of the atmosphere are reconstructed from measurements from all the wavefront sensors. That information is used to build a ‘reconstruction matrix' that determines the deformable mirror correction.

"The most demanding performance specs were built for narrow-field mode," says Arsenault. With four guide stars, the laser tomography algorithm allows the telescope to approach the ultimate correction of a natural guide star.

Very large to extremely large
The MUSE instrument officially starts science operations with the wide-field mode on 1st April, and the narrow-field mode should be completed by the end of 2018. The entire system is a major steppingstone toward an even bigger telescope.

Construction began on the European Extremely Large Telescope (E-ELT) at Paranal Observatory in May 2017. Upon completion in 2024, it will be the largest optical and infrared ground telescope in the world. But constructing an adaptive optics system with laser guide stars for a telescope with a 39m primary mirror couldn't be done without lessons learned while commissioning its smaller neighbor.

The E-ELT will have a 2.5 m-diameter deformable mirror with 7000 actuators, and six laser guide stars, identical to those of the 4LGSF. The VLT will prove the concept of laser tomography for correcting turbulence at different layers. Ultimately, the E-ELT telescope will gather 15 times more light than today's best telescopes, allowing it to image distant exoplanets and helping scientists understand the evolution of distant galaxies.

"This is the first time this kind of adaptive optics has ever been developed for routine science observation" says Arsenault.

Rachel Berkowitz is a US-based freelance science writer. This article originally appeared in the July issue of SPIE Professional magazine.