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The IAP at 75: The early history of our Institute

The IAP at 75: The early history of our Institute

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A grainy black-and-white photograph. A line of men stare into the camera. In the background, bare trees are outlined against the cold winter sky. The ground is thick with dead winter leaves. Four of the men are in suits and hats; three are labourers. One heavy-set man with a pencil moustache leans nonchalantly against a tree, another is there with his hands in his pockets. Behind them, a small truck is parked at an awkward angle. On the far left, one of the well-dressed men holds a pick in his hand. This man is Henri Mineur, who would in the following year become the first director of the IAP. The date is the 6 of January 1938: the date on which construction of the IAP started. Last Friday, we celebrated the institute’s 75th anniversary with a series of talks and presentations, and in addition a short film made by my friend Mr. Jean Mouette.

PremierCoupDePiocheIAP 1938
Henri Mineur starts construction work  for the IAP

The IAP was created out of an urgent need for new structures to carry out scientific research. In the early part of the 20th century, astronomy was undergoing a radical transformation. With the arrival of new instrumentation and new telescopes it became possible for the first time to apply our knowledge of physics to understand astrophysical processes and observation: the science of “astrophysics” came into being. For centuries, astronomy had been concerned with the positions and movements of stars and other objects, but with the arrival of richly quantitative measurements such as spectroscopy, which can provide detailed information concerning the chemical make-up of very distant objects, it became clear that a new approach was possible. However, it was not clear where this kind of new astronomy could be done in France – certainly not at the Universities which were orientated uniquely towards teaching, and saw no place for research. The observatories, steeped in centuries of positional astronomy (and still labouring to accomplish immense tasks like the “Carte Du Ciel”) were not quite ready for the transition.

One man, Jean Perrin, saw the need for a new national institution to carry out astrophysical research – an institution which would not be part of any existing structures but would be independent. A left-leaning government had just arrived in Paris, le “Front populaire”, and they fully supported Perrin’s idea. Jean Zay, a minister at the time, signed a decree on the 30th of October 1936 which led to the creation of the Observatoire de Haute Provence (the OHP, in some ways the “observing station” of the IAP), and the IAP itself, initally designated as a centre of research which would analyse data arriving from OHP and devise new instruments for the telescopes. The IAP would be constructed on a patch of ground in the Jardin de L’Observatoire, which the government had requisitioned for this purpose (leading to tensions between the Observatoire and the IAP which persisted for decades). The front populaire was uniquely disposed to these ideas. In fact, Perrin’s visits to the minister Jean Zay’s office invariably resulted in him receiving all the funds he requested.

Construction of the IAP started soon afterwards, and the building’s skeleton was in place by 1940: the interior, however was unfinished, and with the arrival of the Vichy regime and the German occupation, the construction was halted. Nevertheless, as Daniel Chalonge tells us after the war, building work was carried on in secret. Certainly other concerns occupied the scientists. Some left, others remained. Neither Perrin or Zey would survive the war: Perrin left for New York, where he died in 1942. Jean Zey was arrested by the Vichy government and later assassinated. In Paris, two astronomers, Holweck and Solomon, were arrested and executed by the Nazis. Henri Mineur himself was briefly imprisoned, before being released: he spent the remainder of the war in the resistance. Even astronomers long-dead suffered: the statue of Arago on the place Ile-de-Seine, in front of the site of the observatory, was melted down for bullets and shells. But finally, in 1944, some staff moved to the IAP. The building would not be completed until 1952 (and in the 1980s a third floor would be added to create the building as we know it today).

From almost the beginning, both theoretical and observational subjects were investigated at the IAP: spectrophotometric observations of the sun, stellar atmospheres, and every aspect of physical processes in an astrophysical context. We heard how Evry Schatzmann, aided by a large number of students, investigated almost every kind of astrophysical phenomena, and contributed greatly to the international reputation of our institute (but it made life difficult for the students: as they were all working on different topics, none could help each other). At the same time, machine shops and mirror polishing facilities, together with facilities for numerical calculations has ensured that new observations from OHP and elsewhere could be fully exploited. Today at IAP there are no longer any machine shops, but the importance of computing in astrophysics at the IAP has only grown in the intervening years.

Interlude: Geneva-Paris

Interlude: Geneva-Paris

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I’m on the train, gliding through mountains shrouded in mist and fog, not so far from the border between Switzerland and France. I’m returning from a short trip to Geneva and heading to Paris.

Even today, each place I visit is framed by the books I have read – too many, it seems. It’s the second time in a year I am in Geneva, but before that it was more than twenty years since I last visited. I am reminded of that short story by Borges where the older Borges meets himself, much younger, on a park bench overlooking the lake. What would I have said if I had met the younger H. J. McCracken in the streets of Geneva? Well, I know what he would have said to me: “Hey, what happened to the hair”? He might have also remarked on the wedding ring. But he would have probably been relieved to learn that I was now a astronomer and living in Paris and that I had actually managed to find someone like Marie-Laure. All that would probably have seemed incredible to him then, in that distant summer of 1991 when I first travelled to “continental Europe” as I sometimes called it back then. I don’t think I would have had any useful advice to offer him, other than “keep working, you’ll get there, and along the way you’ll find out where you’re supposed to go”.

I spent three months as an assistant at a research lab in Zurich, the ETH. It was my first to the vast European continent. I was so excited at the prospect of that trip and spending so much time there that for weeks beforehand my dreams were filled with jumbled-up impressions of what I imagined that these cities I had never visited before would look like. Once I arrived, I was disorientated and charmed – I had never seen before a city that was actually pleasant to look at (my thoughts on Zurich have changed a bit in the meantime, when I returned there in 2012: that, again, is a story for another day). On the weekends, I would take the train and visit Swiss cities or wander the streets of Zurich. The last two weeks before I returned to Manchester I made a short tour of a few European cities, strangely selected in retrospect – Munich, Prague, Strasbourg and Paris. I did not visit Italy. I harboured a lingering suspicion of the debauched South at that time in my life, probably as side-effect of my isolated adolescence in rural Ireland. Back in Manchester I vainly searched for traces of the life I had seen in Switzerland, which in general meant lingering for hours over milky tea in greasy spoon cafes after I had eaten beans on toast and imagining that I was in a cafe overlooking Lake Zurich.

It’s strange the details I recall from that summer. I actually remember the first espresso I ever drank. I had somehow been given a ticket for a free drink at Zurich cafe (remember, I am a student coming from Manchester at this point). I presented my ticket, expecting a large mug of coffee I think, and then being surprised to see … what? The smallest cup of coffee I had ever seen in my life. An espresso. I didn’t understand. I didn’t appreciate it either – it probably wasn’t a very good espresso. Ah, I would have to wait a decade or so before I really appreciated espresso.

To return to the present: this afternoon at IAP we are celebrating the institute’s 75th anniversary. It’s interesting to think about how astronomy has changed in intervening years, and to imagine what will be the state of affairs will be in another 75 years. Reporting back soon…

How survey astronomy really got started, part 2: Astronomers realise how much work it takes

How survey astronomy really got started, part 2: Astronomers realise how much work it takes

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(This is the second part of a two-part article. Read the first part here).

Now skip ahead once again another ten or fifteen years. I found this fascinating book “The great star chart” written by a British astronomer, H.H. Turner, about the progress of the “Carte de Ciel” survey. Turner was an astronomer at the University of Oxford, and this short book is his account of the survey and the work that had been accomplished in Oxford by 1911.

It’s interesting to consider his book from a modern perspective: in those distant days our notions of the Universe were very different; cosmology did not exist as a science, Einstein had yet to formulate his theory of General Relativity, and we didn’t know what the true nature of the nebulae — those dim smudges which were picked up on the photographic plates from time to time — really were. That meant interpreting observations on the first deep plates quite challenging. In Turner’s book there is a lot of talk about the “fog” that might exist between the stars — that this fog might be part of an explanation why the numbers of stars varies so much from plate to plate. Were clusters of stars and were there really “stellar streams”? Similar confusion would exist in the coming years when we tried to understand the distribution of the counts of “nebulae” on the plates — was this variation again because of some kind of “fog” or was the distribution of the galaxies really non-uniform?

It turns out, that like a lot of things, the answer was a bit of both: there really is dust, but the distribution of stars and galaxies on the sky really is clustered, for the former because of the shape of our own milky way galaxy, and the latter because … well, that’s a much longer story. But it’s interesting to think of the parallels between counting stars to find out about the Milky Way and counting galaxies to find out about the Universe.

But getting back to the “carte du ciel”… There is the interesting table I have reproduced below, which shows the state of survey after ten years of operations, divided by into catalogue plates (the shallower survey) and “charts published” which are reproductions of the deeper survey plates.
Here it is:

Greatstarmapbein00turn pdf
Who actually got some work done

While some progress has been made in measurement, it is already clear at this stage that printing the plates will be very expensive: based on the techniques used in Paris to reproduce their part of the survey, Turner calculates that a complete set would weight over four tonnes, if it were ever to be completed. Printing the entire set would be staggeringly expensive.

The work was very time-consuming: it had taken four or five astronomers working full time almost ten years in Oxford to complete their part of the survey. The work was mind-numbingly repetitive, involving countless calculations to produce a catalogue for a single plate. Every position of every one of the stars on the plate was measured manually. To guard against errors, the plates were rotated 180 degrees and the measurement made a second time, and the positions compared. In those days “computers” were in fact room-full of workers with slide-rules. In fact, this chronic mismatch between the data-gathering capabilities of telescopes equipped with photographic plates and our ability to process it would last until the 1960s when digital computers finally became fast enough to handle the volumes of data involved. (In fact, the first extragalactic surveys also suffered from a lack of computing power, but that is a story for another day.) Given all this, it’s hardly surprising that very few observatories, more than ten years after the survey started, had completed their quota of plates. It’s interesting to note in passing that it is also said in some quarters the reason why Europe lagged America in the new science of observational cosmology was because all the astronomers on this side of the pond were tied up measuring positions of stars on thousands of photographic plates.

Turner also talks about cost.

Greatstarmapbein00turn pdf page 86 of 178
And how much it cost…

Well, not much has changed in survey operations in the last century or so: today staff costs and maintenance remains the most expensive items in running a survey. What is interesting from a contemporary point of view is that Turner talks about the trade-off between accuracy and speed: it’s obvious that in an undertaking this size, attempting to make the measurements to infinite precision would simply take infinitely long. Better do the job well enough to get the necessary precision — but not too well, otherwise it will never get finished. Tell that to a student finishing their first paper.
How could other observatories with smaller amounts of staff hope to complete such a massive enterprise? In fact, they couldn’t. The deeper survey plates were never printed out — it was simply too expensive. The rest of the survey, the astrographic catalogue, did actually get finished sometime in the 1950s, almost half a century after it started. In the 1980s and 90s, with the arrival of cheap and fast computing power, interest in the survey returned. One group of astronomers recalculated all the positions of the stars in the astrographic catalogue and compared them to those taken a century later with the Hipparcos satellite.

Another group turned to the photographic plates. Although plate-scanning equipment had been around for a while, it was much too slow to scan the plates of the survey, machines like the PdS microdensitometer would take one day to scan a single plate. Instead, another group of astronomers used off-the-shelf photographic film scanners to digitize some of the plates (this was in the last ten years) and compare them to more recent catalogues. In both cases, the age of the old plates becomes their greatest asset, providing an enormous baseline to measure the motions of stars in our galaxy…

Today, the carte du ciel is one of the major attractions at the “journee du Patrimoine” at the observatory. In fact, here you can see interested members of the public waiting to visit the old rusting domes of the carte du ciel this year just to hear this story that I’ve been telling you…

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The public visits the “carte du ciel”! 

We are just getting started. The Gaia satellite will be launched in the next month or so and will provide the most precise measurements of untold numbers of stars in the Milky Way.  Euclid,  further down the line, will do the same thing for galaxies. But we had better make sure the astronomers are properly motivated and that there are enough resources in place to complete the project, and actually do science with the data !

How survey astronomy began, part I: An international conference is held

How survey astronomy began, part I: An international conference is held

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It’s time to get this blog back on track and talk about SCIENCE (….”Mr. White”, if you understand the reference). It’s been too long.

Last weekend (14th-15th of September) was “Journée du patrimoine”, that wonderful day in which buildings normally closed are open to the public. I’ve seen any number of interesting things in Paris over the years. However, this year, I decided it was about time I was on the other side, so to speak, and I volunteered to assist at that august institution the IAP shares its grounds with, the “Observatoire de Paris”. The IAP is the oldest CNRS lab in France, and this year we are celebrating our 75th anniversary, but the Observatoire is much older — it was founded in 1667. It is probably one of the oldest Observatories in the world and probably one of the only ones in which there are still real astronomers doing real research. All the other institutions from that distant epoch have been either demolished or converted to museums and their staff shuttled off to unhappy ugly concrete buildings in the suburbs. So you may imagine what a rich heritage of science and learning there is to talk about when considering Paris Observatory (and how much Paris has transformed itself around the site of the observatory in the last three and half centuries, but that is another story).

There are so many interesting stories concerning the Observatory, but perhaps the most fascinating for me is the “Carte du Ciel” project — because it is no underestimate to say that with this undertaking modern international survey astronomy really began. There is a direct link between the Carte du Ciel and the Euclid project I’m involved in.

Modern survey astronomy started here !

But let’s take a step back. You have just left the IAP and are crossing the observatory gardens. On your left you see two small, rusted domes. The paint is peeling off. These are the domes of the “carte du ceil” observatory. Open the door, and inside the left dome, you can see a small refracting telescope. There is a large square metal box just below the eyepiece. Now take a step even further back. It’s end of the 19th century. Paris under the second Empire. Only a few years previously, in 1882, astronomers in the Cape of good hope, took some of the first images of the heavens with photographic plates — Halley’s comet — and they were astonished to see many, many stars on each plate. The image below shows of one of these first “deep sky” images made with photographic plates (by a Dr. David Gill). I found a scan of this plate (from H. H. Turner’s “Astronomical discovery” and have included it below):

I135
The first deep sky image of all time ! 

It became clear very quickly that the information gathering capacity of photographic plates surpassed anything which was available before then. Moreover, photographic measurements had the great promise of being objective, unlike hand-drawn sketches and notes (think of those “canals” on Mars). Now the only thing missing was a telescope optimised for photographic measurements.
In Paris two opticians, the Henry brothers, working out of their garden shed (as far as I could tell) had a design for a refracting telescope which could provide a wide field of view, 2 degrees on a side. A second-Empire start-up! Admiral Mouchez, the director of the Paris observatory, impressed by their successes, ordered the construction of a much larger telescope with an objective of 34 centimetres. This would later become the first telescope of the “carte du ciel project”. Photographic plates attached this instrument could easily reach stars of V~12 or 13 magnitudes, unheard of at the time, and on each plate hundreds of stars were visible.

Such an instrument would be perfectly optimised to realise a modern survey of sky using photographic detectors. These was one problem — from Paris, only a small fraction of the heavens are visible. To survey the entire sky, observatories would be needed in the four corners of the world. Everyone would have to agree on what parts of the sky they would survey and what instruments they would use. To make progress… there would be to have a meeting. So an international conference in was held in April 1887 in Paris Observatory, under the instigations of the paris Academy of Science (suggested by Mouchez) — “The international astrophotographic congress”. It started on Saturday, 16th of April 1887, at 14:00, (so I should really not complain about meetings starting on Sunday). Was this the first international astronomy meeting? It was certainly the first international meeting to whose principal objective was a sky survey.

Now, today, in the 21st Century, everything has been virtualised … and I found, digging around just a little bit, the conference proceedings for this meeting. Well, not exactly: it is an account of the meeting written by a one A. G Winterhalter, who represented the American Academy of Sciences (and is published here as an annex to the 1887 proceedings of the United States Naval Observatory’s). Steam-ships and trains, natural products of the industrial revolution, meant that such an international conference could take place for the first time (Winterhalter writes in his introduction that his steam-ship voyage from New York to Cherbourg takes 11 days, and during the trip he met another astronomer who was attending the same conference – so nothing changes there!). There is a table listing attendance at the conference, broken down by country:

1891USNOO 3D 1B pdf page 430 of 1 036
Who attended the first international astronomy conference

Winterhalter notes that “the proceeds were conducted entirely in the French language”. Hey, those were the days (sorry, French colleagues)! It’s interesting to read Winterhalter’s account of the meeting: a large part of the proceedings is concerned with finding the best possible technical solutions and fixing the parameters of they survey (to which everyone had to agree to). At the meeting, everyone agreed to use the Henry brothers’ telescope, paired with a standard photographic emulsion.
One important question which had to be addressed was: what would be the limiting magnitude of the survey? It was already clear at the outset that this would be a massive undertaking, because at the faintest limits accessible by the Henrys’ telescope there would be an overwhelming number of stars. Millions and millions. How could catalogues be made on paper containing all those stars? The could never be printed, they would simply be too large.

A compromise solution was adopted: the survey would be in two parts: a catalogue release containing all stars to V < 11 and an “imaging data release” which would consist of reproductions of the plates themselves and reach fainter magnitudes – down to V< 14.

Well, that was the plan… More coming up in the second post..