Le télescope spatial Hubble est sans doute la mission spatiale qui nous a la plus émerveillé par ses images absolument fulgurantes de notr...

Le télescope spatial Hubble est sans doute la mission spatiale qui nous a la plus émerveillé par ses images absolument fulgurantes de notre Univers. Mais parfois, c'est Hubble lui-même qui se fait photographier depuis la Terre !
Voici une vidéo impressionnante montrant le passage du télescope de la NASA devant la lune, enregistrée depuis le sol par un astronome amateur.


Hubble, visible dans le ciel ? Oui, c'est bien vrai

Ce n'est pas une récente découverte, les satellites artificiels orbitant autour de notre planète sont bien observables et une grande partie d'entre-eux sont mêmes visibles à l’œil nu sous la forme de points lumineux se déplaçant rapidement dans le ciel nocturne.
Tout comme la Station Spatiale Internationale, le télescope spatial Hubble est lui aussi situé sur l'orbite basse terrestre, à environ 500 km de la Terre. Par conséquent, lorsque la nuit tombe, son altitude lui permet encore d'être illuminé par le soleil et de réfléchir cette lumière à l'aide de ses panneaux solaires.

Malheureusement, l'orbite de Hubble n'est pas aussi inclinée que celle de l'ISS. Alors que la station spatiale peut être observée depuis une grande partie de la planète, Hubble survole quant à lui une portion limitée, le rendant visible uniquement depuis certains endroits de part et d'autres de l'équateur.

Un rare passage devant la Lune

Superposition d'images représentant le passage du télescope spatial Hubble devant la Lune par Michael Marston

A certains moments et à un endroit donné, la trajectoire du télescope spatial peut croiser celle de la Lune, par simple effet d'optique. Bien que cela pourrait paraître banal, un tel transit reste pour le moins extrêmement rare étant donné la taille minuscule du télescope situé à 500 km d'altitude, dans un ciel des dizaines de milliers de fois plus grand !
Rare ne signifie pas impossible pour autant. En effet, c'est l’événement auquel un astronome amateur Australien a pu assister le 22 mai 2018. La vidéo montre le transit extrêmement bref du télescope devant la lune qui aura duré moins d'une seconde.
Pour réaliser ces images, Michael Marston n'a pas utilisé de télescope mais un objectif photographique de 800 mm de focale sur un boitier de type Reflex.

Vous aussi, tenez-vous informés des transits de la station spatiale devant la Lune en allant sur le site CallSky. Il suffit de rentrer vos coordonnées géographiques et ce site calculera la date ainsi que l'heure précise auquel ce transit aura lieu !

English version  at the bottom of the page ( here ) La nova de la carène est situé tout proche de l'étoile de HIP 51912  et de l...

English version at the bottom of the page (here)
La nova de la carène est situé tout proche de l'étoile de HIP 51912 
et de la nébuleuse qui porte le même nom 

ASASSN-18fv est apparue dans le ciel de l'hémisphère sud le 16 mars 2018, tout près de la célèbre nébuleuse de la Carène. Classée en tant que Nova potentielle, son éclat inhabituel a permis de la rendre visible avec une simple paire de jumelles. Cependant, après plusieurs observations réalisées depuis ce jour, les astronomes ne sont pas encore convaincus quant à la nature précise de cet étrange objet...


Notre voûte céleste est en permanence surveillée par un grand nombre d'observatoires à travers le monde. Leur objectif n'étant pas de saisir de belles images de notre Univers mais de détecter de brefs phénomènes pouvant évoluer en quelques secondes ou quelques semaines. Ces derniers peuvent être des novas, supernovas, transits, sursauts gamma... et sont qualifiés de phénomènes astronomiques transitoires ("transcient").

Le 21 mars 2018, un observatoire du CTIO participant au programme automatisé de la recherche de supernovas (ASAS-SN) remarque la présence d'un objet non répertorié jusqu'à présent. Le nouvel objet porte alors le nom de ASASSN-18fv et les astronomes émettent l'hypothèse d'une Nova. 
Détection de ASASSN-18fv par CTIO, le 21 mars 2018

Cependant, la position exacte ainsi que la luminosité restent difficilement mesurables et les astronomes lancent un appel à la communauté scientifique pour débuter des mesures spectroscopiques et photométriques fiables.

Deux jours plus tard, le télescope de surveillance Evryscope confirme la présence de cet objet et découvre même que ASASSN-18fv était apparue sur des images antérieures datant du 16 mars 2018, avec une magnitude très faible de 10.21.
Détection de ASASSN-18fv réalisée par Evryscope, le 16 mars 2018 (première détection)
Comparaison avant/après l'apparition de ASASSN-18fv (source)

Un extraordinaire pic de luminosité a été atteint le 23 mars 2018, avec une magnitude estimée à 5,7 , soit la visibilité limite d'une étoile par l’œil humain ! Bien qu'une dizaine de novas soient chaque année découvertes, très peu parviennent à libérer autant de lumière. Cela est encore plus rare si une nova est observable avec de simples jumelles, comme ASASSN-18fv en est l'exemple.

Depuis cette annonce officielle, astronomes professionnels comme amateurs se sont empressés d'aller chasser ASASSN-18fv, désormais surnommée la nova de la carène.

L'astronome et astrophotographe français Alain Maury, fut l'un des premiers à la photographier depuis le désert d'Atacama au Chili, accompagné de J. Fabrega (voir la dernière image de cet article).

Nova ou non ?

L'astronome L. Izzo et son équipe à l'observatoire européen de La Silla restent sceptiques quant à cette dénomination de nova. 
Pour rappel, une nova correspond à un bref sursaut de lumière d'une étoile existante provoqué par l'absorption de matière stellaire provenant d'une autre étoile compagnon moins massive. Généralement, un nova se produit au sein d'un système d'étoile binaire.
Dans le cas présent L. Izzo soulève la question du spectre intriguant de ASASSN-18fv, qui ne présente pas les caractéristiques typiques d'une nova classique : " L'absence d'un continuum bleu fort, qui est typique de l'éruption d'une nova classique, et les faibles vitesses d'expansion suggèrent une nature différente possible pour cet objet ".
Selon les derniers rapports, on suspecte également une nature particulière de nova rouge lumineuse ou encore de flash d'hélium.  

La collaboration entre astronomes professionnels/amateurs du monde entier sera encore une fois déterminante pour percer le secret de ASASSN-18fv.
Restez à l'écoute !


************************ENGLISH VERSION*******************

ASASSN-18fv appeared in the Southern hemisphere sky on March, 16 2018, very close to the famous Carina nebula. Classified as a potential nova, its unusual brightness enables to spot it using simple binoculars. However, after several observations led from that day, astronomers are still not convinced of the precise nature of this peculiar stellar object...

Our starry sky is continuously monitored by reams of worldwide observatories. Their main goal does not rest on taking amazing images of our Universe but on detecting some brief phenomena evolving within seconds or weeks. These could be novae, supernovae, transits, gamma ray burst...and are called Transient astronomical events.

On march, 21 2018, an observatory at CTIO (Chile) who takes part in the automated program for the research of supernovae (ASAS-SN) notices a bright object which does not match with any objects already detected. This newly-discovered object is given a name : ASASSN-18fv . At first sight, astronomers think it shows the properties of a likely classical nova.

However the exact position and its brightness remain difficult to evaluate and an alert is launched to the scientific community for further spectroscopic research and measurements.

Two days later, the monitoring Evryscope (in Chile again) confirmed the observation of the potential nova and discovered that it had even appeared on previous images from march 16, 2018, with a low magnitude of 10.21.

An extraordinary peak of luminosity happened on March 23, 2018, reaching a magnitude of 5.7, namely the human's eye visibility limit. Although a dozen of novae are yearly discovered, only a few of them manage to release such an amount of light. It is even rarer if a nova can be observed through binoculars like the ASASSN-18fv.

Since this official announcement, professional and amateur astronomers turn their telescope in a hurry towards ASASSN-18fv, now known as the 2018 Carina nova.

French astronomer and astrophotographer Alain Maury was one of the first to capture it, from the Atacama desert, Chile along with his mate J. Fabrega :
Announcing Nova Carinae 2018 Image Credit & Copyright: A. Maury & J. Fabrega


Nova or not ?

Astronomer L. Izzo and his team at the European observatory ESO in La Silla remain skeptical as far as the denomination of nova is concerned.
As a reminder, a nova is a outburst of brightness coming from an existing star, produced by an absorption of stellar material coming from an other neighbor star (less massive). Typically, a nova is more likely to form inside a binary star system.

In this present case, L.Izzo raises the issue of the intriguing ASASSN-18fv spectrum, which does not show the basic features of a classical nova : "The lack of a strong blue continuum, that is however typical of classical nova outburst, and the low expansion velocities suggest a possible different nature for this object."

According the latest reports, the team also suspects a specific nature such as luminous red nova or helium flash.

The collaboration between professional and amateur astronomers of all around the world will be again rewarding to unveil the secrets of ASASSN-18fv.
Stay tuned !

Currently being built in Chile, since July 2014, the american LSST Telescope could likely revolutionize the astronomical observation and l...

Currently being built in Chile, since July 2014, the american LSST Telescope could likely revolutionize the astronomical observation and lead to an unprecedented level of sky survey by scanning the entire austral sky within three nights only.

Artist's view of the LSST, at the summit of Cerro Pachon (2 715 meters altitude), in Chile. Source : https://www.lsst.org/
The LSST should provide its first images in 2019 and start its scientific missions in 2022. Their goals will be to study the dark matter, quasars, supernovae, variable stars, gravitational lens, and on a larger scale to observe the internal sturcture of the Milky Way and Universe.

In this article, firstly, we are describing and analyzing the outstanding technical specifications of the LSST project led by the AURA. In a second part, we are explaining to you how this innovation could drastically give a support and a dynamism to the astrophysics.

Interactive Outline :

★ Introduction : A cutting-edge Telescope

★  LSST Optics

★ LSST Camera

★ The Technology serving Astrophysics


-------------------------------------------------------------

★ A cutting-edge Telescope ★ 

The code name LSST means "Large Synoptic Survey Telescope". The primary asset of this 8.4-meter-diameter telescope settled in Chile (Cerro Pachon) is its wide vision as the word synoptic suggests it.
Indeed, the field of view scanned by a single image will be the tantamount of 40 full moons, and will need just a 15-second exposure time.

The vision performances of the LSST is simultaneously wide and deep, since its camera is made of the worlds's greatest resolution of 3 200 Millions of pixels, namely 3.2 Gigapixels.

Infographics of the LSST project . Source : AstroSpace and LSST (numbers)

With such features, the LSST could cover the whole Chilean's sky within 4 or 3 nights of observation, and could alert the astronomical community of a suspicious changing object in the next 60 seconds. The service duration provided by this telescope is estimated to 10 years. During this period, scientist expect that 37 Billions of stars, Galaxies, neutron stars will be classified and discovered.

The global cost of the project reaches 850 Millions of dollars, whose main fundraising source comes from the 35 institutional members as well as several noticeable private institutions (Bill Gats, W.M Keck...).


★ Optics ★ 

The optical design selected for the LSST is not similar to the ones we find on semi-professional telescopes (Newtonian, Schmidt-Cassegrain, Richtey-Chrétien...). Engineers decided to use the optical structure of "Paul Baker", namely composed of 3 reflective elements whose 2 of them are located on the same glass (see image below).
Optical structure of the Telescope. Source : https://www.lsst.org


This particular optical structure highlighted by the primary mirror of 8.4 meters diameter, makes the LSST very compact and enables a F/D ratio to 1.23 ! This ratio represents the quantity of collected light by expressing the focal length F over the diameter of the mirror D. The lower this number is, the higher the quantity of light collected is.
Having such a low F/D ratio is not only a beneficial fact. On the other hand, it means also that the mirror aberrations are increased. It is bound to bring a corrective element in order to reach the best performances as possible.
The mount slewing the telescope could point any object of the sky within 5 seconds ! Source : https://www.lsst.org/

The LSST is indeed equipped with an optical correction located before the camera system. Made of 3 lenses, this system corrects the mirror aberrations to obtain an image almost perfectly plane. Of course, these corrected aberrations are not completely deleted but they have been reduced to the point that atmospheric aberrations are far more predominant than optical ones.
The Large Synoptic Survey Telescope will not use an Adaptative Optics system (AO) to offset the atmospheric aberrations, because it is actually impossible today to embed an AO on wide-view telescopes like the LSST. However, an Active Optics has been installed inside the primary mirror, so that the optical defects caused by weight constraints or temperature variations can be corrected.


Auxiliary telescope under construction (foreground).
Source : https://www.lsst.org/

 So, what about the atmospheric aberrations ?
As Active Optics does not affect anything to reduce the effect of the atmosphere, scientists and engineers had to figure out an other way of compensating them. Therefore, they decided to build an auxiliary telescope of 1.2 meters, located just next to the LSST. The principle remains very simple : this smaller telescope will monitor and measure the side effects of the atmosphere on the the part of the sky the LSST is observing. These measurements will serve as calibration frames to remove the aberrations on science images.

Before reaching the camera's sensor, the incident light will pass through 6 large bandwidth filters which are sensitive from the near-infrared spectrum to the near-ultraviolet spectrum. Changing these 78 cm diameter filters requires a complex system, carousel-like. It is one of many other parts of the LSST developed in collaboration with the french laboratory IN2P3.

Animation video of the change filter system
Source : National Accelerator Laboratory
LSST : a collaboration between the United-States, Chile and France ! In addition to the filter system, the french researchers of the IN2P3 lab' are taking part in the development of the CCD sensors, the electronic command of the camera and its software.

★ World's most powerful Camera ★

The optical design and the camera must be coordinated to reach the impressive performances of the LSST as expected.
To get this breathtaking total resolution of 3.2 Gigapixels, the camera is actually stitched by 189 individual CCD sensors (of 10 µm side) of a 16 Megapixels resolution each. The total field of view covered by the camera represents 3,1° x 3,1°, equivalent to the part of the sky occupied by approximately 38 full moons ! (see image below)

Image showing the size the moon would have if projected onto the LSST camera.
Source : National Accelerator Laboratory
The image acquisition electronics will be divided in 31 blocks of 3x3 CCD sensors, enabling to read a captured image within 2 seconds. Each image requires a single 15 seconds exposure. The image acquisition cycle will start every 40 seconds.

The readout noise of the camera is supposed to be extremely low (<10 electrons) and the camera itself will be cooled at -100°C thanks to a liquid-nitrogen-cooling system, having a mass flow of 217 kg/day.

90 % of the whole focal plane surface will be used by the camera, similar to a disk of 63.4 cm diameter. So, the area that the sensor is taking is 0,28 m².

In terms of dimensions and compacity, the entire imaging system can be compared to the size of a mini-van, weighing more than 3 tons !
On the next picture (below), a 3D-illustration of the camera has been established to human-scale, so that we can imagine the size of such component.

Cross-sectional view of the imaging sensor + corrective optics of the LSST. Source : https://www.lsst.org/

★ The Technology serving Astrophysics ★ 

Beyond all of these technical words and principles is hiding a real assistant or astronomical hunter for the scientists, not only with impressive quality but also in a huge quantity of useful data.


State of the art performances !

With its F/1.23 optical structure and its highly resolved and sensitive camera, the LSST will be capable of detecting objects of 27.7 maximum magnitude. For example, observing a pulsar located at more than 1 Gigaparsec (= +3 Billions light years) will not be a stopping point for the LSST.

Comparative video of the deep vision between SDSS telescope and LSST (simulation)
Source : Johan Knapen, LSST


During its 10 years lifetime (beginning in 2022), the telescope will have classified more than 37 Billions celestial objects across the austral sky and will have monitored 800 times the same object. This mind-blowing performances will be necessary in order to describe evolution of astronomical objects and phenomena with an unprecedented accuracy, like :
Dark Matter and Dark energy characterization 
Space distribution of the Galaxies (with photometric measurements of redshifs)
Gravitational lensing
Supernovae, quasars (the LSST will likely discover  ~1000 quasars with z>7 , located at more 3 Bn l.y) 
The internal structure of our Solar System (asteroids)
The Milky Way structure
An interesting fact given by the scientists of the LSST, is that the current number of stars discovered by the SDSS telescope will be equal to the number of variable stars the LSST will discover !

Researchers Paradise

Obviously, the ambitions shown by the LSST project could not be accomplished without a minimum of facilities which can collect and process the huge amount of data collected.


Artistic view of the LSST. Source : https://www.lsst.org/

Scientists estimate the quantity of data stored after every night of observation to 15 Terabytes. They will be then sent to a database in the United States. Half of the data will be processed at the Calculation center of CNRS (CC-IN2P3) in Lyon (France). France will also have a complete copy of the data science of the LSST.
In 10 years of activity, the LSST will produce an archive of data representing around 200 Petabytes, or 200 000 Terabytes !

The LSST project is considered to be one of the most exciting projects for the international astronomical community. Of course, we must not forget the first lights of the E-ELT in Chile too, scheduled in 2024.

Follow AstroSpace blog and on social medias to stay tuned !


Source : https://www.lsst.org