- What is MAVIS?
- Science with MAVIS
- Adaptive Optics and Multi-Conjugate Adaptive Optics
- Expected performance and instrument design
- Path forward
What is MAVIS?
MAVIS is a proposed instrument for the ESO’s VLT AOF (Adaptive Optics Facility, UT4 Yepun). MAVIS stands for MCAO Assisted Visible Imager and Spectrograph. It is intended to be installed at the Nasmyth focus of the VLT AOF and is made of two main parts: an Adaptive Optics (AO) system that cancels the image blurring induced by atmospheric turbulence and its post focal instrumentation, for which the baseline is a 4000x4000 pixel imager and a spectrograph, both covering the visible part of the light spectrum.
The VLT, since its first light in 1998, has already gone through two generations of instruments. In 2015 and 2016, ESO probed the community for ideas for a third generation instrument at its “ESO community days”. Simone Esposito, backed up by scientists and instrument makers from INAF, proposed a visible MCAO system: Taking advantage of the superb performance of the AOF (powerful Laser Guide Stars and Deformable Secondary Mirror), the idea is to push AO toward the visible, and, using the Multi-Conjugate AO concept, to provide a large field of view at an angular resolution close to the diffraction limit of the 8-m aperture.
It is anticipated that ESO will release a call for proposal for such an instrument sometime during spring 2018 (North-hemisphere spring that is). Australia is leading a consortium that includes the ANU, the AAO, INAF and the Laboratoire d’Astrophysique de Marseille, with associated members ONERA, Swinburne University of Technology and Macquarie University. Eventually, all the Australian institutes will be participating under the umbrella of the soon-to-be-formed National Optical Instrumentation Capability; in that sense this is a national effort (as well as, in Italy, this is a national effort lead by and under the umbrella of INAF).
ESO has developed a science case and requirements for this instrument. The MAVIS consortium, with the help of the community at large, is currently enriching the science case and developing an instrumental concept.
MAVIS has the potential to be an extremely novel and powerful facility. With an angular resolution of 15 milliarcseconds (close to 50 times better than the seeing limited conditions encountered without Adaptive Optics) and a powerful and sensitive post-focal instrumentation, MAVIS will be instrumental to bring answers to a number of astrophysical science questions regarding stellar evolution and star formation, physical composition of mid-redshift galaxies, how early galaxies assemble, or closer to us, weather monitoring on our solar system planets and moons.
If you are interested in helping developing the science case, the best opportunity in the near future is to attend the MAVIS Science & Instrumentation workshop, Sydney, 7-9 May 2018. You can also contact the project scientist.
Science with MAVIS
The detailed Science Case for MAVIS is currently under active development, and will extend upon the initial collection of cases put forward by ESO. If you would like to contribute to the MAVIS Science Case, please contact us. A call for science white papers went out in April, already attracting a lot of interest from all consortium members — and beyond.
The key design drivers for MAVIS are to fully exploit the outstanding image quality possible from the Adaptive Optics Facility over a significant field of view, and with the largest possible sky coverage. This will herald a new era of extremely high image quality in the visible from ground-based facilities, which will become commonplace in the near infrared with the advent of Extremely Large Telescopes (ELTs). Below we highlight just some of the key science capabilities that MAVIS will enable. See also the contributions to the first MAVIS Science Workshop in November 2017.
Resolved Stellar Populations Beyond the Local Group
The enhanced point-source sensitivity and faint confusion limit of MAVIS combined with the information-rich optical colour regime enable the exciting prospect of studying galaxies star-by-star out to the nearest galaxy groups and clusters, revolutionising our understanding of stellar populations beyond the Milky Way and Local Group. Through analysis of colour magnitude diagrams, degeneracies that hinder integrated light studies can be broken, allowing the first accurate star formation histories of objects that can only be found in these environments. The excellent crowded-field capabilities of MAVIS will allow such analysis to probe stellar density regimes that will not be accessible by, for example, JWST. Moreover, MAVIS will form a major enhancement to the E-ELT for tackling this science, complementing the outstanding sensitivity of the E-ELT at infrared wavelengths with the incisive diagnostic power of the optical regime, observed, crucially, at the same spatial resolution.
Crowded Field Photometry & Spectroscopy
Access to optical spectral information of resolved point sources in extremely crowded fields, either through multiple optical colours or medium-resolution multi-object spectroscopy, opens new environment regimes for studying the age and chemical composition of stars. Such environments include the dense core regions of Milky Way stellar clusters and the Galactic bulge, where star formation processes may have differed significantly from the solar neighbourhood. The light-gathering power of the VLT will furthermore allow multi-object spectroscopy of such regions, as well as probing below the main-sequence turn off in star clusters of the S/LMCs.
Precision Astrometry and Proper Motions
With high spatial resolution (~20 mas), a large field, and the light gathering power of the VLT, MAVIS will give access to a new regime of proper motion studies. MAVIS will greatly exceed the crowded-field capabilities of GAIA, and with higher sensitivity than HST, will be able to observe fainter stars thus enabling accurate proper motions for more distant dwarfs and sparse Milky Way satellites and streams. The detailed dynamical properties of such systems are key for testing the predictions of different models of dark matter, and exploring the existence of intermediate-mass black holes. Combined with multiplexed radial velocity measurements and resolved stellar population information, this allows a complete chemo-dynamical picture of these long-lived low mass systems, allowing detailed archaeological studies of the epoch of reionization.
Morphology of Young Galaxies
Rest-frame UV observations of galaxies at the peak of cosmic star formation (z~1-4) exhibit distinctly irregular morphologies due to ‘clumps’ of enhanced star-formation. These clumps appear to be significantly larger and more massive than typical star forming complexes in galaxies today, and may play an important role in building the bulges of present day galaxies. The limited resolution and sensitivity of current observations, however, prevent an accurate picture of the intrinsic size and mass distribution of these clumps. The combined light-collecting power and spatial resolution of MAVIS will be able to constrain the rest-frame UV morphology of high-redshift clumpy galaxies down to smaller spatial and mass scales than is currently possible, giving stringent constraints on models of feedback and bulge growth. Targeting low-redshift analogues of these galaxies with MAVIS will further extend this to even lower mass scales, giving insight into fragmentation scales in turbulent disks, with the possibility of medium resolution integral field spectroscopy to obtain dynamical and chemical insight into massive star formation clumps.
Probing the Edge of Reionization
For probing the highest redshift objects, the optical capabilities of MAVIS will play a crucial role in complementing the outstanding sensitivity of infrared-optimised facilities such as JWST and E-ELT. While these facilities will be crucial in pushing the limits of candidate highly-redshifted (z>6) source detections that lie within the expected epoch of reionization, optical data with comparable sensitivity and (crucially) spatial resolution it will be essential to confirm the presence of the Lyman break in such systems. MAVIS can also open up new parameter space in understanding the properties of (spectroscopically confirmed) Gamma Ray Burst (GRB) host galaxies. GRBs are highly energetic events associated with the core collapse of massive stars, and thus provide a probe of star formation (as well as other host and IGM properties) that is independent of the host luminosity, and unique insight into the UV luminosity function near the epoch of reionization. Photometric properties such as size and luminosity are crucial to understand the nature of these sources, however most are missed with the insufficient sensitivity and astrometric accuracy of current facilities – both limitations that MAVIS will alleviate.
Solar System Science
While much solar system science is focussed on probe missions, many aspects are better and more cost-effectively done from the ground. This includes long-term remote monitoring of planetary bodies for weather system information or volcanic activity; rapid follow-up of unexpected events; and building up statistical samples of e.g. asteroid morphologies.
Right: Hubble Space Telescope image showing the presence of a dark vortex in the atmosphere of Neptune, indicating a high pressure system. The image at the bottom right shows that the vortex is best seen at blue wavelengths. Credits NASA, ESA, M.H.Wong and J.Tollefson (UC Berkeley)
Adaptive Optics and Multi-Conjugate Adaptive Optics
Adaptive Optics (AO) is a technique to compensate quickly varying optical aberrations in an optical system. In astronomy, telescope images from astronomical objects are blurred by atmospheric turbulence. By using AO, astronomers can cancel out these blurs and restore images close to the ultimate image quality imposed by the telescope, something called diffraction limit (and given by the ratio of the imaging wavelength and the telescope aperture diameter, ). The diagram on the right illustrates the principle of AO: The light coming from a star through atmospheric turbulence, after being collected by the telescope, bounces off a deformable mirror (DM) that is shaped to compensate the corrugation of the light wave. A Wavefront Sensor (WFS) measures the aberrated light wave, and, through a control computer, is used to control the shape of the DM. The resulting compensated light can be captured by regular post focal instrumentation, imagers or spectrographs, with greatly improved clarity (angular resolution) with respect to the natural seeing limit imposed by atmospheric turbulence.
One of the limitation of AO is that the correction is only valid in a very small patch of sky (depending on the wavelength, from a few arcsec to a few tens of arcsec). Multi-Conjugate Adaptive Optics, or MCAO, solves this problem by using a series of DMs to compensate the turbulence in volume. To do so, MCAO uses several guide stars to measure the light wave aberrations in several directions, and using a tomographic reconstruction process, determine the best commands to apply to the DMs, as illustrated in the figure on the right.
The net result of MCAO is that the corrected field of view is much enlarged with respect to what can be produced with regular AO, typically by a factor of 10 to 20 in area. In addition, one can tweak the uniformity of the output image quality (IQ) to obtain the most uniform IQ, something important for the extraction of quantitative photometry and astrometry from the post-focal instrumentation data (images and spectra).
Is MCAO possible in the visible?
MAVIS proposes to merge two demonstrated techniques:
- On the one hand, AO in the visible has been demonstrated by several teams; most notably by Forerunner (FLAO) at the Large Binocular Telescope – a team led by Simone Esposito. Forerunner produced images with Strehl ratio (a measure of the image quality) of 50% at 650nm and image size of 18mas, which is extremely good and demonstrate very high correction performance at this wavelength. The image on the right is an example of an image at 650nm, showing many diffraction rings. MAG-AO, the AO system of the Magellan telescope, has also been producing high quality visible data for a number of years.
- On the other hand, MCAO in the Near-Infrared has been demonstrated by MAD (ESO) and GeMS (Gemini). The latter is a facility instrument on the Gemini South telescope, and has been producing images close to the diffraction limit over field of view of since 2011.
MAVIS will combine these two demonstrated techniques to provide unprecedented angular resolution in the visible part of the spectrum. Of course, there are challenges, of which the tight error budget is certainly the one we will have to watch the most closely. Expected performance are reported below.
Expected performance and instrument design
ESO’s call for proposal includes Top Level Requirements (TLRs), available on the ESO website (public document). These requirements are important, but they could change, as a result of science cases completion, re-evaluation, and as a result of AO simulations and instrument design. The numbers below represent our best estimates to date.
Performance parameters (subject to adjustment after the phase A)
- Field of view (FoV):
- Wavelength coverage: VRI, extended to UBz (goal)
- Angular resolution (FWHM): 20 milliarcsec at V
- Strehl of 15% in V under median seeing conditions
- Sky coverage: > 50%
System parameters (subject to adjustment after the phase A)
- Number of Laser Guide Stars:
- Number of Natural Guide Stars 3
- Zenith angle restrictions: 2 airmasses
The sky coverage is of the utmost importance for an AO system in general, and a MCAO system in particular. To compensate for Tip, Tilt and plate scale modes (variation of the plate scale across the field of view i.e. “breathing” of the image), a minimum of three natural guide stars (NGS) is needed. Several studies of NGS availability have been done in the context of GeMS, the TMT NFIRAOS and the E-ELT MAORY. All of this studies conclude that the sky coverage can be reasonably high if certain conditions are met:
- An acquisition field of view for the NGS that is significantly larger than the science field. For MAVIS, the acquisition field is envisaged to be 90-120 in diameter.
- There is a significant gain to be had by using the NGSs in the Near Infrared (typically 1 to 1.7m). At this wavelength, the NGSs are quite well corrected and the guiding can be done using their tight core, which improve accuracy and SNR (here and here). For instance, NFIRAOS anticipates a sky coverage of 70% over the whole sky (it is NIR, so easier than visible, but ELT, so more difficult as the tolerable rms TT error is smaller).
- Using as short and simple an optical train prior to the NGS wavefront sensors to maximise throughtput (e.g. as done in NGS2, the new generation sensor for GeMS at Gemini)
A preliminary analysis of sky coverage will be done for the phase A proposal, and a full analysis for the phase A. For now, what is important to realise is that the performance degradation is gentle when going to fainter guide stars: A larger tip-tilt error means essentially a broadening of the Point Spread Function core, but the flux stays in the core, which means that this has little effect on the limiting magnitude of the images produced by the system, and almost zero effect for spectroscopic applications —to a certain extent— for which in general the aperture is larger than the diffraction limit.
- Imager: 4k4k covering the 30 FoV. Filters will include a set of broad band filters and narrow band filters TBD.
- Spectrograph: TBD. One concept that seems to be picking up interest for the stellar applications proposes single fibres + fibre bundle with a Starbug positioner and a spectral resolution of 5000 to 10000 TBC. Another concept that is favoured by ESO is an image slicer IFU covering about 3 at 25mas spaxel size (“1/4 of MUSE”).
This is being defined right now by discussions within the science team (you are welcome to join). The AO performance is very much set by the atmosphere and the technology. The post-focal instrumentation is much more open, although it will inevitably be constrained by funding and compromises between the various instrumental options. The characteristics listed above are a snapshot of the current thinking.
For the MAVIS consortium, and all scientists willing to join, the first and most important thing to do is to get ready for the expected call for phase A. It is expected that the proposal will be due by July 2018. The proposal should outline the case for such an instrument, complete convincingly the science case put together by ESO, sketch instrument designs, establish preliminary error budget and performance. In a word, present a convincing case that the instrument is doable, that it will produce excellent and unique science, and that the consortium is up to the task of producing a phase A. The team should start working on the proposal ASAP, and discussions have started to that effect. The May 7-9 workshop intent is to:
- Mature/advance our instrument and science case ideas with the view of writing the proposal;
- Exchange and interaction between scientists and instrument builders;
- Review the various instrumental concepts (optical design, alternative design ideas); and
- Consolidate the science case and prepare the detailed writing of the science case in the proposal.
The Australian team is going through a European MAVIS tour March 12-16 2018, visiting European consortium partners to consolidate and formalise the team agreement, and visiting ESO on March 15-16.
If the MAVIS Phase A proposal is selected, it is expected that the phase A will last 15 months. If the project is approved beyond phase A, it is expected that the development time for MAVIS will be 7 years (including phase A), which puts MAVIS on sky mid-2025 (post HST, roughly in line with ELTs first light).