Dernier point sur l'avancement de la mise au point de James Webb (blog Nasa)
Normalement, la mise en phase des miroirs devrait être terminée .
Après, ce sera les réglages fins (phase 5)
Webb Will Use Spectroscopy to Study Composition of Distant Galaxies
Author: Alise Fisher
March 10, 2022
This week the Webb team continued to make progress in aligning the telescope to the NIRCam instrument. Between taking the data to understand the optical components, we continue to check out the science instruments. The NIRSpec instrument includes a microshutter array of a quarter-million miniature movable windows, each 0.1 by 0.2 millimeters in size. The microshutter array allows scientists to target specific galaxies in fields they are studying, while closing the windows on the background or other objects which would contaminate the spectra. We have begun testing the mechanism and electronics that control and actuate the microshutters.
In recent weeks, we shared a technique for theoretically modeling the early universe. Today, we will discuss an observational program to help us answer some of those questions. Massimo Stiavelli, the Webb Mission Office head at the Space Telescope Science Institute, tells us about his planned investigations of the first stars and galaxies:
“The chemical composition of the early universe, just after the big bang, is the product of the nuclear processes that took place in the first few minutes of the universe’s existence. These processes are known as ‘primordial nucleosynthesis.’ One of the predictions of this model is that the chemical composition of the early universe is largely hydrogen and helium. There were only traces of heavier elements, which formed later in stars. These predictions are compatible with observations, and are in fact one of the key pieces of evidence that support the hot big bang model.
“The earliest stars formed out of material with this primordial composition. Finding these stars, commonly dubbed as the ‘First Stars’ or ‘Population III stars,’ is an important verification of our cosmological model, and it is within reach of the James Webb Space Telescope. Webb might not be able to detect individual stars from the beginning of the universe, but it can detect some of the first galaxies containing these stars.
“One way to confirm whether we are finding the first stars is to accurately measure metallicities of very distant galaxies. The astronomical term, metallicity, is a measurement of the amount of material heavier than hydrogen and helium – so a low metallicity galaxy would indicate it was made up of these ‘First Stars.’ One of the most distant galaxies discovered so far, known as MACS1149-JD1, is confirmed to be at redshift 9.1 and emitted the light we see when the universe was only 600 million years old. The light from this distant galaxy has been traveling ever since then and is just reaching us now.
“In the first year of Webb science, I have an observing program to study this galaxy and determine its metallicity. I will do this by attempting to measure the ratio in the strength of two spectroscopic lines emitted by oxygen ions, originally emitted at violet-blue and blue-green visible light (rest frame wavelengths at 4,363 angstroms and 5,007 angstroms). Thanks to cosmological redshift, these lines are now detectable at the infrared wavelengths that Webb can see. The use of a ratio of two lines of the same ion can provide an exquisite measurement of the gas temperature in this galaxy and, through relatively simple theoretical modeling, will provide a robust measurement of its metallicity.
“The challenge is that one of these lines is usually extremely weak. However, this line tends to get stronger at lower metallicity. So if we failed to detect the line and measure metallicity for MACS1149-JD1, that would likely mean that it has already been enriched by the heavier elements, and we need to look further and harder. Whether using my data or with future programs, I fully expect that during its operational lifetime Webb will be able to find objects with metallicity sufficiently low to hold keys for understanding the first generation of stars.”
–Massimo Stiavelli, Webb Mission Office head, Space Telescope Science Institute
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Il est aussi expressément à noter, que tout démarchage commercial - de quelque nature qu'il soit - n'est pas autorisé et sera modéré.
Merci de bien respecter la Charte HCFR, l'espace de loisirs proposé ici n'étant pas destiné à être un défouloir ou une foire d'empoigne.
Et pour mémoire, la Charte HCFR est directement consultable ici : Charte Forum HCFR
Il est aussi expressément à noter, que tout démarchage commercial - de quelque nature qu'il soit - n'est pas autorisé et sera modéré.
Le post des nouvelles des étoiles...
- Robert64
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on attends la première vrai image !!!
La configuration dans mon profil
Ceux qui ne savent rien en savent toujours autant que ceux qui n'en savent pas plus qu'eux
Abonnez vous à votre groupe régional: "Résidents de ............."
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Thierry.P - Modération Rencontres
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Thierry.P a écrit:on attends la première vraie image !!!
D'après le planning, l'alignement et les dernières corrections sont prévus pour la 18ème semaine après le lancement, ce qui nous amène début Mai.
En principe, s'ils pointent un objet proche, ce ne devrait pas être très différent de ce que montre Hubble, puisque les résolutions des deux télescopes sont voisines (0,1" d'angle).
Mais c'est sur des objets lointains, de lumière fortement décalée vers le rouge, que l'observation en infra rouge montrera du jamais vu.
Wait & see.....
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- Robert64
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La mise en phase des miroirs entre eux est maintenant terminée!
Reste maintenant à faire l'alignement global
L'image de l'étoile qui sert de référence commence à ressembler à quelque chose de plus familier:
Pour plus de détails, le blog Nasa:
Mar 16, 2022
NASA’s Webb Reaches Alignment Milestone, Optics Working Successfully
Following the completion of critical mirror alignment steps, NASA’s James Webb Space Telescope team expects that Webb’s optical performance will be able to meet or exceed the science goals the observatory was built to achieve.
On March 11, the Webb team completed the stage of alignment known as “fine phasing.” At this key stage in the commissioning of Webb’s Optical Telescope Element, every optical parameter that has been checked and tested is performing at, or above, expectations. The team also found no critical issues and no measurable contamination or blockages to Webb’s optical path. The observatory is able to successfully gather light from distant objects and deliver it to its instruments without issue.
While the purpose of this image was to focus on the bright star at the center for alignment evaluation, Webb's optics and NIRCam are so sensitive that the galaxies and stars seen in the background show up.
At this stage of Webb’s mirror alignment, known as “fine phasing,” each of the primary mirror segments have been adjusted to produce one unified image of the same star using only the NIRCam instrument. This image of the star, which is called 2MASS J17554042+6551277, uses a red filter to optimize visual contrast.
Although there are months to go before Webb ultimately delivers its new view of the cosmos, achieving this milestone means the team is confident that Webb’s first-of-its-kind optical system is working as well as possible.
“More than 20 years ago, the Webb team set out to build the most powerful telescope that anyone has ever put in space and came up with an audacious optical design to meet demanding science goals,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington. “Today we can say that design is going to deliver.”
While some of the largest ground-based telescopes on Earth use segmented primary mirrors, Webb is the first telescope in space to use such a design. The 21-foot, 4-inch (6.5-meter) primary mirror – much too big to fit inside a rocket fairing – is made up of 18 hexagonal, beryllium mirror segments. It had to be folded up for launch and then unfolded in space before each mirror was adjusted – to within nanometers – to form a single mirror surface.
“In addition to enabling the incredible science that Webb will achieve, the teams that designed, built, tested, launched, and now operate this observatory have pioneered a new way to build space telescopes,” said Lee Feinberg, Webb optical telescope element manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
With the fine phasing stage of the telescope’s alignment complete, the team has now fully aligned Webb’s primary imager, the Near-Infrared Camera, to the observatory’s mirrors.
“We have fully aligned and focused the telescope on a star, and the performance is beating specifications. We are excited about what this means for science,” said Ritva Keski-Kuha, deputy optical telescope element manager for Webb at NASA Goddard. “We now know we have built the right telescope.”
This new “selfie” was created using a specialized pupil imaging lens inside of the NIRCam instrument that was designed to take images of the primary mirror segments instead of images of the sky. This configuration is not used during scientific operations and is used strictly for engineering and alignment purposes. In this image, all of Webb’s 18 primary mirror segments are shown collecting light from the same star in unison.
Over the next six weeks, the team will proceed through the remaining alignment steps before final science instrument preparations. The team will further align the telescope to include the Near-Infrared Spectrograph, Mid-Infrared Instrument, and Near InfraRed Imager and Slitless Spectrograph. In this phase of the process, an algorithm will evaluate the performance of each instrument and then calculate the final corrections needed to achieve a well-aligned telescope across all science instruments. Following this, Webb’s final alignment step will begin, and the team will adjust any small, residual positioning errors in the mirror segments.
The team is on track to conclude all aspects of Optical Telescope Element alignment by early May, if not sooner, before moving on to approximately two months of science instrument preparations. Webb’s first full-resolution imagery and science data will be released in the summer.
A+
Reste maintenant à faire l'alignement global
L'image de l'étoile qui sert de référence commence à ressembler à quelque chose de plus familier:
Pour plus de détails, le blog Nasa:
Mar 16, 2022
NASA’s Webb Reaches Alignment Milestone, Optics Working Successfully
Following the completion of critical mirror alignment steps, NASA’s James Webb Space Telescope team expects that Webb’s optical performance will be able to meet or exceed the science goals the observatory was built to achieve.
On March 11, the Webb team completed the stage of alignment known as “fine phasing.” At this key stage in the commissioning of Webb’s Optical Telescope Element, every optical parameter that has been checked and tested is performing at, or above, expectations. The team also found no critical issues and no measurable contamination or blockages to Webb’s optical path. The observatory is able to successfully gather light from distant objects and deliver it to its instruments without issue.
While the purpose of this image was to focus on the bright star at the center for alignment evaluation, Webb's optics and NIRCam are so sensitive that the galaxies and stars seen in the background show up.
At this stage of Webb’s mirror alignment, known as “fine phasing,” each of the primary mirror segments have been adjusted to produce one unified image of the same star using only the NIRCam instrument. This image of the star, which is called 2MASS J17554042+6551277, uses a red filter to optimize visual contrast.
Although there are months to go before Webb ultimately delivers its new view of the cosmos, achieving this milestone means the team is confident that Webb’s first-of-its-kind optical system is working as well as possible.
“More than 20 years ago, the Webb team set out to build the most powerful telescope that anyone has ever put in space and came up with an audacious optical design to meet demanding science goals,” said Thomas Zurbuchen, associate administrator for NASA’s Science Mission Directorate in Washington. “Today we can say that design is going to deliver.”
While some of the largest ground-based telescopes on Earth use segmented primary mirrors, Webb is the first telescope in space to use such a design. The 21-foot, 4-inch (6.5-meter) primary mirror – much too big to fit inside a rocket fairing – is made up of 18 hexagonal, beryllium mirror segments. It had to be folded up for launch and then unfolded in space before each mirror was adjusted – to within nanometers – to form a single mirror surface.
“In addition to enabling the incredible science that Webb will achieve, the teams that designed, built, tested, launched, and now operate this observatory have pioneered a new way to build space telescopes,” said Lee Feinberg, Webb optical telescope element manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.
With the fine phasing stage of the telescope’s alignment complete, the team has now fully aligned Webb’s primary imager, the Near-Infrared Camera, to the observatory’s mirrors.
“We have fully aligned and focused the telescope on a star, and the performance is beating specifications. We are excited about what this means for science,” said Ritva Keski-Kuha, deputy optical telescope element manager for Webb at NASA Goddard. “We now know we have built the right telescope.”
This new “selfie” was created using a specialized pupil imaging lens inside of the NIRCam instrument that was designed to take images of the primary mirror segments instead of images of the sky. This configuration is not used during scientific operations and is used strictly for engineering and alignment purposes. In this image, all of Webb’s 18 primary mirror segments are shown collecting light from the same star in unison.
Over the next six weeks, the team will proceed through the remaining alignment steps before final science instrument preparations. The team will further align the telescope to include the Near-Infrared Spectrograph, Mid-Infrared Instrument, and Near InfraRed Imager and Slitless Spectrograph. In this phase of the process, an algorithm will evaluate the performance of each instrument and then calculate the final corrections needed to achieve a well-aligned telescope across all science instruments. Following this, Webb’s final alignment step will begin, and the team will adjust any small, residual positioning errors in the mirror segments.
The team is on track to conclude all aspects of Optical Telescope Element alignment by early May, if not sooner, before moving on to approximately two months of science instrument preparations. Webb’s first full-resolution imagery and science data will be released in the summer.
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- Robert64
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La phase 6 qui constituera l'alignement final du télescope est maintenant commencée:
Telescope Alignment Over All Instrument Fields Of View
Step 6
Nominal Event Time: Starts - Launch + ~3 Months
Status: ONGOING
After Fine Phasing (step 5), the telescope will be well aligned at one place in the NIRCam field of view. Now we need to extend the alignment to the rest of the instruments.
In this phase of the commissioning process, we make measurements at multiple locations, or field points, across each of the science instruments, as shown below. More variation in intensity indicates larger errors at that field point. An algorithm calculates the final corrections needed to achieve a well-aligned telescope across all science instruments.
Source Nasa
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Telescope Alignment Over All Instrument Fields Of View
Step 6
Nominal Event Time: Starts - Launch + ~3 Months
Status: ONGOING
After Fine Phasing (step 5), the telescope will be well aligned at one place in the NIRCam field of view. Now we need to extend the alignment to the rest of the instruments.
In this phase of the commissioning process, we make measurements at multiple locations, or field points, across each of the science instruments, as shown below. More variation in intensity indicates larger errors at that field point. An algorithm calculates the final corrections needed to achieve a well-aligned telescope across all science instruments.
Source Nasa
A+
- Robert64
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Webb Begins Multi-Instrument Alignment
Alise Fisher
Posted onMarch 17, 2022
CategoriesJames Webb Space Telescope
After meeting the major milestone of aligning the telescope to NIRCam, the Webb team is starting to extend the telescope alignment to the guider (the Fine Guidance Sensor, or FGS) and the other three science instruments. This six-week-long process is called multi-instrument multi-field (MIMF) alignment.
When a ground-based telescope switches between cameras, sometimes the instrument is physically taken off the telescope, and a new one is installed during the daytime when the telescope is not in use. If the other instrument is already on the telescope, mechanisms are in place to move part of the telescope’s optics (known as a pick-off mirror) into the field of view.
On space telescopes like Webb, all the cameras see the sky at the same time; to switch a target from one camera to another, we repoint the telescope to put the target into the field of view of the other instrument.
After MIMF, Webb’s telescope will provide a good focus and sharp images in all the instruments. In addition, we need to precisely know the relative positions of all the fields of view. Over last weekend, we mapped the positions of the three near-infrared instruments relative to the guider and updated their positions in the software that we use to point the telescope. In another instrument milestone, FGS recently achieved “fine guide” mode for the first time, locking onto a guide star using its highest precision level. We have also been taking “dark” images, to measure the baseline detector response when no light reaches them – an important part of the instrument calibration.
Webb’s mid-infrared instrument, MIRI, will be the last instrument that is aligned, as it is still waiting for the cryogenic cooler to chill it to its final operating temperature, just under 7 degrees above absolute zero. Interspersed within the initial MIMF observations, the two stages of the cooler will be turned on to bring MIRI to its operating temperature. The final stages of MIMF will align the telescope for MIRI.
You might be wondering: If all of the instruments can see the sky at the same time, can we use them simultaneously? The answer is yes! With parallel science exposures, when we point one instrument at a target, we can read out another instrument at the same time. The parallel observations don’t see the same point in the sky, so they provide what is essentially a random sample of the universe. With a lot of parallel data, scientists can determine the statistical properties of the galaxies that are detected. In addition, for programs that want to map a large area, much of the parallel images will overlap, increasing the efficiency of the valuable Webb dataset.
By Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center
And Stefanie Milam, Webb deputy project scientist for planetary science, NASA Goddard
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Alise Fisher
Posted onMarch 17, 2022
CategoriesJames Webb Space Telescope
After meeting the major milestone of aligning the telescope to NIRCam, the Webb team is starting to extend the telescope alignment to the guider (the Fine Guidance Sensor, or FGS) and the other three science instruments. This six-week-long process is called multi-instrument multi-field (MIMF) alignment.
When a ground-based telescope switches between cameras, sometimes the instrument is physically taken off the telescope, and a new one is installed during the daytime when the telescope is not in use. If the other instrument is already on the telescope, mechanisms are in place to move part of the telescope’s optics (known as a pick-off mirror) into the field of view.
On space telescopes like Webb, all the cameras see the sky at the same time; to switch a target from one camera to another, we repoint the telescope to put the target into the field of view of the other instrument.
After MIMF, Webb’s telescope will provide a good focus and sharp images in all the instruments. In addition, we need to precisely know the relative positions of all the fields of view. Over last weekend, we mapped the positions of the three near-infrared instruments relative to the guider and updated their positions in the software that we use to point the telescope. In another instrument milestone, FGS recently achieved “fine guide” mode for the first time, locking onto a guide star using its highest precision level. We have also been taking “dark” images, to measure the baseline detector response when no light reaches them – an important part of the instrument calibration.
Webb’s mid-infrared instrument, MIRI, will be the last instrument that is aligned, as it is still waiting for the cryogenic cooler to chill it to its final operating temperature, just under 7 degrees above absolute zero. Interspersed within the initial MIMF observations, the two stages of the cooler will be turned on to bring MIRI to its operating temperature. The final stages of MIMF will align the telescope for MIRI.
You might be wondering: If all of the instruments can see the sky at the same time, can we use them simultaneously? The answer is yes! With parallel science exposures, when we point one instrument at a target, we can read out another instrument at the same time. The parallel observations don’t see the same point in the sky, so they provide what is essentially a random sample of the universe. With a lot of parallel data, scientists can determine the statistical properties of the galaxies that are detected. In addition, for programs that want to map a large area, much of the parallel images will overlap, increasing the efficiency of the valuable Webb dataset.
By Jonathan Gardner, Webb deputy senior project scientist, NASA’s Goddard Space Flight Center
And Stefanie Milam, Webb deputy project scientist for planetary science, NASA Goddard
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- Robert64
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Iterate Alignment For Final Correction
Step 7
Nominal Event Time: Starts - Launch + ~4 Months
Status: Future
After applying the Field of View correction, the key thing left to address is the removal of any small, residual positioning errors in the primary mirror segments. We measure and make corrections using the Fine Phasing process (step 5). We will do a final check of the image quality across each of the science instruments; once this is verified, the wavefront sensing and controls process will be complete.
As we go through the seven steps, we may find that we need to iterate earlier steps as well. The process is flexible and modular to allow for iteration. After roughly three months of aligning the telescope, we will be ready to proceed to commissioning the instruments.
Source : Nasa
A+
Step 7
Nominal Event Time: Starts - Launch + ~4 Months
Status: Future
After applying the Field of View correction, the key thing left to address is the removal of any small, residual positioning errors in the primary mirror segments. We measure and make corrections using the Fine Phasing process (step 5). We will do a final check of the image quality across each of the science instruments; once this is verified, the wavefront sensing and controls process will be complete.
As we go through the seven steps, we may find that we need to iterate earlier steps as well. The process is flexible and modular to allow for iteration. After roughly three months of aligning the telescope, we will be ready to proceed to commissioning the instruments.
Source : Nasa
A+
- Robert64
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Sur le JWST, la mise au point se poursuit:
Source: blog Nasa
Webb Continues Multi-Instrument Alignment
Thaddeus Cesari
Posted onMarch 24, 2022
CategoriesJames Webb Space Telescope
While telescope alignment continues, Webb’s Mid-Infrared Instrument (MIRI) is still in cooldown mode. MIRI, which will be the coldest of Webb’s four instruments, is the only instrument that will be actively cooled by a cryogenic refrigerator, or cryocooler. This cryocooler uses helium gas to carry heat from MIRI’s optics and detectors out to the warm side of the sunshield. To manage the cooldown process, MIRI also has heaters onboard, to protect its sensitive components from the risk of ice forming. The Webb team has begun progressively adjusting both the cryocooler and these heaters, to ensure a slow, controlled, stable cooldown for the instrument. Soon, the team will turn off MIRI’s heaters entirely, to bring the instrument down to its operating temperature of less than 7 kelvins (-447 degrees Fahrenheit, or -266 degrees Celsius).
In the meantime, after achieving alignment with the Near-Infrared Camera (NIRCam), Webb engineers have begun aligning the telescope to the remaining near-infrared instruments. For more about this six-week process, we hear today from Michael McElwain and Charles Bowers, members of the Webb team at NASA’s Goddard Space Flight Center:
“Webb’s alignment at the NIRCam field showed some spectacular diffraction-limited images, producing a tantalizing glimpse of the capabilities this observatory will carry for its science program. This was a major milestone because it required nearly all of the observatory systems to be functioning as designed. It all worked as well as we dared to hope, and it was certainly a moment to celebrate.
“The next step is to ensure the telescope is well-aligned to the instruments other than NIRCam, including the guider (the Fine Guidance Sensor, or FGS) and the other three science instruments: the Near-Infrared Slitless Spectrograph (NIRISS), Near-Infrared Spectrometer (NIRSpec), and MIRI. All the near-infrared instruments have already been passively cooled, are approaching their operational temperatures, and are participating in this next alignment stage. MIRI requires active cooling by a cryocooler, which is now underway, and it will be ready for alignment in a few weeks.
“This is the sixth stage of our telescope alignment plan, the Telescope Alignment Over Instrument Fields of View. Each of the instruments occupies a part of the telescope focal plane, just slightly offset with respect to each other. NIRCam was intentionally placed at the center of the telescope field where the telescope’s optical performance is best due to its demanding imaging performance requirements. Additionally, NIRCam was equipped with some specialized optical tools used to align the telescope. However, the initial alignment using only NIRCam could lead to an incorrect placement that compensates errors from primary-to-secondary mirror misalignments with the primary mirror itself. Small misalignments of this type will be evident in images in instruments farther from the center of the telescope field of view.
“The first step was to simply look at star fields as seen by NIRCam, NIRISS, FGS, and NIRSpec to see whether they were in focus. The stars looked nearly in-focus, which was a sign that the primary to secondary mirror alignment was already very good. A more accurate optical error measurement has been carried out at five to 10 field positions within each operational science instrument, using data taken with the secondary mirror positioned out of focus. This dataset provided a conclusive determination of the telescope alignment state.
“The Webb optics team analyzed the multi-instrument dataset and determined that only minor focus adjustments are needed on the secondary mirror and science instruments. Since the telescope is still cooling along with the MIRI instrument, we will not apply the corrections at this time and will defer them until the next round.
“When MIRI is available, an additional round of measurements will be conducted by each science instrument to determine the final state of the telescope alignment. We will iterate this process as needed to ensure the telescope performance is optimized for all of the instruments. After the telescope alignment to all instruments is complete, we will transition to the final two months of commissioning, where we will carry out optical stability tests and measure the science instrument performance before embarking on the Cycle 1 science program.”
–Michael McElwain, Webb observatory project scientist, NASA Goddard
–Charles Bowers, Webb deputy observatory project scientist, NASA Goddard
A+
Source: blog Nasa
Webb Continues Multi-Instrument Alignment
Thaddeus Cesari
Posted onMarch 24, 2022
CategoriesJames Webb Space Telescope
While telescope alignment continues, Webb’s Mid-Infrared Instrument (MIRI) is still in cooldown mode. MIRI, which will be the coldest of Webb’s four instruments, is the only instrument that will be actively cooled by a cryogenic refrigerator, or cryocooler. This cryocooler uses helium gas to carry heat from MIRI’s optics and detectors out to the warm side of the sunshield. To manage the cooldown process, MIRI also has heaters onboard, to protect its sensitive components from the risk of ice forming. The Webb team has begun progressively adjusting both the cryocooler and these heaters, to ensure a slow, controlled, stable cooldown for the instrument. Soon, the team will turn off MIRI’s heaters entirely, to bring the instrument down to its operating temperature of less than 7 kelvins (-447 degrees Fahrenheit, or -266 degrees Celsius).
In the meantime, after achieving alignment with the Near-Infrared Camera (NIRCam), Webb engineers have begun aligning the telescope to the remaining near-infrared instruments. For more about this six-week process, we hear today from Michael McElwain and Charles Bowers, members of the Webb team at NASA’s Goddard Space Flight Center:
“Webb’s alignment at the NIRCam field showed some spectacular diffraction-limited images, producing a tantalizing glimpse of the capabilities this observatory will carry for its science program. This was a major milestone because it required nearly all of the observatory systems to be functioning as designed. It all worked as well as we dared to hope, and it was certainly a moment to celebrate.
“The next step is to ensure the telescope is well-aligned to the instruments other than NIRCam, including the guider (the Fine Guidance Sensor, or FGS) and the other three science instruments: the Near-Infrared Slitless Spectrograph (NIRISS), Near-Infrared Spectrometer (NIRSpec), and MIRI. All the near-infrared instruments have already been passively cooled, are approaching their operational temperatures, and are participating in this next alignment stage. MIRI requires active cooling by a cryocooler, which is now underway, and it will be ready for alignment in a few weeks.
“This is the sixth stage of our telescope alignment plan, the Telescope Alignment Over Instrument Fields of View. Each of the instruments occupies a part of the telescope focal plane, just slightly offset with respect to each other. NIRCam was intentionally placed at the center of the telescope field where the telescope’s optical performance is best due to its demanding imaging performance requirements. Additionally, NIRCam was equipped with some specialized optical tools used to align the telescope. However, the initial alignment using only NIRCam could lead to an incorrect placement that compensates errors from primary-to-secondary mirror misalignments with the primary mirror itself. Small misalignments of this type will be evident in images in instruments farther from the center of the telescope field of view.
“The first step was to simply look at star fields as seen by NIRCam, NIRISS, FGS, and NIRSpec to see whether they were in focus. The stars looked nearly in-focus, which was a sign that the primary to secondary mirror alignment was already very good. A more accurate optical error measurement has been carried out at five to 10 field positions within each operational science instrument, using data taken with the secondary mirror positioned out of focus. This dataset provided a conclusive determination of the telescope alignment state.
“The Webb optics team analyzed the multi-instrument dataset and determined that only minor focus adjustments are needed on the secondary mirror and science instruments. Since the telescope is still cooling along with the MIRI instrument, we will not apply the corrections at this time and will defer them until the next round.
“When MIRI is available, an additional round of measurements will be conducted by each science instrument to determine the final state of the telescope alignment. We will iterate this process as needed to ensure the telescope performance is optimized for all of the instruments. After the telescope alignment to all instruments is complete, we will transition to the final two months of commissioning, where we will carry out optical stability tests and measure the science instrument performance before embarking on the Cycle 1 science program.”
–Michael McElwain, Webb observatory project scientist, NASA Goddard
–Charles Bowers, Webb deputy observatory project scientist, NASA Goddard
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- Robert64
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tiens pour ceux que ça interesse,
ce soir à 21h00 sur la 5 un beau reportage sur l'Univers dans sciences grand format![:bravo: :bravo:](https://www.homecinema-fr.com/forum/images/smilies/thumbs_up.gif)
ce soir à 21h00 sur la 5 un beau reportage sur l'Univers dans sciences grand format
![:bravo: :bravo:](https://www.homecinema-fr.com/forum/images/smilies/thumbs_up.gif)
La configuration dans mon profil
si tu ne crois pas au son des câbles, essaies sans...
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mariofan de triangle - Membre HCFR
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à bah c'est ballot, j'ai pas pu regarder parce que foot...
je vais le voir en replay![:D](https://www.homecinema-fr.com/forum/images/smilies/icon_biggrin.gif)
je vais le voir en replay
![:D](https://www.homecinema-fr.com/forum/images/smilies/icon_biggrin.gif)
La configuration dans mon profil
si tu ne crois pas au son des câbles, essaies sans...
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mariofan de triangle - Membre HCFR
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J'en ai vu une bonne partie, intéressant !
En gros ils disaient qu'on allait tout doucement vers l'ère de la fin des étoiles et qu'il ne resterait qu'un univers complètement sombre et sans vie
Bon, on a le temps, c'est pour dans quelques milliards d'années.
En gros ils disaient qu'on allait tout doucement vers l'ère de la fin des étoiles et qu'il ne resterait qu'un univers complètement sombre et sans vie
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Bon, on a le temps, c'est pour dans quelques milliards d'années.
- arnuche
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arnuche a écrit:J'en ai vu une bonne partie, intéressant !
En gros ils disaient qu'on allait tout doucement vers l'ère de la fin des étoiles et qu'il ne resterait qu'un univers complètement sombre et sans vie![]()
Bon, on a le temps, c'est pour dans quelques milliards d'années.
bon, ça me rassure
![:mdr:](https://www.homecinema-fr.com/forum/images/smilies/icon_mrgreen.gif)
La configuration dans mon profil
si tu ne crois pas au son des câbles, essaies sans...
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mariofan de triangle - Membre HCFR
- Messages: 9614
- Inscription Forum: 07 Juil 2005 15:13
- Localisation: Chez moi, sauf quand je suis dehors
arnuche a écrit:J'en ai vu une bonne partie, intéressant !
En gros ils disaient qu'on allait tout doucement vers l'ère de la fin des étoiles et qu'il ne resterait qu'un univers complètement sombre et sans vie![]()
Bon, on a le temps, c'est pour dans quelques milliards d'années.
On parle de 10000 milliards d'années
![:mdr:](https://www.homecinema-fr.com/forum/images/smilies/icon_mrgreen.gif)
La configuration dans mon profil
Ceux qui ne savent rien en savent toujours autant que ceux qui n'en savent pas plus qu'eux
Abonnez vous à votre groupe régional: "Résidents de ............."
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Thierry.P - Modération Rencontres
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On en a déjà causé là: (post du 10 janvier 2021 à 11h 25 pour avoir les images)
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Robert64 a écrit:Un petit parfum d’éternité
(c’est long, surtout vers la fin)![]()
(D’après un dossier Science & Avenir/La Recherche réalisé par Azar Khalatbari et Fabrice Nicot avec Marine Benoit et Franck Daninos)
A propos de l’Univers, on parle beaucoup de son hypothétique début mais beaucoup moins de sa fin.
Des astrophysiciens, en partant des lois physiques connues, ont essayé d’imaginer ce que pourrait être la mort de notre petit chez nous.
Selon les hypothèses prises, on arrive à 5 scénarios différents, certains plus probables que d’autres, mais qui ont en commun de mettre en jeu des durées absolument impossibles à concevoir pour un esprit humain.
LE GRAND GEL OU LE GRAND ECRASEMENT ?
L'Univers prendra fin dans 101100 années par un gigantesque feu d'artifice !
C'est l'hypothèse du « Big Freeze » décrit par un jeune théoricien américain.
Avec le « Big Crunch », le « Big Slurp » , le Big Bunce et « Big Rip », les scénarios de fin des temps sont variés et ont tous l'intérêt de pousser la cosmologie dans ses derniers retranchements.
UN GIGANTESQUE FEU D'ARTIFICE! Voilà à quoi pourrait ressembler la fin de l'Univers, marquée par une explosion fantastique d'étoiles, inconnues jusqu'alors, appelées naines noires. Une succession de formidables jets de matières avant que tout ne s'éteigne et ne se fige à jamais. Ce scénario de la fin des temps, personne n'avait encore osé l'écrire ainsi, sinon le théoricien Matt Caplan, de l'univerité d'État de l'Illinois (États-Unis), qui vient de publier ses travaux dans la revue britannique MNRAS (Comptes rendus mensuels de la société royale d'astronomie). Pour ce faire, le scientifique a dégainé un arsenal théorique en accord avec les lois de la physique, mais riche en hypothèses décoiffantes, pour décrire le dernier événement astrophysique de l'Univers. Mais nul humain n'assistera à cette fête monstrueuse! Car celle-ci surviendra lorsque l'Univers sera âgé de 101100 années, soit un temps hors de toute imagination, proche de l'éternité!
En regard, les 13,8.milliards d'années (soit 13,8. 109 années) de son âge actuel sont moins qu'un battement de cil.
Les travaux originaux de Matt Caplan s'inscrivent dans une longue recherche sur le futur de l'Univers. Jusqu'au début des années 2000, les réflexions dans ce domaine étaient de deux types : soit le cosmos connaît un Big Crunch (un grand effondrement), soit il se fige dans un Big Freeze (un grand gel) (voir schémas ci-dessus). Dans le premier scénario, après une période d'expansion, l'Univers s'effondre sur lui-même, pour atteindre un état extrêmement chaud et dense, similaire à celui qui avait cours aux toutes premières fractions de seconde de son existence. Dans le second, il connaît au contraire une expansion éternelle et un lent refroidissement. Deux hypothèses rendues possibles grâce à la cosmologie relativiste, celle née à la suite de la théorie d'Einstein au début du xxe siècle. Car l'un des résultats les plus importants sur lequel se fonde cette discipline est que l'Univers est en expansion : l'espace se dilate, emporté par l'énergie héritée du Big Bang. Mais la matière, qui agit par la force gravitationnelle, s'y oppose. Et le résultat de ce bras de fer dépend, selon les théoriciens, de la densité moyenne de matière dans l'espace. Au-delà d'une certaine valeur appelée densité critique, estimée à quelques atomes par mètre cube d'espace, la gravité devrait l'emporter sur l'expansion : c'est le Big Crunch; en deçà, la gravité est vaincue par l'expansion devenant éternelle : c'est le Big Freeze.
Mais coup de théâtre en 1998! Cette année-là, les astrophysiciens prouvent que l'expansion va en s'accélérant, signe qu'un ingrédient inconnu jusque-là domine le contenu d l'Univers. Parmi les suspects, une énigmatique énergie noire. Dès lors, le scénario du Big crunch s’effondre au profit du Big Freeze, devenu le futur le plus probable de l’Univers.
LE BIG SLURP (LA GRANDE ABSORPTION )
L'avantage est que nous, humains, ne sentirons rien à l'instant fatal. L'inconvénient, c'est qu'il peut se produire... n'importe quand ! « Slurp », c'est le bruit que l'on fait en avalant une cuillerée de soupe. Et il s'agit bien de cela. Dans ce drôle de scénario, notre région d'univers serait engloutie par une autre, à la vitesse de la lumière.
Pour comprendre, il faut revenir 10-35 seconde après le Big Bang. L'Univers à peine né connaît alors une inflation formidable, durant laquelle sa taille est multipliée de l'ordre de 1029 fois.
L'origine de cette croissance exponentielle est toujours discutée, mais l'une des hypothèses repose sur l'existence d'un champ scalaire, dont on ignore encore la nature physique concrète, mais qu'il faut voir comme une source d'énergie baignant l'Univers primordial. Il exerce sur son environnement une pression négative, contrairement à la matière ordinaire dont la pression est toujours positive. Si les chercheurs injectent ce champ dans les équations de la relativité générale, qui relie la géométrie de l'espace-temps à la matière et à l'énergie qu'il renferme, ils montrent alors qu'il provoque bel et bien une expansion accélérée de l'Univers.
« Mais cette inflation est de très courte durée. En effet, pour que la pression soit négative, il faut des conditions énergétiques particulières, avec une énergie potentielle du champ supérieure à son énergie cinétique. Avec l'inflation, l'énergie potentielle diminue puis lorsque cette dernière équivaut à l'énergie cinétique, l'inflation cesse : une partie de l'énergie du champ scalaire se convertit alors en matière. C'est ce qui s'est passé pour notre univers.
Toutefois, ces mêmes modèles n'excluent pas qu'en certains endroits, cette égalité ne soit jamais atteinte. Dans ce cas, l'inflation s'y poursuivrait éternellement. »
Résultat : un « Big Slurp », qui correspondrait à la collision entre une de ces parties de l'Univers en pleine inflation et la nôtre. « L'ennui, c'est que cette région de l'Univers n'aurait sans doute pas les mêmes lois physiques que la nôtre. Les constantes fondamentales de la physique dépendent de la valeur du champ scalaire dans la bulle d'inflation. Or, ce champ étant fluctuant, rien ne dit que sa valeur serait identique à celle qui prévalait lors de notre inflation ». De cette rencontre entre deux portions d'univers aux lois physiques différentes ne sortira qu'une seule gagnante... « La région la plus stable devrait "avaler" l'autre à la vitesse de la lumière ».
Cette théorie a pris une importance particulière avec la découverte du Boson de Higgs en 2012. Cette particule, responsable de la masse de toutes les autres, est justement la manifestation d'un champ scalaire, le tout premier découvert. Est-ce celui à l'origine de l'inflation? Pour l'instant, ce candidat tient la corde. Ce qui pourrait avoir quelques inconvénients fatidiques... En effet, à partir de la masse du boson de Higgs, il est possible de calculer le niveau d'énergie fondamental du champ scalaire associé. Les calculs ont montré que le champ de Higgs peut se trouver dans deux énergies fondamentales, l'une basse dans laquelle il se trouve aujourd'hui, et une autre bien plus élevée. Or, il pourrait passer spontanément d'un état à l'autre... Ce qui entraînerait un changement brutal de la masse de toutes les particules. Et finalement, un bouleversement de la physique telle que nous la connaissons. « Le "slurp" viendrait "de l'intérieur" en quelque sorte, et pas d'une collision extérieure. L'anéantissement serait tout aussi rapide.
LE BIG RIP (LA GRANDE DECHIRURE )
Comme son nom l'indique, « la Grande Déchirure » n'augure rien de bon. Une sorte de gigantesque écartèlement ! Dans un premier temps, les étoiles s'éloigneront de plus en plus rapidement les unes des autres, entraînant le délitement des galaxies. Puis ce sera au tour des planètes de quitter leur orbite. Enfin, tous les corps se disloqueront, jusqu'aux atomes eux-mêmes. » Quel programme !
En fait, ce serait l'emballement d'un phénomène déjà à l'oeuvre : l'accélération de l'expansion de l'Univers, mise en évidence en 1998 par Saul Permutter et Adam Riess, prix Nobel de physique en 2011. Depuis, différents modèles tentent d'expliquer cette accélération. L'un d'eux, imaginé par le cosmologiste américain Robert Caldwell et publié en 2003, envisage l'existence d'une énergie « fantôme » baignant l'Univers tout entier et dont la pression négative dilaterait celui-ci, agissant comme une force répulsive.
Jusqu'à présent, cela ressemble à l'énergie noire, très en vogue pour expliquer les observations de 1998. Mais dans ce cas particulier, la densité d'énergie augmenterait au fil du temps. Elle deviendrait tellement forte qu'elle disloquerait jusqu'aux quarks constituant les protons et les neutrons. L'Univers finirait éparpillé « façon puzzle ».
Est-ce probable? La clé pour le savoir se niche dans les caractéristiques de l'accélération actuelle de l'expansion de l'Univers. Et notamment la constante de Hubble, qui détermine le taux d'expansion. Sa valeur fait aujourd'hui l'objet de plusieurs estimations, sans qu'il y ait consensus. Toutefois, certaines d'entre elles, les plus élevées, ne sont pas incompatibles avec le Big Rip. « En fait, les propriétés de l'expansion accélérée que l'on connaît sont à la limite entre une densité d'énergie diminuant avec le temps, et une densité augmentant. On privilégie a priori la première option, mais aujourd'hui rien ne permet vraiment de trancher. » La Grande Déchirure n'est peut-être pas une fatalité, mais elle reste une option... Dans quelques dizaines de milliards d'années, heureusement!
LE BIG BOUNCE (LE GRAND REBOND)
Dans l'hypothèse du « Grand Rebond », l'Univers, après une phase d'expansion, ferait machine arrière !
Mais contrairement au Big Crunch , c'est un univers tout neuf qui jaillirait de ce nouveau Big Bang! Pour l'instant, l'expansion s'accélérant, notre univers n'en prend pas le chemin. Pour que cette dernière s'inverse, il faudrait en effet que la force qui dilate l'Univers change radicalement de nature, et les chercheurs en savent encore trop peu sur elle pour spéculer.
Il n'empêche que la théorie du rebond est née d'une démarche intéressante : décrire ce qui s'est passé au moment du Big Bang. En effet, la seule relativité générale décrit un Univers infiniment petit, doté d'une densité d'énergie infiniment grande, ce qui est impossible. Pour décrire correctement cet instant si particulier du Big Bang, les chercheurs doivent alors recourir à la physique quantique. Et les tentatives sont nombreuses. Parmi elles, la théorie de la gravitation quantique à boucles, développée par l'Américain Lee Smolin et l'Italien Carlo Rovelli notamment, postule un espace-temps quantifié, le quantum d'espace ne pouvant descendre en dessous de I0-35 mètre. L'Univers à ses débuts était donc très étroit, mais pas infiniment petit. En suivant ce modèle jusqu'au bout, les chercheurs ont établi que... le Big Bang disparaît tout à fait ! Dans ce scénario, l'Univers serait issu du stade final d'un précédent en contraction. Dès lors, pourquoi notre Univers, à son tour, ne serait-il pas l'ancêtre d'un prochain cosmos?.
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- Robert64
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arnuche a écrit:...
En gros ils disaient qu'on allait tout doucement vers l'ère de la fin des étoiles et qu'il ne resterait qu'un univers complètement sombre et sans vie![]()
.....
et à -273,15°C !
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- Robert64
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