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Olivier Le Fèvre

Olivier Le Fèvre

It was the winter of 1998. I was reaching the end of my thesis in Durham, England, and I knew it was time to start looking for a new city to live in, a new place to go. By this point in my life I’d already spent almost three years in North America (Socorro, New Mexico, and Victoria, BC) and six years in England (Durham and Manchester), and I knew that I really didn’t want to live in those kinds of of places any more, I knew that they weren’t for me. But was there any place in Europe that did the kind of science I wanted to do, what I had done in Canada and England? By that I meant surveys of the Universe with tens of thousands or hundreds of thousands of galaxies, studying what everything looked like on the largest scales, finding new objects and galaxies that no-one had ever gazed upon before. So I was more than just intrigued when I saw the job advertisement for a postdoctoral research assistant at the Laboratoire d’Astrophysique Spatiale (LAS), in Marseille. “Observational cosmology / VLT-VIRMOS deep redshift survey” it said. They were looking for someone to help out with a survey of the distant Unvierse that would be an order of magnitude larger than anything attempted before using a new instrument on one of the four European Very Large Telescopes, in Chile. And I although in my ignorance I had never heard of the LAS, I had certainly heard of the person offering the position — Olivier Le Fevre. He had already authored or co-authored many papers on distant galaxies, surveys, clustering, all the kinds of topics that I had been immersed in during my studies. In my anglocentric innocence I wasn’t sure if this science was being done in the Old World. It seemed like a wonderful opportunity to do some exciting work on the shores of the Meditteranean. I had never been to the south of France before and after long years in the North of England I was ready for sunshine. I applied, and was invited for an interview.

I remember very clearly that I had arrived early at the LAS and was waiting in the lobby for Olivier to arrive. The building dated from the 1970s; outside the facade was all lightly-tarnished metal and glass and inside there were narrow corridors with worn linoleum on the floor. It had been well lived in. Outside, the sun shone brightly even on that early in the morning in winter and from the lobby it was hard to see who was coming through the doors. I stood with Vincent Le Brun, waiting, and I saw the silhouette of a tall man walking towards me, ah there he is now … and I was surprised. He seemed to be only slighly older than me. He was tall, handsome, athletic and impeccably dressed. But from his impressive publication record I was expecting a greybeard and not this man I saw before me.

During my one-day stay at the LAS I was very well looked after. Olivier and Alain Mazure took me to lunch at a restaurant nearby surrounded by the rolling green fields of a golf course. I remembered Roger Davies’ advice and spoke slowly during my talk which detailed our painful efforts during my thesis in Durham to map a tiny part of the sky with hundreds of hours of telescope time. I soon learned that Olivier’s under-construction VIMOS instrument, combined with the new wide-field cameras coming online at the Canada-France-Hawaii-Telescope, planned to make all this instantly redundant and open up a completely new window on the Universe. Precise distance measurements would be possible for tens of thousands of galaxies and there would be photometry for millions more. The galaxy-counting skills I learned at Durham were what the team needed to help make the input catalogues for this new instrument. After my talk, I spent some time with Olivier in his office. I was immediately taken by the his motivation and the vast amount of data he intended to collect and the chance that it might answer all those hanging questions we’d had until now. It was very exciting and it was in France!

A few months later I had finished my thesis, and on the first week in January 1999, I started my postdoctoral position at the LAS with Olivier. Incredibly, they had offered me the position; I was certain that there must have been crowds of people banging on the doors of the LAS. Only later did I discover that there was only one other applicant. “Observational cosmology”, as Olivier’s job application promised, had yet to really come to the LAS which was not yet on the post-doc radar. A colleague from the UK even confided in me that he wouldn’t ring a telephone number in a French laboratory in case the person picking up the telephone spoke French to him! I soon discovered I was the only post-doc in the lab and in those early days almost the only person in the building after 19:00. The LAS had a certain charm: and there was a long table outside under the trees where you could eat lunch most days. After a few months there I got to know some wonderful people and my French steadily improved. However, the “observational cosmology group” promised in Olivier’s job announcement for the moment didn’t comprise more than five people, including myself and two or three students. I mention all this here only to insist that the LAM (the fusion of the LAS and Marseille Observatory) has become the great force that is today in surveys largely thanks to Olivier’s work.

In 1999, however, such a happy ending was far from certain. Data was steadily arriving from CFHT telescopes in Hawaii and the computer in my office had attached disks piling up to the ceiling. Could we keep with the data? Worse yet, VIMOS turned out to be a very challenging instrument to build and commission. It took all of Oliver’s skills in management and persuasion to get the instrument on the sky and keep to the schedule. The team worked very well together and although there was a very long period before the spectra arrived Olivier kept us motivated. At an observing run in Hawaii I met Yannick Mellier who put the resources of TERAPIX at our disposal which helped us a lot with the early data. In the end, despite these difficult early years, tens of thousands of spectra were collected and VIMOS has gone on to be one the most successful instruments at ESO.

Already, in the first few years of new century, the context was changing: in the space of a few short years observational cosmology was gaining in importance in the community. The group at LAS was growing. The skills I had learned were becoming increasingly important, important enough that in the summer of 2003 I was recruited as a staff astronomer at the Institute d’Astrophysique de Paris. Olivier played a very important role in that change through his tireless support of countless other projects and instruments. For example, the VIMOS spectrograph turned out to be a crucial instrument for spectroscopic follow-up of the COSMOS survey, one of the largest-ever allocations of Hubble Space Telescope time. As well as that, Olivier brought TERAPIX into the COSMOS project to help with initial imaging at CFHT. That was the origin of my own highly fulfilling involvement with the COSMOS project, a collaboration which is still going strong after a decade.

In the winter of 2017 at a meeting in Paris I told him that I had been diagnosed with thyroid cancer. The treatment was going well, I said. I am sure I told him (as I told everyone) that if you want to have a cancer, that’s the one you should get; treatment is straightforward. Unfortunately, only a week or so later, he would fall from his bicycle and be diagnosed with a brain tumor. It didn’t slow him down. He worked as tirelessly as ever to realise his countless projects even as his body weakened. He still came to meetings. He was there at at our COSMOS meeting in Copenhagen in the summer of 2018. One evening, Olivier, and Olivier Ilbert and myself ate together in a restaurant, outside on the terrace. Olivier was unfailingly positive even though he must have known his chances of survival were slim.

Today, more than twenty years after my first meeting with him in that distant winter of 1998, observational cosmology and survey astronomy is now firmly established in France. And this is in large part due to Olivier’s work.

52 photographs (2018) #25: In the round tower

52 photographs (2018) #25: In the round tower

Now, Copenhagen. Walking around on the city on a Saturday evening just after I arrived, I came across a round tower in the city centre. Inside, it looked like this:

In the round tower…

A smooth brick floor curves resolutely to the top of the tower. Some soft evening light shone through the windows.

Of course, I discovered once I got to the top that the Rundetaarn was once an astronomical observatory, built in 1642, thirty years before the Paris Observatory. Peering through a window near the top one can see instruments and telescopes. Today, the centre of Copenhagen, like the center of Paris is no longer an ideal location for observations, at least of celestial objects. 

The Strand magazine, 1896: a Howard Grubb Illustrated interview

The Strand magazine, 1896: a Howard Grubb Illustrated interview

After writing about telescopes in space in my last article, I was reminded of this “illustrated interview” of Howard Grubb, published in the Strand Magazine in 1896. It starts perhaps not very promisingly: “The poverty of Ireland is such that the superficial observers are apt to wonder whether any good thing can really come out of that distressful country”. It does improve from there! It was sent to me by a descendant of Grubb. It is very interesting, especially the part at the end about future large telescopes which, of course, will be floating in water. The image below is supposed be “casting the mirror for the great Melbourne telescope” but it doesn’t look like any kind of “astronomical” ceremony to me!

“Casting the mirror for the great Melbourne telescope”

Read the PDF here:

TheStrandMagazine-1896-b-Vol.XII-Jul-Dec-Sir-Howard-Grubb-illustrated-interview-1

Surveying the Universe from space

Surveying the Universe from space

The Euclid space mission (which I have already written about here) plans to stringently test our cosmological model by precisely measuring the shapes and positions of a billion faint galaxies. But you are not going to take pictures of each galaxy individually, obviously, that would be too slow! You need to do a survey, which means with each image you want to take a picture of largest possible area of the sky. So, you need a survey telescope, and because you have a limited budget you need to be able to launch your telescope on the smallest possible rocket. How do you do this? That is what I am going to write about here.

Designing survey telescopes has always been challenging. Photographic plates made possible for the first time measuring the positions and brightnesses of a large number of objects. The field-of-view (the amount of sky you see) for these telescopes was comparatively small. The search soon started in earnest for a telescope which would allow astronomers to even more rapidly the sky. The Schmidt telescope design comprises a spherical mirror paired with an aspherical correcting lens. One of the most famous such telescopes, the 48-inch Palomar Schmidt telescope, covered the entire northern sky using thousands of 14-inch photographic plates and provided an invaluable discovery tool for astronomers.

In the 1970s, the advent of automated plate-scanning machines meant that the first digital surveys of the sky were in fact made with photographic plates by scanning all these plates! Similar southern sky surveys were made by the UK Schmidt telescope in Australia and such surveys were only surpassed by the arrival of true digital sky surveys like the SDSS. The SDSS, incidentally, bypasses the need for extremely wide-field optics by cleverly reading out the camera at exactly the rate of the earth’s rotation, but that’s another story.

The UK Schmidt telescope, built in 1973, is a wide-field telescope, but much too large to be sent into space! (c) AAO

However, a significant disadvantage with Schmidt telescopes is that the focal plane — where the image is recorded — is curved, making them impractical for use with flat electronic detectors unless heavy corrective optics are installed (yes, I grudgingly admit that photographic plates are not practical in a space observatory (although amazingly there were once spy satellites which used film).

In addition to this, don’t forget that also that one key requirement for Euclid is not simply to measure the positions, brightnesses and distances of all objects but also their shapes. For faint galaxies seen through ground-based telescopes, object shapes are dominated by atmospheric effects. For exposures longer than a few seconds, object light profiles are smeared out, severely limiting our ability to extract useful information.

For these reasons, Euclid is space: above the atmosphere, the telescope’s shape-measuring capabilities are limited only by the satellite’s optics and detectors. This is also why telescope designs which might have been fine for an instrument lying at the bottom of the murky soup of Earth’s atmosphere are simply not good enough for space. Essentially we need a design which preserves as much as information as possible concerning the intrinsic light profile of objects. And we also need to be able to calculate how much this light profile has been distorted by presence of telescope and detector optics – usually this is done by making observations of perfect point-like sources. In astronomy terms, stellar sources fit this bill very well. But what design is this?

Automated ray-tracing revolutionized telescope design in the second half of the 20th century. Without having to cut glass, computer programs could calculate the optical performances of a telescope even before it was built. In series of papers and described at length in a classic book, the optical engineer Detrich Korsch used these new techniques to perfect a compact, three mirror design which had the great advantage that it features a wide field of view, few optical surfaces, and almost no aberrations over the entire field of view. It’s worth mentioning that knowing the optical performance of Euclid requires an intimate knowledge of optical properties of all surfaces. For this reason, ground testing and qualification of all components are an important part of verifying the Euclid optical design.

The optical path of the Euclid survey telescope. (c) ESA

But it’s not enough to have an excellent optical design if it is not stable and image quality cannot be maintained during normal operations. So, thermal expansion and contractions must be minimised. Euclid features a silicon carbide baseplate on which all the instruments and telescope are mounted. Silicon Carbide has the unique feature that it expands and contracts very little with changes in ambient temperature, meaning that the path length of the whole telescope can be rigidly controlled. The baseplate is actually created from a mould of particles of silicon carbide which are stuck together under pressure.

What’s the best place to put Euclid? At first, one might think, well, in orbit around the Earth, right? It turns out that a low-Earth orbit is a surprisingly hostile environment. In addition to constant sunrises and sunsets, there are also bands of nasty charged particles, in particular in the region called the South Atlantic anomaly. Hubble Space Telescope observatory is in such an low orbit, and the unfortunate consequence is that there are “blackout” periods which no observations cannot be made. In addition, there a not inconsiderable amount of background light.

L2 orbit of Euclid

There is a much better place to put a space observatory — the second Sun-Earth Lagrangian point, or L2, shown below. Here, a satellite’s location can be maintained with only a minimal expenditure of propellant. At L2, the gravitational pull of the Earth-Sun system almost perfectly balances inertial forces. Moreover, an object maintains an approximately constant distance to the Earth, making it ideal for high-bandwidth satellite communications: Euclid will need to send a lot of data to earth. In this orbit it will also be possible to control rigorously angle the sun’s rays fall on the telescope’s sunshield: this is essential to maintain the optical stability of the telescope. These factors have made L2 one of the best locations in the solar system to place an observatory, and many future telescopes will be placed there. The Planck and Herschel satellites clearly demonstrated all the benefits of this.

With this unique telescope design, capable of taking a high-resolution one-degree image of the sky with each exposure, Euclid’s primary mission will be completed in around six years of observations. No space satellite has ever flown before with such a unique set of instruments and telescope, and Euclid’s images of our Universe will be one of the most lasting legacies of the mission.

This article is one of a series of articles which will be appearing on the official blog of the Euclid Consortium before the end of the year!