MAVIS: overpowering space telescopes from the ground

A new generation of astronomical telescopes are currently under development - and in the next few years, will open up the cosmos, unlike anything we have seen before. These telescopes are giants, with primary mirror diameters ranging from 10 metres up to almost 40 metres. 

This includes the European Southern Observatory (ESO) Extremely Large Telescope (E-ELT; 39.3m), the Thirty Metre Telescope (TMT; 30m), the Giant Magellan Telescope (GMT; 24.5m) and the soon to achieve first-light, Vera C. Rubin Observatory with its 8.4m mirror. 

These giant telescopes will join existing telescopes, such as the Keck Telescopes, the Very Large Telescopes (VLTs), the Gemini Telescopes, and the Subaru Telescope, located across the world, all of which average primary mirrors with light-collecting diameters of approximately 8m - 11m. 

Building giant telescopes on Earth has both advantages and disadvantages, as opposed to developing space-based telescopes, like the Hubble Space Telescope (HST), or the soon to (hopefully) launch James Webb Space Telescope (JWST). 

The main advantage is that the telescope can be big. Very big. Mirrors that are 8m - 10m in size can be built, and several of these can be configured together to make a larger primary mirror. Attempting to lift these sensitive objects into orbit is both costly (as they are extremely heavy) and risky (vibrations during launch and then configuration in space introduces too many variables). 

However, remaining on the ground means that telescopes are defined by their ability to resolve targets through a turbulent medium (the atmosphere), as well as limited by the wavelengths of light that the atmosphere allows to reach the surface or certain elevations (e.g. higher energy frequencies like x-rays and UV rays, cannot penetrate down to the surface - a good thing for life on this planet!). Even water in the lower parts of Earth’s atmosphere, such as vapour and clouds, presents a problem for the infrared range, and so these telescopes need to be built on top of mountains. 

Until recently, technology was a constraining factor in the development of these giant telescopes, and in particular the ability to resolve images clearly. With larger primary mirrors comes more amplification of atmospheric turbulence, and thus more blurry imaging. 

But a clever technique, known as Adaptive Optics, utilises laser technology and deformable mirrors to make corrections in rapid real-time allowing the ability to utilise these giant ‘eyes on the sky’ more efficiently. 

Now, a new instrument called MAVIS (multi-conjugate-adaptive-optics Assisted Visible Image and Spectrograph) - developed by an international consortium which is led by the Australian Astronomical Optics (AAO) - will help some of the largest telescopes in the world (located in Chile) see the light of stars and galaxies better, helping provide some context to some exciting research questions - such as when the first stars formed, or what the weather is like on the planets of our Solar System. 

The atmosphere on Earth can be considered a dynamic fluid that is continually undergoing variations and changes across a number of parameters, like air pressure, temperature, wind speed and direction, humidity and the number of particles that are trapped or moving within it. 

When light from astronomical objects arrives at Earth-based telescopes, it needs to pass through this medium which causes the appearance of the object to jump around, twinkle and distort in and out of focus.

Technology has now however found a way to counter these effects, through Adaptive Optics (AO). The basis of the technology is a feedback loop that continuously measures how the atmosphere above the telescope is behaving, then countering this in real-time. How this works is usually by measuring the light of a guide star, or even creating a fake star in the sky using powerful lasers. Then, a determination is made on how the guide star or fake star behaves and moves, which is then fed back into a computer system rapidly and in real-time. 

Specialised sophisticated reflecting apparatus, called deformable mirrors, are then this data and counter the distortion in the exact opposite manner, thereby neutralising the effect of the atmosphere in real-time. 

Advanced AO systems now also measure not just the atmosphere at one location above the telescope, but at numerous locations - firing fake laser stars to different altitudes to continually measure the column of air above the telescope and its behaviour. 

The benefit of sharpening images against atmospheric distortions not only assists in taking higher detailed images of deep space objects by these big telescopes but also in collecting important scientific data, such as the spectrum components of light from instruments, like MAVIS.

MAVIS is a state-of-the-art instrument being built for the ESO 8-meter VLT. The instrument will be installed at the Nasmyth focus of the telescope and is composed of two main parts: 1) an AO system to avoid the atmospheric turbulence causing blurring of an image and 2) an imager and an integral field-unit (IFU or a.k.a 3D) spectrograph. As it is situated on one of the four VLT telescopes on the ground, MAVIS will be covering the visible part of the electromagnetic spectrum from 370-880nm. 

One of the most exciting outcomes of the MAVIS instrument is that it will provide images three times sharper than the 2-meter HST, which has been orbiting the Earth and revolutionising how we see the Universe for several decades now. 

This will be achieved by taking advantage of the VLT Adaptive Optics Facility (AOF) which has powerful laser guide stars and a secondary deformable mirror to correct the turbulence of the atmosphere. 

MAVIS will in addition use two more adaptive mirrors which can be deformed hundreds of times per second and other systems to correct the atmospheric distortions even further. This multi-conjugate AO concept will be using around 3 natural guide stars and 8 laser guide stars and will provide a field-of-view of 30”x30” at an angular resolution of 18 milliarcseconds (mas) in the V-band. This makes the spatial resolution sharper than the HST. 

MAVIS will further have a 4kx4k imager compromising different visible broadband filters and a spectrograph to perform low-resolution and high-resolution spectroscopy in the spectral resolution ranges of R=4k-15k. The spectrograph IFU sampling is similar to another beast instrument of ESO called the MUSE (Multi-Unit Spectroscopic Explorer) with 20-25 mas spaxels but further has an imager with 7.4 mas/pixel as the highlight. 

The AAO Macquarie University is responsible for the imager and spectrograph components of the MAVIS. The unique capabilities of MAVIS at visible wavelengths make it an avant-garde instrument of current times and an extraordinary complement to future facilities like the space-based James Webb Space Telescope and the above-mentioned ground-based 30-40 meter Extremely Large Telescopes.

The MAVIS project idea started in early 2018 for the ESO call for proposal for a Visible Adaptive Optics Imager in May 2018. MAVIS consortium consists of the Australian National University and Macquarie University, the National Institute for Astrophysics (INAF) Italy, the Laboratoire d’Astrophysique (LAM) France, and ESO. 

François Rigaut, a professor at the Australian National University is leading the consortium for MAVIS. Science white papers showing scientific interest and demand to strengthen the MAVIS case making the successful Phase A bid in early 2019. 

In the last two years, a lot of work on the conceptual design of the instrument was done and several workshops and busy weeks were held. The wait was finally over when both parties signed the agreement after the set up of Technical Requirement Specifications and the Statement of Work. 

ESO signed the agreement for the MAVIS project with the instrument consortium on 1st June 2021. This kicks off the official beginning of Phase B (Preliminary Design phase) of the project. The MAVIS project signing is a fruit of the strategic decadal partnership between the ESO and Australia signed in 2017. Australia again demonstrates with such projects their leadership in science and instrumentation. With the agreement signed, MAVIS is expected to receive its first light in 2027.