Historique de Marseille
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le 02/08/24
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| Hypertelescopes | |||||||
Making telescope mirrors bigger and bigger has always been hard. The difficulty with casting ever larger lumps of glass, very homogenous and without air bubbles, has slowed down the construction of telescopes around 4 meters in size. To go further the 6 meters of the Bolchoï telescope, one had to give up the rigidity due to the thickness of the glass, which implies a far too high weight. Flexible mirrors or mosaic dynamically shaped by computer have given the solution, allowing to reach the 10 meters. But further, we face another problem : it's not possible to make a monolithic mirror in situ, and no more easy to bring it to the observatory. The solution were to build the mirror in segments ; these are named composite mirrors, or mosaics.
In a composite mirror, one minimize the gaps between the segments, in order to lack the minimum of light, et reduce the diffraction spikes. Obviously, the shape and position of the mirrors are computer-driven, in order to ensure that their juxtaposed surfaces constitute always a unique spherical surface. This surface would be that of a monolithic equivalent mirror. It's difficult to build vary large mirrors, et furthermore their cost would be prohibitive. The telescopes planed now, named ELT (Extremely Large Telescope), will reach 40 meters in size. But for such an instrument (E-ELT, Europeen Extremely Large Telescope), a thousand mirrors are needed (exactly 798 mirrors). This implies a great difficulty in positioning each segment precisely on the theoretical sphere.
Another method of interferometers has been explored before. An interferometer is an instrument made of some distinct telescopes, working as a whole on the same target. Mixing their beams, they produce interferences the treatment of which by numerical methods (aperture synthesis and deconvolution) allows to reconstitute an image of the target. The benefit is to obtain with telescopes of reasonable size (8,20 meters for VLTI for example), the resolution of a telescope the diameter of which would be the distance between the components. The main disadvantage of this technique is a loss of fidelity of the image, especially if the telescopes are rather few.
The hypertelescope concept is an interferometer constituted of a lot of mirrors, with a pupil densification system. One can make an easy experience, with a hand mirror we all have in our bathroom. Oriented towards a sunny landscape, it projects an inverted image on a screen. Cover the mirror with a sheet of black paper, drilled with some small round holes (you can use cutting dies to drill). The image is always visible, however it is less luminous. Starting out from this observation, the idea is to cut the mirror in several round pieces, and arrange these pieces together so as to rebuild the mirror the exact form it was before being cut. The pieces must form the same global spherical surface. It may therefore be somewhat surprising to see that the holes are no longer visible on the image. But this is a fact.
A mirror made of several segments is called diluted mirror. Making a diluted mirror is obviously easier and cheaper than making a monolithic or mosaic one. Its luminosity will be proportional to the total surface of the segments, but its resolution is determined by the edge of the outside envelope of the mirror (not taking into account the turbulence). One can imagine building very large diluted mirrors constituted of many small mirrors which are easy and cheap to make.
This option has been considered for a long time but it is unfeasible due to the diffraction. Let us look at the landscape through a mousseline chiffon : you will see its blurred image in a bright halo produced by the diffraction. A rule to guide the construction of an interferometer was established : the entrance pupil must be similar to the exit pupil. But Antoine Labeyrie realized that this was far too restrictive. He proved that they (the 2 pupils) must be on the same layout, but there is no need for them to be similar. The light beams coming from different apertures must keep their relative position, but their size can be modified. If we increase the size of the exit pupil, thanks to the pupil densification, the brightness of the image will increase, while the brightness of the halo will decrease.
The downside is that the field for direct imaging will be narrower.
We could consider either taking a few big mirrors or many small mirrors, in order to reach the same collecting surface, so that we obtain the same amount of light energy from the source. Considering a given collecting surface, what is the best solution ? To answer that question Antoine Labeyrie made some theoretical calculations, and then computer simulations. The result is obvious : it is best to use several small mirrors. The more pieces there are, the better the quality of the image and its contrast are, and the larger the field of direct imaging is. We will now examine how the image is formed.
 | Let us take an example. The light coming from a point-like star creates a spherical wave which is quasi-plane at the aperture of the instrument. The wave reaches the center of the lens first, then it penetrates the glass. The speed of light in glass is reduced compared to what it is in the air - it is only 200,000 km/s - hence causing the wave to be delayed. In contrast, on the edge of the lens, the thickness of the glass is insignificant, and the wave will not be delayed. This will bend the wave after it has crossed the lens.
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The light coming from two stars reach the lens as a system of plane waves (?) (shown in two colors, blue and red in the illustration).
The two diffraction spots are separated by a distance that is proportional to the separation angle between the two stars. If this separation angle is sufficient you will see really two distinct spots. If not you will observe only one spot. |  |
Focusing the image is obtained by delaying some parts of the wave, in order to produce a concentric wave. The shape of a mirror produces the same result.
  Monolithic mirror |
  Diluted mirror, with Regular layout |
  Mirror reduced to two segments |
On the bottom left, we can see what would be the image of a single star produced by a giant telescope. If we replaced the mirror by a set of little mirrors regularly spaced, the image would be recognizable, even if it would be more complex in that case. The light spreads over the image, forming secondary spots. We can wonder whether it would be possible to recognize a star cluster if each of them gave such an image.
On the right, you can see the image produced by only two segments ! These are the Young interference fringes. They contain the same information, but perfectly hidden. However that is how interferometers are made.
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| Fizeau interferences with 2, 3, 5 and 9 apertures respectively, regularly laid in a circle. Credits : Antoine Labeyrie | |||
The images obtained, depending on the number of mirrors, show that the quality increases proportionally. Note that the central spot, invisible for two apertures, is more and more brighter as their number increases. This shows that the light is more and more concentrated in the diffraction spot, instead of spreading out in secondary spots, which are grow darker and darker.
The first experience of this kind has been conducted at the Marseilles Observatory by Edouard Stephan who used the Foucault Telescope. It made use of only two apertures, and it did not allowed him to measure the diameter of a star, because the telescope is too small. But he managed to determine an upper limit of 0,158 arcsecond.
The hypertelescope, as defined by Antoine Labeyrie, is a multi-aperture Fizeau interferometer equipped with a pupil densifier. This definition specifies all the elements that constitute it.
Scheme of a hypertelescope, credit Antoine Labeyrie
Let us consider a diluted lens, -represented here by a segmented lens,- which co-focuses the beams towards the Fizeau focus. If the lens segments are laid out regularly, the image will be like the one we can see in the center of the scheme : one central and secondary spots. Beyond the focus, a second lens makes the beams parallel. Then, for each beam corresponding to a fragment of the first lens, there is an inverted galilean refractor telescope. Its eyepiece is diverging, being directed towards the light, whereas the objective lens is convergent.
The eyepiece makes the beam divergent, and the objective makes it parallel again (this telescope is an afocal instrument). Notice that the gap between the beams is reduced. This is why this device is named pupil densifier. Finally, a convergent lens reconstitutes the global image. The diffractive frame, produced by the sub-apertures, is narrowed, attenuating the secondary images. Their energy is concentrated in the central peak, which is strongly amplified. If the star has a companion, its image is similar but shifted except if the shift is greater than the distance of the peaks. If the pupil is completely densified, this spacing corresponds to the field of direct imaging, the angular diameter of which is &lamda;/s where s is the minimal spacing of the mirrors.
Notice that the pupil densifier was also created by Antoine Labeyrie.
This process is less efficient when the star spreads out from the optical axis, because the densified wave tends to be distorted step by step (figure ajoutée). The limit of the direct imaging field is reached when the height of the steps exceeds the quarter of the wavelength ; this phenomemon is named Rayleigh tolerance. Beyond, the resolution is decreased by the enhanced secondary peaks. The pupil densification restricts the usable field, and hence the hypertelescope is unusable for extended objects. However multiple channels eventually plentiful, can be separated from the Fizeau focus to give images from, for example, stars belonging to a star cluster if their angular diameter is less than the value λ/d of the direct imaging field.
Present interferometers, including VLTI, are constituted of independent telescopes by mixing their lights. But these telescope are not equidistant from the target. Their mirrors don't constitute a unique spherical surface. Hence, to restore a plane wave it is necessary to insert optical delay lines in the optical path, synchronizing all the beams. These delay lines are complex and expensive, requiring ultra-precise motoring, and limit the number of conceivable apertures (4 Auxiliary Telescopes only, or 4 Auxiliary Telescopes and 4 Unit Telescopes for the VLTI).
credit A. Labeyrie
An aluminium foil creased and then smoothed, placed in the sunlight, produces an image similar to that of a constellation. Another foil is disposed behind the lens of a digital camera drilled by some pin holes, through which the light goes. Notice that :
In this easy experience, the pupil was not densified. It produces the halo surrounding the image produced by 600 pinholes. It brings to light the fact that the quality increases with the number of segments of the diluted mirror.
Then, a question arises : what is the diameter of the segments to obtain the best quality ? Another experience helps find the result : it is best to increase the number of fragments rather than their diameter. For a same collecting area, the maximum number of mirrors is the most efficicent.
Credit A. Labeyrie
This comparison concerns two mirrors of the same total surface S. The first is fragmented in 6 segments, the second in 600. The radius of the components are R2 = S / 6 π and r2 = S / 600 &pi. Therefore R = 10 r. For example, let's consider 6 mirrors of one meter or 600 mirrors of 10 centimeters. It seems no more difficult to make 600 mirrors of 10 cm than 6 of a meter.
The images obtained with theses diluted mirrors are located in the centre. It's easy to notice that the 600 mirror segments give a readable image, which is not the case of the other. On the right side, two images produced by the same mirrors can be seen, but with a rotation of the field during the exposure. The six segments diluted mirror show an amelioration with rotation, but for the 600 segments one, the improvement is limited. Yet we'll see its utility.
Credit A. Labeyrie
On the right, the exposure without rotation is unusable. The rotation produces an impressive enhancement of the quality. Finally, the substraction of the background gives a very useful image.
The theory tells us that n segment mirrors are necessary to resolve n2 stars. For instance, 33 mirrors are necessary to resolve a set of 1,000 stars.
Of course, the resolution is proportional to the diameter of the diluted mirror, which must be constituted of a great number of segments.
We mentioned also that the secondary peaks are attenuated by the pupil densification. These peaks are introduced by dilution. Therefore, in order to use the dilution to reduce the cost, the pupil densification is essential. The only disadvantage of this densification is the diminution of the field. If the image grows up by a factor γ, the field is reduced by this same factor γ, while the central peak is increased by the factor γ2.
It remains a limitation due to atmospheric turbulence. The turbulence cells at different temperature shift the phase of the light arriving from a star. It is thus not possible to have a constructive interference as expected.
To correct this turbulence, two methods, which are now considered as being classic, exist : speckle interferometry and adaptive optics. The second one needs an important and expensive equipment. So it will be interesting to use speckle interferometry as a first step. This technique, developed by Antoine Labeyrie, only requires a very sensitive camera.
Speckles produced by a star
Speckles are images of stars appearing on very brief snapshots. By an appropriate numerical treatment applied on thousands of snapshots, it is possible to build an image similar to the image which would be made without the atmosphere. This is the principle of this technique named speckle imaging. Indian astronomer Arun Surya has just showed that this method is suitable for hypertelescopes. But adaptive optics promises to do better, giving directly a high resolution image on the camera.
A Schmidt telescope is made of a spherical primary mirror bigger than the correcting plate placed at the opening. So the light beams from stars, angularly spaced apart and going through the correcting plate, reach the mirror in different places. For a given star, only a part of the mirror is used. But the other parts allow the imaging of other stars, giving the telescope a large field.
The angular spacing of two stars in a Schmidt telescope can be compared to two locations of the same star at different times.
Among the different types of hypertelescope, some use a fixed giant diluted mirror, taking advantage of the natural shape of a crater or valley. They need no mount which would be very expensive at the kilometer scale, but are not orientable. A star may be followed by moving the camera on the focal surface of the metamirror, a half radius spherical cap concentric with it. It is useful to make a metamirror bigger than its part used at a time, which is limited by the light cone defined by the camera.
Let us consider an unused part of the mirror (at a given time). To improve it, you just have to collect the light it reflects. This is done by placing a second nacelle bearing an optical device similar to the first.
It is then possible to construct a hypertelescope with a spherical mirror and many nacelles, to make several observations simultaneously. The inefficiency becomes a benefit. The increased number of the segments allows to track an object for a long time, without an expensive mount for the instrument. Furthermore, many different observations may be conducted in parallel. Different parts of the mirror may be used simultaneously for several different observations.
In the space, a fleet of spherical mirrors bubble-shaped would be usable by a lot of camera placed on the focal sphere, observing an equal number of different sources.
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The first hypertelescope was built to prove the theory. The diameter of its diluted lens is… 10 cm ! Hence, it is sparsely affected by atmospheric turbulence. No great discovery is expected with this instrument of course.
It is made of a refractor telescope of 10 cm of aperture, behind which is located a mask drilled by 64 holes of 0.8 mm regularly spaced. The eye piece of the telescope is focused to infinity, and give of each hole an image of 0.1 mm of diameter. These images are separated by 1 mm. The telescope is equipped with a pupil densifier made of microlens, with a ratio γ = 10. Hence the images of the holes have a diameter of 1 mm, and are quite contiguous. We have seen that the central peak magnification is proportional to γ2, a hundred times here.
Scheme of the first hypertelescope
Notice the difference between the images obtained directly by the masked lens (left) and after densification (right). This show clearly the effect of the densification.
This miniature instrument allowed to validate the concepts of pupil densifier and hypertelescope.
We noted that the hypertelescope principle may be implemented in different ways, in particular by avoiding any mount in order to allow a meta aperture largely exceeding that of Extremely Large Telescopes, the construction of which begins. If this was precisely true, the hypertelescope could only observe a star at the zenith, for a very brief time. To make a long exposure, the device put at the focus has to move along the focal sphere, rotating around the centre of curvature C1 of the meta-mirror. Doing so, the instrument can track the star, but some segments will be too offset, out of the cone of acceptance. The diluted mirror must be larger than the part used at a given time.
Déplacement de la nacelle credit A. Labeyrie
The size limit would be on Earth of 1,000 to 1,200 m of diameter, because there is no bigger natural site allowing to build the mirror for a reasonable price !
A first prototype has been built at the Observatoire de Haute Provence (OHP). It included initially two mirrors and its aimed at develop the nacelle hanging from an airship. All the device allowing the nacelle to be precisely positioned was to conceive. It has been achieved with a set of cables driven by robotic winch pulling the airship in three directions at 120°. The nacelle must also move gently to track the star. The device is inspired by that used at Arecibo in radioastronmy. |  Composed flower of Carlina Acanthifolia thistle embeded in the ground and containing hundreds of little flowers, which give his name to the concept. photo A. Labeyrie |
 Prototype from OHP, airship and nacelle credit Antoine Labeyrie |
 The complex viewed from below. credit Antoine Labeyrie |
reflection in one mirror credit Antoine Labeyrie
A third mirror has been added yielding the prototype more realistic.
Since this beginning, a full-scale prototype is evaluated in the Vallon de la Moutière in the Ubaye Valley, near Barcelonnette (French Alps). The project starts with an aperture of 57 meters. The meta-mirror is spherical. The spherical aberration, resulting from an imperfect crossing of the light beams in the focus of a non parabolic mirror, is rectified by an aspherical optical system named Mertz corrector, constituted by two mirrors.
The site - Located at an altitude of 2,000 m in the southern Alps, the Vallon de la Moutière is not far from Barcelonette and fulfill the required criterions. It benefits from a sky without stray lights, with a good transparency and a week atmospheric turbulence. The median part of the valley, east-west oriented, in shape of cylinder, is protected from prevailing winds. Thermal winds are practically null, which is very good for the hanging structure supporting the optical device at the focus.
road map of the site du site (carte Michelin)
The site of this instrument is part of the Parc du Mercantour, a protected national park where no permanent establishment is allowed and wildlife must not be bothered. However, this project will be developed until its achievement, which will take some years. At the end of the project, no trace will remain.
The mirror - The mirror is made of segments of 20 cm in diameter, laying on tripod mounts imbeded in the ground. The diluted mirror is of 200 meters in diameter (57 m in the beginning) and its components are all on a same spherical surface. The optical nacelle weigh only some kilogram, and may be supported by a cable stressed between the two hillsides. The valley being oriented east-west, the cable is north-south.
The cable - In order to consider the park demands the cable is only stressed during the observation periods. It is made of Kevlar of 6 mm in diameter, which give it a great strength and a good stiffness. Its length is 800 meters.
implantation (scheme Antoine Labeyrie, background Google Earth)
The nacelle - The nacelle, hanged at this cable a thousand meters above the ground, may roll like a miniature cable car along this cable in the north-south direction. The cable may also swing from east to west in order to give the nacelle a freedom of movement east-west to track a star, and to access to stars more or less close to the celestial pole. It is positioned and oriented by six little cables driven by winches, the motors of which are stepper motors. To synchronize the operation of the winches, a wifi net has been deployed on the site. It allows a computer to drive the necessary actions. Of course, a solar power is needed to obtain the electric power.
west motor credit Antoine Labeyrie
The nacelle reflects the light towards the south point of the site, where an equatorial mounted telescope is set to gather it. The polar axis of the mount goes through the C1 center of curvature of the mirror, and by the telescope in order to facilitate the positioning.
Performance - Thanks to its numerous little mirrors, this prototype will have the same resolution as the VLTI (made of only four great mirrors) for a cost having nothing to do with its own. Furthermore, Carlina gives an image immediately visible, while a classical interferometer needs to reconstruct the image by computer after data collection. To obtain every spatial frequencies, one must observe with a lot of couples of telescopes differently positioned (orientation and distance). These observations are made sequentially, excluding rapidly changing objects.
The hypertelescope resolution in its first phase (57 m) will reach 2 milli arcsecond. This value is to compare with the 40 milli arcsecond resolution of the HST (Hubble Space Telescope), 20 times betterr ! In ist extended version, with a diameter of 200 m, it will reach 0.5 milli arcsecond. This is 80 times better than HST, and 120,000 times better than the observations of Tycho Brahe…
Where Tycho Brahe saw one pixel, this hypertelescope will see 120,000 x 120,000 = 14 millions pixels. Or 11,039 17" computer monitors (1,440 × 900 each) presented as a matrix of 83 x 133 screens.
This is only the beginning…
ELHyT means Extremely Large HyperTelescope. It represents the achievement of the concept on the ground, which should reach 1,000 to 1,200 m in diameter. For this purpose, it will be constructed in one of the exceptionnel suitable valleys on Earth. One of these sites is in the indian Himalaya. The hypertelescope could include a thousand mirrors.
Despite of these impressive dimensions, the cost of such an instrument would be very moderate since all the mirrors are fixed and open air. No mount is needed, and this represent a key saving for the project. Installing the adaptive optics device is an important step which seem achievable. And so is the adjunction of an artificial laser star allowing the observation of faint stars for which the laser power is to specify. We hope it affordable. Waiting for the spatial versions, free of atmospheric turbulence, these improvements will allow to get direct pictures with a resolution of 100 micro arcsecond, 10,000 times better than the resolution ordinary imposed by atmospheric turbulence.
An important aspect is the ability to ad mirrors incrementally. So scientific goals needing few mirrors will be acquired before the achievement of the instrument.
To return to Tycho : the resolution of 0.1 milli arcsecond is 600,000 times enhanced ! 278.000 17" screens…
And last, but not least :
Two ways are possible to build a spatial hypertelescope : :
The first one has been foreseen by NASA with some studies : TPF (Terrestrial Planet Finder) is a beam of length 100 meters holding some mirrors. The second one is european, but this project where presented to the European and american space agencies.
The project Darwin, now discarded, was a spatial interferometer involving six 2 meters mirrors configured on a surface of 100 to 1,000 meters in diameter. This was not an hypertelescope since it involves only a few mirrors, ans didn't use pupil densification.
For a free-flyers configuration, the problem which arise is the stabilization of the mirrors which was guaranteed by the rigidity of the beam in the other solution. Stabilization in space may be considered in three ways :
Driving by microrockets has been evaluated by the French-Swedish experience Prisma. It has proven the feasibility of the concept already with a centimetric precision. To be operational, the precision should reach a fraction of micrometer in the axial direction.
Perhaps, the solar sail is not the best solution due to its response time to change the mirror configuration. But it has a critical advantage : for some reason, a mirror may be lost (unreachable by radio) and disoriented. It may compromise the total system. The solar sails may be paraboloid shaped in such a manner that, if the mirror is disoriented, the solar radiation pressure will put it again approximately in the right direction after a certain amount of time.
Laser driving is close to solar sail in principle, because it uses a passive sail. In place of the solar light, this sail is designed to receive a laser beam produced by the central laboratory (which contains the optical device). Some studies have proved that the precision would be better than that of microrockets, with a life time unlimited.
Epicurus is a spatial hypertelescope project conceived by Antoine Labeyrie, and embeding 6, then 18 and finally 36 mirrors of 30 cm in diameter, spread over several hundreds or thousands of meters, giving a resolution of the order of one milli arcsecond.
Luciola is a next step following Epicurus. It is another spatial hypertelescope project designed by Antoine Labeyrie, reaching a kilometer in diameter. It embeds a pupil densifier. Is has been sumitted to the European Space Agnecy in june 2007. It is in a simplification phase in order to make it operational.
Luciola project illustration Antoine Labeyrie
The luminosity will be sufficient instead of the diluted aperture. Indeed, with 100 mirrors of 25 cm in diameter, the same limit magnitude as that of the HST will be reached. Each mirror has a surface of s = π r2 r being its radius, and hence the total surface of the 100 mirrors is S = 100 π r2. This surface S is equivalent to that of a single mirror of radius R such that S = π R2. Thus 100 π r2 = π R2 and R = 10 r. Since r = 25 cm we obtain R = 2,50 m. The diameter of the HST is 2,40 m. Luciola will have the same sensibility with a resolution 400 times better (ratio of the diameters 1,000 m / 2,4 m).
Above the atmosphere, no adaptive system is necessary and there is no absorption. It will be possible to observe from the ultraviolet (spectral line Lyman α 120 nm) to the infrared (20 μm).
The methods appropriate to increase the dynamics of the picture such as coronography and spectroscopy, may be used and we will dispose of an exhaustive instrument. It meets the conditions for the program Comso Vision proposed by ESA, which planed the programs for the period 2015 - 2025. This scheduling is essential due to the length of development (20 years for the program Huygens).
This concept, like its ground counterpart, is extensible. The number of mirrors may increase with successive launch, and many optical devices (equivalent to the Carlina nacelle) may be added. The number of segment mirrors could reach 1,000, increasing significantly the sensibility. This number, with segment mirrors of 25 cm, would reach the surface of one UT of the VLT (8,20 m).
To get a star image, only the first elements will be enough. But to image an exoplanet a greater number of mirrors will be necessary. These observations will begin in the infrared where the contrast is better and imaging easier. But for an image in the visible radiations, we will have to wait the achievement of a more complete instrument. Then, combining infrared and visible spectrum will allow the detection of life on an exoplanet.
Luciola is the spatial equivalent of the Ubaye project : a prototype which will certainly give important results, but is only a demonstrator, a step in the development. The continuation is already foreseeing :
The possible progress is incredible.
There is a technical argument to minimise the size of the segments : if d is the mirror's diameter, the mass of the micro satellite is proportional to d3, its surface proportional to d2. Then the miniaturization decrease by a factor 1/d the necessary acceleration to put the micro satellite in movement (to reconfigure the telescope, the acceleration needed is proportional to the mass, therefore to its volume). Finally, the time needed to reconfigure the telescope (in order to observe another object) is proportional to d1/2.
With an hypertelescope on the ground, the imaging of stars and planets is enhanced. It is also possible to look for life -eventually evolved-, galaxies and far universe.
It would be possible to observe the transit of a planet in front of its star, if the star is not too far from Earth. By spectroscopy the composition of the atmosphere may be measured.
With a coronograph, some close and large exoplanets have already been imaged. This technique will obviously apply to an hypertelescope. A terrestrial planet is 1 to billions times fainter than its star.
All that we observe today would be obviously potential targets : black holes, active galactic nuclei, quasars, micro-quasars (in rder to verify if the model derived from its properties is realistic), gravitational lens, gamma ray bursts (optical counterpart)…
With a spatial hypertelescope of 100 km in diameter, made of 100 three meters mirrors, it's possible to image a planet like Earth in 30 minutes. The image will show the continents (if any) end therefore to discover many information on the planet. In particular color changes depending on the season and localized, could highlight potential life.
Simulated image of an exoearth, as seen by a 150 km hypertelescope in space.
The image above show what would be the picture of a planet looking alike Earth taken by this instrument 100 km in diameter at a distance of 10 light years from us. One can see the seas, continents, the colors. A potential change of color with season would be visible. This would not constitute a proof of life on this planet, but not so far.
It is obvious that the Lagrange point L2 is a perfect location for such an instrument. A very uniform microgravity takes place there, hence the tidal forces are very weak. These forces are responsible for the dissociation of the cluster of microsatellites.
A solar-powered nanosatellite
Supergiant stars are well resolved since a diameter of 18 m.
An ELT has a field largely greater than that of an hypertelescope. But of course, its diameter beeing very inferiour, its resolution is inferiour as well.
An hypertelescope may embed many nacelles, and then constitute several telescopes operating simultaneously.
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Antoine Labeyrie envisage jusqu’à un hypertélescope formé de miroirs espacé sur un diamètre de 100.000 km ! Un tel instrument, forcément spatial, permettrait d’imager la surface d’une étoile à neutrons (20 km de diamètre).
