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From mirror petals to laser stars: ELT adaptive optics milestones reached

6. oktober 2021

The largest adaptive mirror ever built, the M4 mirror for ESO’s upcoming Extremely Large Telescope (ELT), has reached an important milestone in its development: all six petal-shaped segments that make up the mirror are now completed.

M4, the fourth mirror in the light path of the telescope, can change shape quickly and very precisely, and is a key part of the ELT’s adaptive optics system. Light from cosmic objects is distorted by our planet’s atmosphere, producing blurred images. To correct for these distortions, the ELT will use advanced adaptive-optics hardware and software, some of which has been specially developed for the telescope. This includes powerful lasers that create artificial reference stars when there aren’t stars bright enough close to the object of interest to allow measurements of atmospheric distortions, and fast and accurate sensing cameras that measure these distortions. The measurements are then passed to extremely fast real-time computers that can calculate the necessary shape corrections to be applied to M4. In addition to the completion of the M4 petals, these systems have also all recently achieved key milestones.

Thanks to its adaptive-optics system, ESO’s ELT will deliver images sharper than those taken by current and future telescopes in space, like the NASA/ESA Hubble Space Telescope and the James Webb Space Telescope.

Final petal of M4 adaptive mirror delivered

With a diameter of 2.4 metres, M4 is the largest deformable mirror ever made and one of the most challenging and exciting components of what will be the largest visible and infrared telescope in the world. It is made up of six ultra-thin segments, the last two of which have now been finalised.

The six petals of M4 are made from Zerodur©, a special glass-ceramic material manufactured by SCHOTT in Germany. The French company Safran Reosc began polishing the M4 petals in 2017, turning each 35 mm-thin sheet of Zerodur© into a flexible segment less than 2-mm thick. All petals were checked by ESO engineers before being sent to Italian company AdOptica, who received the final one just a few months ago.

During the final production stages AdOptica have applied a coating to the mirror’s back surface and put in place lateral supports to connect the petals to the M4 mechanical structure. In addition, the companies’ technicians have glued more than 5000 magnets onto the mirror’s back surface, which play a role in deforming the flexible segments of M4, making adjustments 1000 times per second to an accuracy of 50 nanometers — as small as the tiniest viruses.

Now Safran Reosc is working to produce an identical set of petals, bringing the total number to 12. These will serve as spares and will be swapped out with the original six petals when they require recoating after a few years of use, minimising disruption to telescope observing time.

M4 reference body progressing

Because the M4 petals are so thin and need to be deformed to incredible accuracy, they require a very stable structure to support them: a reference body with attached magnets that support the mirror and adjust its shape. This reference structure is made by French company Mersen from Boostec® silicon carbide, one of the stiffest light materials available, and then polished by Belgian company AMOS, who are in the final stages of this process.

Getting the reference body to its final shape is extremely challenging. AMOS aims to get the structure flat to an accuracy of 5 microns, which has been made difficult by the fact that its surface has many holes to fit the M4 actuators.

Once the reference body is finished and delivered, AdOptica can begin the lengthy process of integrating the complete M4 unit, the structure comprising the mirror, its reference body and all support and connection elements. AdOptica is expected to do the first tests on the fully integrated M4 mirror in the final quarter of 2022.

Guide star laser accepted

One of the most visible components of the ELT’s adaptive optics systems will be its “laser guide star system” consisting of 6 lasers generating artificial guide stars in the upper atmosphere. To measure the distortions caused by the Earth’s atmosphere, the ELT’s adaptive optics system requires bright stars close to the object of study. Because these stars are not always conveniently placed, the ELT’s laser systems will allow astronomers to create artificial stars anywhere in the sky that is required, by exciting sodium atoms located at around 90km altitude in the atmosphere.

The first laser source for ESO’s ELT was finished by German company TOPTICA Projects in May 2021 and was delivered to ESO where it has now completed acceptance tests.

Progress for sensing cameras and ultra-fast computers

Another essential component of ESO’s ELT adaptive optics system are the so-called wavefront sensing cameras, which act as the “eyes” of the telescope by sensing the light from guide stars. ESO’s ELT will be equipped with three complementary types of wavefront sensing cameras, each with a distinct image sensor or detector, which will be used both by the telescope itself and by the science instruments.

These cameras are considered so critical for the functioning of the ELT adaptive optics that ESO has decided to do much of the work in-house. Two types of cameras, called ALICE and LISA, are designed at ESO while the third camera type, FREDA, is an adaptation of a commercially available camera (C-RED One) made by the French company First Light Imaging and modified to ELT standards by ESO engineers. In addition, ESO has developed, in collaboration with international company Teledyne, the detector for LISA, which will be ready for production by the end of this year. The design and prototyping activities for all three cameras should be completed next year.

Special computers on the ELT, called adaptive optics real-time computers, will then use the signals from the cameras to calculate how mirrors like M4 need to be deformed to correct for distortions caused by turbulence in the Earth’s atmosphere. A prototype computer developed at ESO has recently shown it can receive data from camera sensors and transmit it to the actuators that deform the mirror in just a few hundreds of microseconds.

While we are still a few years away from seeing the ELT’s complex adaptive optics systems completed, these recent advances show that significant progress is being made towards the scientific first light set for 2027. Once in operation, ESO’s ELT will dramatically change what we know about our Universe and will make us rethink our place in the cosmos.