[Image: Courtesy of Domenico Bonaccini Calia]
Domenico Bonaccini Calia, a physicist at the European Southern Observatory (ESO) in Munich, Germany, will deliver a plenary talk titled “The Ongoing Adaptive Optics Revolution” at the OSA Imaging and Applied Optics Congress in Munich, Germany this June. Calia gave OPN a preview of the program, offering insights into what laser guide stars and adaptive optics have accomplished so far—and what is yet to come.
OPN: As we’re speaking now you’re at the ESO’s Paranal Observatory in Chile. What are you working on there?
Domenico Bonaccini Calia: I’m working in laser-guide-star adaptive optics. This observatory has several adaptive-optics systems and, since 2016, several laser guide stars as well. After building and deploying the systems now in operation, I’m here to check them, propose changes regarding the preventive maintenance and to verify how things are going during operations, which are almost every night. We also propagate the lasers and observe the sky to do periodic calibrations.
This is all part of a continuing learning curve that relates to the next 40m class telescope project, the European Extremely Large Telescope, or ELT, which is going to be built about 25 kilometers from here by 2024 and will host 6–8 laser guide star units.
What do you think are some of the most impressive observations that adaptive optics and laser guide stars have enabled?
In astrophysics, the most impressive is the confirmation of the existence of a black hole in the core of our galaxy—a 4.3-million-solar-mass black hole. To confirm this, we essentially have to look in the center of the galaxy, where there are not many reference stars that we can use. So we create an artificial star with the laser, 90 km above the telescope, and then we use this reference star to measure the atmospheric turbulence and cancel it out.
Laser-guide-star adaptive optics is enabling lots of science, especially extragalactic—but also in solar system, exoplanet searches and a whole spectrum of fantastic, super-energetic extragalactic phenomena.
Also, in solar physics, there’s a lot going on with adaptive optics. The Sun is the only star where we can clearly see the surface and what’s going on. A lot of what we know about how stars work comes from our study of the Sun. And there is this internal fight on the surface of the Sun between the magnetic forces and the turbulence-induced pressure forces. High resolution is mandatory to understand what’s going on there. We are trying to predict solar flares, which can be very threatening for modern society, based on electricity and power grids. Adaptive optics is achieving very beautiful discoveries in solar physics at 0.1 and 0.05 arcsec resolution.
The title of your upcoming talk for the Imaging and Applied Optics Congress refers to an ongoing adaptive-optics revolution. What advances are being worked on today?
[Image: Getty Images]
My field is astronomy but I am keeping a broader view of this technology’s potential. What I mean with adaptive optics is the idea that, depending on the speed and variation of the wavefront, you can correct an optical wavefront in a dynamic way. So it’s something which reacts, in real time, to whatever deformation happens.
When I say a revolution, it’s because I see that now adaptive optics is penetrating in several other disciplines—this idea that you can actually control in real time and achieve diffraction-limited performance is applicable in a multitude of applications. And the list is expanding as the technology matures.
For example, in ophthalmology and with laser surgery you want to image the retina. It’s quite difficult because you have this water in your eyeball, and it’s very turbulent, so you can’t really see directly, and in the necessary detail, what goes on inside. And the patient is also moving their eye. You have to correct adaptively the imaging wavefront and also the laser if there is an operation ongoing.
Another area that is progressing is laser communication. In fact, I am involved in a project for satellite laser communication, and this involves correcting with adaptive optics for the atmosphere. You’re talking to a satellite, which is moving, but the atmosphere it’s crossing is constantly changing. So you have to point ahead; you have to think where it will be when the light reaches it and use a laser guide star to sample the atmosphere ahead of time. It’s a more complex problem than an astronomical application, and that’s what we are trying to do now, not only to talk to low-orbit satellites but also to more remote satellites.
There are applications now in industry as well, from the car industry to even the glass of your iPhone. Processes that use lasers always provoke localized heat, deforming optical surfaces, such as soldering and mechanically ablating surfaces. This heat provokes turbulence, and this turbulence means that wavefront arriving at the target is not perfect, and you would like to have it perfect.
What are some of the biggest challenges we face today with moving the adaptive-optics revolution forward?
There are many different challenges depending on the application. In astrophysics, laser guide stars work very reliably and solve a number of problems that we have with natural guide stars. But we are not able, at the moment, to determine the image motion from the laser guide star signal.
And this is what we are tackling experimentally now—how we can derive the image motion from a laser guide star? Right now we’re losing the information of the tilt, and this is one of the big challenges in getting full sky coverage.
Another challenge is to use laser guide stars in daytime. In satellite laser communications systems, for example, you want to use it to correct for atmospheric turbulence for satellites day and night. In astrophysics, we want to use laser guide stars in daytime for observations of the Sun—not the Sun directly, but its edge for coronal mass ejections, a consequence of the big solar flares that happen on the edge of the Sun. The adaptive optics we have now for the Sun don’t work after the sun limb, because there is no reference—there is no guide star and no sun surface.
In astrophysics, we also have the possibility of using adaptive optics for thermal imaging. If you image in the infrared, in the bands from 3 µm downward, you can observe in daytime—it’s already being done. And if you have a laser guide star at these wavelengths, you’re king! You make fast frames to reduce the sky background and can do the adaptive optics correction in daytime (usually in the early hours of the day).
The challenge is to go to visible adaptive optics correction in daytime for large telescopes at a very high resolution. Adaptive optics allows the concentration of the flux coming from an extraterrestrial object, such as from planets. What we want to do is to measure the composition of the atmosphere on these planets using a spectrometer. If you find oxygen spectral lines in that atmosphere, you’re one step from being 100 percent sure that there is life. Because oxygen is so chemically active that it would combine with everything within a 1,000 years on Earth if there were not phytoplankton and vegetation to produce it.
Any other opportunities or challenges you’d mention?
Well, on the opportunity side, there are going to be some very interesting applications that can be used for industry—in medicine, in industrial production processes, in microscopy and in miniaturized quantum optics. Industry will want to really keep an eye on this, and foster opportunities for cross-fertilization with academic institutes.
Challenges will include developing the specialized photonics devices needed for different AO applications. One last challenge is that adaptive optics is really a discipline that requires a lot of knowledge to be implemented and evolved. It’s so multidisciplinary—you have to know too many things to be a one-person shop. I think we can only go on together and advance this technology if we truly and openly collaborate. We have to give up our ‘ownership’ ego a little bit and act as a team—a challenge for many of us, especially for successful, competitive scientists and engineers!