Browsed by
Tag: Euclid

On seeing the Euclid launch

On seeing the Euclid launch

In July, I had the great luck to visit the Kennedy Space Flight Center to see the launch of the Euclid satellite. I wrote this a few days after the launch, but with the great amount of work that we have all been doing since then, I have not had time to publish this! I was thinking I had better get caught up…

My trip to Florida started inauspiciously enough, with a text message telling me that my flight to Newark, the first leg of my journey, had been delayed. This meant that I would miss my connection to Orlando. At CDG the united staff told me that, although they couldn’t rebook my flight from Paris, don’t worry, in Newark they will look after you. I imagined disembarking from the plane and walking right to the smiling United representative at the service desk who would immediately put me on the next available flight. The reality was a four-hour wait in the line-up at Newark until finally a very helpful lady booked me on a flight to Orlando the next morning. I arrived in the humid Florida heat on Friday with enough time to pick up my tickets and those of my colleagues. I will pass in silence over the night spent in a hotel in the grey post-industrial suburbs of New Jersey.

I admit that I had a certain ‘frisson’ seeing the signs for ‘Kennedy Space Flight Center’ (KSFC) as I drove away from Orlando airport. As a child in Ireland in the 1970s I wrote to KSFC and asked them to send me stuff about space and planets. Soon enough, a big wad of old press releases as well as some nice colour pictures of planets arrived from America in a big yellow envelope. It was wonderful. I couldn’t understand why more people were not interested in astronomy given the Universe was so large and the Earth was so small. And so now, 40 years later, I was on a highway heading right to KSC. Soon enough, I was flying over a large river inlet and in the distance I could see what I knew was the famous Vehicle Assembly Building, the largest building in the world. But right now, I was not going to KSC, I was only going to the hotel to get the tickets…

The next morning, I left the hotel at 07:20. The launch was scheduled for 11:12AM, but I knew that My bag was full of cameras and factor 50 sunscreen. I had a big hat I bought at the surf shop near my hotel, ready for the intense heat and light of a summer morning in Florida. I picked up a colleague at his nearby hotel.

Although we had left early, there were already many people at Kennedy for the launch. It was blisteringly hot, and the sun shone brightly in a blue sky without clouds. Although I have been working on the Euclid project for more than a decade and have been to every consortium meeting (except one), there were many people I had never seen before.

Soon enough, we left on the first bus carrying everyone to the bleachers at banana creek, a prime viewing location across the water from the launch towers. The bleachers, however, had not an inch of shade, and it was more than an hour until the launch; no question of staying outside. But next to the bleachers was the Saturn V Apollo building, and we spent a good hour there looking at this impressive space hardware from the past. Soon it was time to go outside again

At the bleachers, everyone was finding their places. There was blinding heat and light. To the right, there was a big screen relaying the SpaceX/ESA livestream, and a local commentator provided us some additional context. I stared out at the horizon at where the Euclid will soon leave the Earth. Confusingly, there were several different launch towers.

Waiting for the Euclid Launch

And suddenly, we were only a few minutes before launch. We were in the bleachers. I had promised to do a livestream with IAP auditorium where everyone was gathered to watch the launch. I put in my AirPods and my colleague Amadine tried to film me with my iPhone. There was so blinding light everywhere it was impossible to see the screen, but I could hear the questions from Paris and I tried to formulate some intelligent answers.

We were in the final minutes before the countdown. No sign of a hold or a delay. There were no clouds in the sky. We were told there was a delay between the livestream and the real world, a few seconds, not much but enough to make everyone chanting countdown pointless. We didn’t know the real-world number of minutes left. Suddenly, there was an enormous cloud on the horizon, low down, and rising from the clouds we could see the Falcon 9 rocket atop a bright orange-yellow flame. It was one of the brightest things I have ever seen. But at first it was completely silent as the rocket arched up into the sky. Then the sound came, a great rolling rumbling wave of sound energy. My cheap straw hat vibrated in time to the roar of the nine merlin rocket engines. Clearly, it takes a lot of energy to get a one-ton rocket to L2, I thought to myself.

I was transfixed. In the bag at my feet I had my two Leica cameras, I had my telephone and a Ricoh compact camera too, but I didn’t want to do anything but look at this bright orange candle as it disappeared into the cloudless sky. On the callout from the screen we heard ‘maxq’ indicating the rocket had passed through the zone of maximum aerodynamic pressure. And then, it was gone, and there was finally just a cloud in the sky, a cloud of water vapour made by the Falcon 9 rocket.

The livestream from ESA and SpaceX continued. We saw the booster coming back, landing on the drone ship. There in orbit was a short coast phase and the second stage ignited again. By this time, all the public had left and there were only a few of us in the bleachers or sheltering nearby. Then, on the “jumbotron” — the big screen they have there — we saw Euclid deploy, the silvery yellow foil catching the sunlight as at it separated from the SpaceX second stage. But still, the story was not over. Was the satellite alive and communicating with the Earth? Then finally on the screen we saw the first signal from the spacecraft. Euclid was alive and heading to L2. And the real work would start very soon.

Waiting for the signal from Euclid at KFC
The road to the space coast (looking back on the ideas that led to Euclid)

The road to the space coast (looking back on the ideas that led to Euclid)

On Florida’s Space Coast, the Euclid satellite is undergoing the final preparations for launch on a Falcon 9 rocket next Saturday, July 1st at 11:12 EDT. Although the Euclid mission was approved by ESA in 2011, the origins of the project date back more than a decade before that, starting with the realisation that the expansion of the Universe is accelerating.

In cinema, great discoveries are usually accompanied by the leading lights throwing their hands in the air and exclaiming, “This changes everything!” But in real life, scientists are cautious, and the first reaction to any new discovery is usually: is there a mistake? Is the data right? Did we miss anything? You need to think carefully about finding the right balance between double-checking endlessly (and getting scooped by your competitors) or rushing into print with something that is wrong. At the end of the 1990s, measurements of distant supernovae suggesting the accelerated expansion of the Universe were initially greeted by scepticism.

Conceptually, what those measurements were saying was simple: the further away an object is, the faster it is receding from us. Edwin Hubble’s early observations of galaxies demonstrated that there was a straight-line relationship between the distance of an object and the speed of movement. The most simple explanation (although one that scientists took a while to accept) was that the Universe was expanding.

Over the next few decades, researchers embarked on a long quest to find different classes of objects for which they could estimate distances. Supernovae were one of the best: it turned out that if you could measure how the brightness of supernovae changed with time, you could estimate their distances. You could then compare how the distance depended on redshift, which you could measure with a spectrograph. Wide-field cameras on large telescopes allowed astronomers to find supernovae further and further away, and by the end of the 90s, samples were large enough to detect the first tiny deviations from Hubble’s simple straight-line law. The expansion was accelerating. The origin of the physical process of expansion was codified by “Lambda”. Or “dark energy”.

First measurements of distant supernovae from two teams. The most distant measurements are above the straight-line measurements by ~20%.

But those points on the right-hand side of the graphs which deviated from Hubble’s straight-line law had big error bars. Everyone knew that supernovae were fickle creatures in any case, subject to a variety of poorly understood physical mechanisms that could mimic the effect that the observers were reporting.

Initially, there was a lot of resistance to this idea of an accelerating Universe, and to dark energy. Nobody wanted Lambda. Not the theorists, because there were no good theoretical explanations for Lambda. And not the simulators, because Lambda unnecessarily complicated their simulations. And not even the observers, because it meant that every piece of code used to estimate physical properties of distant galaxies had to be re-written (a lot of boring work). Meanwhile, the supernovae measurements became more robust and the reality of the existence of Lambda become harder and harder to avoid. But what was it? It was hard to get large samples of supernovae, what other techniques could be used to discover what Lambda really is? Soon, measurements of the cosmic microwave background indicated that Lambda was indeed the preferred model, but because the acceleration only happens at relatively recent epochs in the Universe, microwave background observations only have limited utility here.

Meanwhile, several key instrumental developments were taking place. At the Canada France Hawaii Telescope and other observatories, wide-field cameras with electronic detectors — charge coupled devices, or CCDs — were being perfected. These instruments enabled astronomers for the first time to survey wide swathes of the sky and measure the positions, shapes and colours of tens of thousands of objects. At the same, at least two groups were testing the first wide-field spectrographs for the world’s largest telescopes. Fed by targets selected from the new wide-field survey cameras, these instruments allowed the determination of the precise distances and physical properties of tens of thousands of galaxies. This quickly led to many new discoveries of how galaxies form and evolve. But these new instruments would also allow us to return to the still-unsolved nature of the cosmic acceleration, using a variety of new techniques which were first tested with these deep, wide-field surveys.

In the 1980s, observations of galaxy clusters with CCD cameras led to the discovery of the first gravitational arcs. These are images of distant galaxies which are, incredibly, magnified by the passage of light near the cluster. The deflection of light by mass is one of the key predictions of Einstein’s theory of general relativity. The grossly distorted images can only be explained if a large part of the mass of the cluster is concealed in invisible or ‘dark’ matter. However, in current models of galaxy formation, the observed growth of structures in the Universe can only be explained if this dark matter is distributed throughout the Universe and not only in the centres of galaxy clusters. This means also that even the shapes of galaxies of the ‘cosmic wallpaper’ throughout the night sky should be very slightly correlated, as light rays from these distant objects pass close to dark matter everywhere in the Universe. The effect would be tiny, but it should be detectable.

Simulation of the passage of light rays through the Universe, passing close to dark matter (S. Colombi, IAP).

Around the world, several teams raced to measure this effect in new wide-field survey camera data. The challenges were significant: the tiny effect required a rigorous control of every source of instrumental error and detailed knowledge of telescope optics. But by the early 2000s, a few groups had measured the ”correlated shapes” of faint galaxies. They also showed that this measurement could be used to constrain how rapidly structures grow in the Universe. At the same time, other groups, using the first wide field spectroscopic surveys, found that measurements of galaxy clustering could be used to independently constrain the parameters of the cosmological model.

Halfway through the first decade of the 21st century, it was beginning to become clear that both clustering combined with gravitational lensing could be an excellent technique to probe the nature of the acceleration. Neither method was easy: one required very precise measurements of galaxy shapes, which was very hard to do with ground-based surveys which suffered from atmospheric blurring; the other required spectroscopic measurements of hundreds of thousands of galaxies. And both techniques seemed highly complementary to supernovae measurements.

In 2006, the report from a group of scientists from Europe’s large observatories and space agencies charted a way forward to understand the origin of the acceleration. Clearly, what was needed was a space mission to provide wide-field high-resolution imaging over the whole sky to measure the shapes, coupled with an extensive spectroscopic survey. Both these ideas were submitted as proposals for two satellites: one would provide the spectroscopic survey (SPACE) and the other would provide the high-resolution imaging (Dune). The committee, finding both concepts compelling, asked the two teams to work together to design a single mission, which would become Euclid. In 2012, the mission was formally approved.

Euclid in the clean room
Euclid in the clean room at Thales Alenia

Euclid aims to make the most precise measurement ever of the geometry of the Universe and to derive the most stringent possible constraints on the parameters of the cosmological model. Euclid uses two methods: galaxy clustering with the spectrograph and imager NISP (sensitive to dark energy) and gravitational lensing with the imager VIS (sensitive to dark matter). Euclid‘s origins in ground-based surveys makes it unique. Euclid aims to make a survey of the whole extragalactic sky over six years. But unlike in ground-based surveys, no changes can be made to the instrument after launch. After launch, Euclid will travel to the remote L2 orbit, one of the best places in the solar system for astronomy, to begin a detailed instrument checkout and prepare for the survey.

I have been involved in the team which will process VIS images for more than a decade. The next few weeks will be exciting and stressful in equal measures. VIS is the “Leica Monochrom” of satellite cameras: there is only one broad filter. The images will be in black-and-white. It will (mostly) not make images deeper than Hubble or James Webb: Euclid‘s telescope mirror is relatively modest (there are some Euclid deep fields, but that is another story). But to measure shapes to the required precision to detect dark matter, every aspect of the processing must be rigorously controlled.

VIS images will cover tens of thousands of square degrees. Over the next few years, our view of the Universe will dramatically snap into high resolution. That, I am certain, will reveal wonders. Those images will be one of the great legacies of Euclid, together with a much deeper understanding of the cosmological model underpinning the Universe that will come from them and the data from NISP.

This Thursday, I’ll be travelling to Florida to see Euclid start its journey to its L2 orbit for myself. I’ll be awaiting anxiously with many of my colleagues for our first glimpse of the wide-field, high-resolution Universe that will arrive a few weeks later.

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:


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!