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!

52 Photographs (2018) #18: In St. Cloud, hommage to Atget

52 Photographs (2018) #18: In St. Cloud, hommage to Atget

After reading about Eugene Atget’s life I was more than little be interested to visit once again the parks and gardens he photographed in. They are almost all still there and have not changed much. In the last hundred years in Luxembourg gardens they have changed the chairs, but that’s about it.

St. Cloud is an interesting place. It’s easy to get to from here, around half an hour by bus from Porte d’Orleans. There was once a palace there which burned down during the Prussians war at the end of the 19th century. On one side there is the Seine; at the beginning of the 18th century this location was a favoured spot for country houses for rich Parisians. The village of St. Cloud today is quite picturesque. Unfortunately, today it hemmed in on all sides by motorways and elevated flyovers and one is never far from the rumble of traffic. But the park is still intact, and one can still walk rom the town to the park without crossing a motorway. Inside the park, in fact, one can walk for miles, and there are even forest trails, as much as one could hope for in the middle of an enormous city.

In St. Cloud

The statues that Atget photographed are still there. I brought with me a roll of slow, fine-grained film (still much faster than the film Atget used of course). I took pictures with my Leica. Certainly, there is no way a small portable camera such as a Leica can produce anything like an image from a heavy view camera, but at least there were no pixels involved, and I tried to imagine the photographs I was taking before I took them. There is still a special feeling in this park (I think I prefer it to Sceaux) and I’ll try to return there more often.

Read my two articles about Atget here and here.

Chris Marker at the Cinémathèque française

Chris Marker at the Cinémathèque française

Ceci est l’histoire d’un homme marqué par une image d’enfance. And so begins Chris Markers’ La Jetée, a story of memory, time, the past and the future. This is a story of man, marked by a childhood image. That could be, perhaps, Marker’s own story. At the new exhibition at La Cinémathèque française , one has the opportunity to discover all of Markers’ works, and not just his films, but also his writing, his activism — and his drawings of cats.

Chris Marker and Guillaume-en-Égypte. (c) Chris Marker

Marker was a singular figure in the history of French cinema, and one not easy to classify. After early experiences of being misinterpreted by journalists, he became reluctant be interviewed or profiled. He was not, however, a recluse: one may be reminded of Thomas Pynchon, who said: “’recluse’ is a code word generated by journalists … meaning, ‘doesn’t like to talk to reporters”. On his library card, he substituted he picture with that of his cat, Guillaume-en-Égypte, who would serve as his public representative.

His interests were vast. He was at least novelist, director, publisher and photographer. Marker died in 2012, and the Cinémathèque were able to acquire the contents of his studio (all 500 boxes of it!). This show is a result of the last few years’ worth of cataloguing and sorting.

His most famous film in English-speaking countries is La Jetée, a remarkable short film composed almost entirely of static black-and-white images. It tells the story of a man in a post-apocalyptic Paris who is forced to voyage to the past and future and who discovers the true significance of a traumatic event remembered from his own childhood. But at the same time Marker was assembling the striking still images for La Jetée on the floor of his Parisian apartment, with Pierre Lhomme he was filming Le Joli Mai, a revolutionary documentary showing life in Paris in May 1962. Like Werner Herzog he had an amazing ability to get people to speak openly towards the camera. In the film, a young Algerian man tells of the casual racism he suffered at work; a stockbroker complains on how the Algerian war will affect his stock prices.

Investigating Chris Marker. Note the cat.

Today the details of Marker’s are reasonably well-known, but in the past he certainly liked to tell every person he met different and conflicting stories concerning his origins, and at times he had formative childhoods in both Cuba and Inner Mongolia. In reality, he was born in one of the more bourgeois Parisian suburbs and soon changed his name to “Chris Marker” because he’d already decided he wanted to travel. A name like Chris Marker, he reasoned, would be easier to pronounce in many different languages. Travel did indeed play an important part of his life: and in the 50s he edited a series of well-regarded travel books for Seuil. William Klein, who met Marker at this time, remarked that he was “the kind of person you’d expect to have a laser pistol tucked into his belt”.

Technically, Marker was always worked at the forefront of what was possible with the technology available at the time. Le Joli mai was one of the first documentary films for which the sound was recorded with the images, removing the need for a separate sound recording machine. At the one can discover many items from Marker’s vast collection of imaging-recording devices. He was always fascinated by the possibilities that new technologies created for recording and experiencing the word around us. The entrance to the exhibition itself is modelled around the virtual gallery he created inside Second Life.

Inside the expo, exploring Marker’s later works

In an interview on france culture Costa-Gavras (director of the Cinémathèque) explains that Marker understood immediately the potential of digital technology. Already during industrial disputes in the east of France in the 1960s, Marker and and a group of like-minded film-makers showed striking workers how to make films and how to use cameras and recording equipment to their advantage. The collage reproduced here from the exhibition catalogue seems remarkably prescient for our “citizen reporter” times.

Marker was deeply interested in understanding what the consequences were of the revolutionary fervour which spread around the world in the 1960s. This story is chronicled in his epic documentary film, “Le fond de l’air est rouge” which is amusingly translated into English as a “A grin without a cat”. The end results of these revolutionary upheavals? Not a lot. As Marker notes pessimistically in the DVD liner notes, for French intellectuals the bottom of the air would always be red but despite this, red would always be at the bottom.

Film is a weapon ! A collage (attributed to Chris Marker).

Like many of his generation he saw cities in ruins and expected the next war to come soon enough; his first film was made in the rubble of Berlin. The images of “Paris in Ruins” of La Jetée are in fact other European cities destroyed by wartime bombing. At the cinematheque exhibition, many of Markers’ films are shown on continuous loops in separate rooms without sound isolation. In La Jetée, the beautiful choirs of a Parisian Russian orthodox church are heard at the moment of the destruction of the city. These heavenly sounds resonate throughout the whole exhibition at regular half-hourly intervals, conditioned by the length of the film. Ceci est l’histoire d’un homme marqué par une image d’enfance? Perhaps.

For those not familiar with Marker, is wonderful source of information about the man and his work, in both French and English. Thanks also to Mr. J. Seagull for the loan of the exhibition catalogue.