# Can ground based telescopes use a starshade in space?

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1) Could a starshade in orbit be used by ground based telescopes? If so, what kind of orbit should it have?

2) Could the same starshade be used by two different space telescopes, or does the positioning make it impractical?

If the starshade was to stay in place over the ground telescope, it would have to be in a geosynchronous orbit, which puts it almost 36,000 km from Earth's surface. To cover an angular distance of 1 arcsecond (1/3600 of a degree), it would have a size of around 10km. Certainly not impossible, but technologically impractical at the moment. Also, the starshade would only shade a small region of the sky, so it wouldn't be useful from other observing sites.

As for the space telescopes using a starshade in geosynchronous orbit; that would be impossible, at least for scopes orbiting Earth, like Hubble is. The starshade is orbiting Earth at a different distance, and therefore speed, than the telescopes. As far as I can see, it would be near impossible to keep them aligned for anything more than a few minutes. I haven't done any number-crunching on this, though.

## Ground Telescopes

The moon setting behind the Very Large Telescope in Chile
Credit:G.Gillet/ESO

Most of the telescopes used by astronomers are known as ground-based, this means that they are located here on Earth at some of the best observing sites in the world.

It is obviously easier to have a telescope here on Earth as it can be visited much more frequently and is much easier to fix if things go wrong, it is also substantially cheaper to build. However, there are down sides as well. When a telescope is placed on the ground it has to look up through the Earth's atmosphere to see into space, and the atmosphere can blur our images.

The atmosphere also blocks out light from certain parts of the electromagnetic spectrum, this means we cannot see this light from the ground and instead need to put a telescope into space to get this information.

The gaps in our atmosphere where light is able to get through to the ground are called atmospheric windows and they can be seen below where the opacity, or how much light is let through, is 0% (transparent to the light), or a low number (translucent to light). Where the opacity is 100% no light of that wavelength can get through, or it is opaque to that light.

The opacity of the Earth's atmosphere to light of different wavelengths.
Credit: NASA

## Flower power: NASA starshade petal prototype

Astronomers have been indirectly detecting exoplanets for more than 15 years, but actually taking a picture of one has proven an immensely difficult task. Picking out the dim light of a planet from a star billions of times brighter is akin to finding a needle in a cosmic haystack, especially when the planet in question is a small, rocky world similar to Earth. In order to achieve this feat, researchers are developing techniques to block out the starlight while preserving the light emitted by the planet. This is called starlight suppression.

It&rsquos a task that NASA&rsquos flower-shaped starshade is designed to make easier. Working in conjunction with a space-based telescope, the starshade is able to position itself precisely between the telescope and the star that&rsquos being observed, and can block the starlight before it even reaches the telescope&rsquos mirrors..

With the starlight suppressed, light coming from exoplanets orbiting the star would be visible. Using this technology, astronomers would be able to take actual pictures of exoplanets &ndash images that could provide clues as to whether such worlds could support life as we know it.

The flower-shaped petals are part of what makes the starshade so effective. &ldquoThe shape of the petals, when seen from far away, creates a softer edge that causes less bending of light waves,&rdquo said Dr. Stuart Shaklan, JPL&rsquos lead engineer on the starshade project. &ldquoLess light bending means that the starshade shadow is very dark, so the telescope can take images of the planets without being overwhelmed by starlight .&rdquo

The starshade is also unique in that, unlike most space-based instruments, it&rsquos one part of a two-spacecraft observation system. &ldquoWe can use a pre-existing space telescope to take the pictures,&rdquo explains Shaklan. &ldquoThe starshade has thrusters that will allow it to move around in order to block the light from different stars.&rdquo

This process presents a number of engineering challenges that Shaklan and his team are working hard to unravel, from positioning the starshade precisely in space, to ensuring that it can be deployed accurately. &ldquoOur current task is figuring out how to unfurl the starshade in space so that all the petals end up in the right place, with millimeter accuracy,&rdquo said Professor Jeremy Kasdin, a Princeton researcher who is the Principal Investigator of the starshade project. Kasdin&rsquos group will create a smaller scale starshade at Princeton to verify that the design blocks the light as predicted by the computer simulations. Concurrently, the JPL team will test the deployment of a near-full scale starshade system in the lab to measure its accuracy.

Despite these challenges, the starshade approach could offer planet-hunters many advantages. &ldquoOne of the starshade&rsquos strengths is simplicity,&rdquo said Kasdin. &ldquoLight from the star never reaches the telescope because it&rsquos blocked by the starshade, which allows the telescope system to be simpler.&rdquo Another advantage of the starshade approach is that it can be used with a multi-purpose space telescope designed to make observations that could be useful to astronomers working in fields other than exoplanets.

NASA&rsquos starshade engineers are optimistic that refining their technology could be the key to major exoplanet discoveries in the future. &ldquoA starshade mission would allow us to directly image Earth-size, rocky exoplanets, which is something we can&rsquot do from the ground,&rdquo says Kasdin. &ldquoWe&rsquoll be able to show people a picture of a dot and explain that that&rsquos another Earth.&rdquo

## Starshade Would Take Formation Flying to Extremes

To hunt for exoplanets, the flowerlike shade would need to stay aligned with a space telescope over vast distances. Recent work demonstrates how that's possible.

Anyone who's ever seen aircraft engaged in formation flying can appreciate the feat of staying highly synchronized while airborne. In work sponsored by NASA's Exoplanet Exploration Program (ExEP), engineers at the Jet Propulsion Laboratory in Pasadena, California, are taking formation flying to a new extreme.

Their work marks an important milestone within a larger program to test the feasibility of a technology called a starshade. Although starshades have never flown in space, they hold the potential to enable groundbreaking observations of planets beyond our solar system, including pictures of planets as small as Earth.

This artist's concept of a starshade shows how the technology can block starlight and reveal the presence of planets. The video also shows the unfurling of a starshade model built by NASA's Jet Propulsion Laboratory, in an Astro Aerospace/Northroup Grumman facility in Santa Barbara in 2013. Credit: NASA/JPL-Caltech

A future starshade mission would involve two spacecraft. One would be a space telescope on the hunt for planets orbiting stars outside of our solar system. The other spacecraft would fly some 25,000 miles (40,000 kilometers) in front of it, carrying a large, flat shade. The shade would unfurl like a blooming flower - complete with "petals" - and block the light from a star, allowing the telescope to get a clearer glimpse of any orbiting planets. But it would work only if the two spacecraft were to stay, despite the great distance between them, aligned to within 3 feet (1 meter) of each other. Any more, and starlight would leak around the starshade into the telescope's view and overwhelm faint exoplanets.

"The distances we're talking about for the starshade technology are kind of hard to imagine," said JPL engineer Michael Bottom. "If the starshade were scaled down to the size of a drink coaster, the telescope would be the size of a pencil eraser and theyɽ be separated by about 60 miles [100 kilometers]. Now imagine those two objects are free-floating in space. They're both experiencing these little tugs and nudges from gravity and other forces, and over that distance we're trying to keep them both precisely aligned to within about 2 millimeters."

Researchers have found thousands of exoplanets without the use of a starshade, but in most instances scientists have discovered these worlds indirectly. The transit method, for example, detects the presence of a planet as it passes in front of its parent star and causes a temporary drop in the star's brightness. Only in relatively few cases have scientists taken direct images of exoplanets.

Blocking out starlight is key to performing more direct imaging and, eventually, to carrying out in-depth studies of planetary atmospheres or finding hints about the surface features of rocky worlds. Such studies have the potential to reveal signs of life beyond Earth for the first time.

The idea of using a space-based starshade to study exoplanets was initially proposed in the 1960s, four decades before the discovery of the first exoplanets. And while the ability to point a single spacecraft steadily at a distant object is not new, either, keeping two spacecraft aligned with each other toward a background object represents a different kind of challenge.

Researchers working on ExEP's Starshade Technology Development, known as S5, have been tasked by NASA with developing starshade technology for possible future space telescope missions. The S5 team is addressing three technology gaps that would need to be closed before a starshade mission could be ready to go to space.

The work done by Bottom and fellow JPL engineer Thibault Flinois closes one of those gaps by confirming that engineers could realistically produce a starshade mission that met these stringent "formation sensing and control" requirements. Their results are described in the S5 Milestone 4 report, available on the ExEP website.

The specifics of a particular starshade mission - including the exact distance between the two spacecraft and the size of the shade - would depend on the size of the telescope. The S5 Milestone 4 report looked primarily at a separation range of between 12,500 to 25,000 miles (20,000 to 40,000 kilometers), with a shade 85 feet (26 meters) in diameter. These parameters would work for a mission the size of NASA's Wide Field Infrared Survey Telescope (WFIRST), a telescope with a 2.4-meter-diameter primary mirror set to launch in the mid-2020s.

WFIRST will carry a different starlight-blocking technology, called a coronagraph, that sits inside the telescope and offers its own unique strengths in the study of exoplanets. This technology demonstration will be the first high-contrast stellar coronagraph to go into space, enabling WFIRST to directly image giant exoplanets similar to Neptune and Jupiter.

Starshade and coronagraph technologies work separately, but Bottom tested a technique by which WFIRST could detect when a hypothetical starshade drifted subtly out of alignment. A small amount of starlight would inevitably bend around the starshade and form a light-and-dark pattern on the front of the telescope. The telescope would see the pattern by using a pupil camera, which can image the front of the telescope from inside - akin to photographing a windshield from inside a car.

Previous starshade investigations had considered this approach, but Bottom made it a reality by building a computer program that could recognize when the light-and-dark pattern was centered on the telescope and when it had drifted off-center. Bottom found that the technique works extremely well as a way to detect the starshade's movement.

"We can sense a change in the position of the starshade down to an inch, even over these huge distances," Bottom said.

But detecting when the starshade is out of alignment is an entirely different proposition from actually keeping it aligned. To that end, Flinois and his colleagues developed a set of algorithms that use information provided by Bottom's program to determine when the starshade thrusters should fire to nudge it back into position. The algorithms were created to autonomously keep the starshade aligned with the telescope for days at a time.

Combined with Bottom's work, this shows that keeping the two spacecraft aligned is feasible using automated sensors and thruster controls. In fact, the work by Bottom and Flinois demonstrates that engineers could reasonably meet the alignment demands of an even larger starshade (in conjunction with a larger telescope), positioned up to 46,000 miles (74,000 kilometers) from the telescope.

"With such an unusually large range of scales at play here, it can be very counterintuitive that this would be possible at first glance," Flinois said.

A starshade project has not yet been approved for flight, but one could potentially join WFIRST in space in the late 2020s. Meeting the formation-flying requirement is just one step toward demonstrating that the project is feasible.

"This to me is a fine example of how space technology becomes ever more extraordinary by building upon its prior successes," said Phil Willems, manager of NASA's Starshade Technology Development activity. "We use formation flying in space every time a capsule docks at the International Space Station. But Michael and Thibault have gone far beyond that, and shown a way to maintain formation over scales larger than Earth itself."

## Contents

Astronomical observatories are mainly divided into four categories: space-based, airborne, ground-based, and underground-based.

### Ground-based observatories Edit

Ground-based observatories, located on the surface of Earth, are used to make observations in the radio and visible light portions of the electromagnetic spectrum. Most optical telescopes are housed within a dome or similar structure, to protect the delicate instruments from the elements. Telescope domes have a slit or other opening in the roof that can be opened during observing, and closed when the telescope is not in use. In most cases, the entire upper portion of the telescope dome can be rotated to allow the instrument to observe different sections of the night sky. Radio telescopes usually do not have domes.

For optical telescopes, most ground-based observatories are located far from major centers of population, to avoid the effects of light pollution. The ideal locations for modern observatories are sites that have dark skies, a large percentage of clear nights per year, dry air, and are at high elevations. At high elevations, the Earth's atmosphere is thinner, thereby minimizing the effects of atmospheric turbulence and resulting in better astronomical "seeing". [2] Sites that meet the above criteria for modern observatories include the southwestern United States, Hawaii, Canary Islands, the Andes, and high mountains in Mexico such as Sierra Negra. [3] Major optical observatories include Mauna Kea Observatory and Kitt Peak National Observatory in the US, Roque de los Muchachos Observatory in Spain, and Paranal Observatory and Cerro Tololo Inter-American Observatory in Chile.

Specific research study performed in 2009 shows that the best possible location for ground-based observatory on Earth is Ridge A — a place in the central part of Eastern Antarctica. [4] This location provides the least atmospheric disturbances and best visibility.

Beginning in 1930s, radio telescopes have been built for use in the field of radio astronomy to observe the Universe in the radio portion of the electromagnetic spectrum. Such an instrument, or collection of instruments, with supporting facilities such as control centres, visitor housing, data reduction centers, and/or maintenance facilities are called radio observatories. Radio observatories are similarly located far from major population centers to avoid electromagnetic interference (EMI) from radio, TV, radar, and other EMI emitting devices, but unlike optical observatories, radio observatories can be placed in valleys for further EMI shielding. Some of the world's major radio observatories include the Very Large Array in New Mexico, United States, Jodrell Bank in the UK, Arecibo in Puerto Rico, Parkes in New South Wales, Australia, and Chajnantor in Chile.

#### Highest astronomical observatories Edit

Since the mid-20th century, a number of astronomical observatories have been constructed at very high altitudes, above 4,000–5,000 m (13,000–16,000 ft). The largest and most notable of these is the Mauna Kea Observatory, located near the summit of a 4,205 m (13,796 ft) volcano in Hawaiʻi. The Chacaltaya Astrophysical Observatory in Bolivia, at 5,230 m (17,160 ft), was the world's highest permanent astronomical observatory [5] from the time of its construction during the 1940s until 2009. It has now been surpassed by the new University of Tokyo Atacama Observatory, [6] an optical-infrared telescope on a remote 5,640 m (18,500 ft) mountaintop in the Atacama Desert of Chile.

## Conclusion

Development of this efficient algorithm allows for more accurate estimates of completeness using the starshade. It is able to compute a completeness estimate for a user defined set of astrophysical parameters. Its significance lies in that for the first time, we used simulated images instead of averaged values for variables. By mirroring the process of an astronomer, our algorithm generates a more realistic estimation of completeness. With improvements, this algorithm may be very useful for future JPL starshade mission planning, to calculate the probability of a successful mission.

## Seeing the Future with Imaging Science: Interdisciplinary Research Team Summaries (2011)

### CHALLENGE SUMMARY

The world in which we live is the only planet we know that harbors life. Is our planet unique? We have not yet found life on Mars, despite ample evidence of the existence of water, nor have we found evidence of life anywhere else in our solar system. A tantalizing possibility is that life may yet exist under the ice of the moons of Jupiter&mdashyet there is no proof. Is there life elsewhere in the universe? If we were able to image planetary systems around neighboring stars, and in addition, characterize the surfaces and atmospheres of constituent planets, we would be one step closer to answering this question.

To date, more than 400 planets have been detected around other stars through a combination of radial-velocity techniques, transit experiments, and microlensing. Low-resolution spectra of a number of planets have also been found using the Hubble Space Telescope, the Spitzer Space Telescope, and a few ground-based observatories in these cases, the planets have been objects unlike anything in our solar system, being mostly Jupiter-like planets in Mercury-like orbits. Images of several planetary systems have also been collected from the ground and space these have shown planets in orbits much wider than even the bounds of our solar system and with planetary companions of extreme size, 3&ndash20 times Jupiter&rsquos mass.

Planetary systems like our own around other stars are too small to be imaged by conventional telescopes. If we wanted to search around the nearest 150 stars, we would need a telescope with an angular resolution better

20 mas this would allow us to distinguish objects such as Earth and Venus in solar system analogues at a distance of 15 pc from Earth. Our turbulent atmosphere limits ground-based telescopes to resolutions no better than 50 mas&mdasheven with the best available adaptive optics. Furthermore, the Hubble Space Telescope, with its 2.4 m mirror also has a resolving power no better than 50 mas. New advanced space telescopes are needed to image planetary systems similar to our own.

Beyond angular resolution limitations, a more difficult challenge is that planets are extremely faint as compared to the stars around which they orbit. An Earth-like planet would be about 10 billion times fainter than a Sun-like star when viewed at optical wavelengths, albeit somewhat brighter at infrared wavelengths&mdashthen only a factor of 10 million fainter. Because of this, scattered starlight within a telescope, caused by what would otherwise be negligible imperfections in mirror surfaces, can completely overwhelm the light from a planet. Telescopes must be significantly oversized compared to the required diffraction limited resolution so that planets could be seen beyond the glare of scattered starlight. Space telescopes with diameters of 8 m or more are needed to look for terrestrial planets around just the nearest dozen or so stars.

Building an 8-m optical space telescope is a formidable technical and engineering challenge. The largest telescopes on Earth are only slightly larger namely the twin 10-m telescopes of the W. M. Keck Observatory. The largest telescope that can fit easily inside a launch vehicle is much smaller: only about 3.5 m in diameter. Innovative approaches to telescope design and packaging are therefore needed. In addition the telescope must have optics capable of suppressing starlight by a factor of 10 million to 10 billion&mdashwhich is yet beyond the state of the art. Although this approach is certainly feasible with sufficient investment, it would provide images of only a handful of nearby planetary systems. Other innovative approaches have also been under study.

A potentially simpler approach might be to use a starshade to block starlight even before it enters the telescope, and have it an appropriate size and distance so that planet light could yet be seen. A starshade would need to be several 10&rsquos of meters in diameter and situated at several 10,000 km away from the telescope. This approach may greatly relax the engineering requirements on the telescope itself, but at the same time introduces other logistical challenges. It also would not significantly increase the number of planetary systems that could be imaged.

The limitations in angular resolution of a single telescope can be overcome if multiple telescopes are used simultaneously as an interferometer in a synthesis array. This provides an increase in resolution proportional to the telescope-telescope separation, not simply the telescope diameter. Since the late 1950s, radio astronomers have used arrays of radio telescopes for synthesis imaging, realizing that it would never be possible to build steerable telescopes larger than about 100 m (such as the National Radio Astronomy Observatory&rsquos Green Bank Telescope in West Virginia), nor fixed telescopes larger than

300 m (the extreme example being Cornell&rsquos Arecibo telescope in Puerto Rico). Combining signals from separated telescopes is relatively straightforward at radio and millimeter wavelengths, because radio receivers with adequate phase stability and phase references are readily available. At optical and infrared wavelengths the problem is significantly more difficult, because of the increased stability requirements at these shorter wavelengths. Nonetheless, this approach seems to be a promising long-term path to imaging other planetary systems and finding life on other worlds.

An optical or infrared telescope array in space is also a formidable technical and engineering problem. Nonetheless, the required starlight suppression of a factor of 10 million (in the infrared) has been demonstrated in the lab. Telescope separations of up to 400 m are needed to survey the nearest 150 or so stars. The largest ground-based arrays, such as Georgia State University&rsquos Center for High Angular Resolution (CHARA) Array on Mount Wilson, California, have telescope separations of up to 300 m. However, atmospheric turbulence limits their sensitivity to objects brighter than 10&ndash14th magnitudes. A space telescope array, above the atmosphere, would have a sensitivity limited primarily by the collecting area of each telescope, but there would be no single platform large enough on which to mount it. The telescopes would need to be operated cooperatively as a formation-flying array: this was for many years the baseline design of NASA&rsquos Terrestrial Planet Finder (TPF) mission. Although experiments in space have demonstrated rendezvous and docking of separate spacecraft, no synthesis array has yet been flown. There is no precedent for a mission like TPF.

#### Key Questions

What innovative new ways and approaches might there be from other disciplines that could reduce the cost and increase the science of a planet-imaging mission?

## LUVOIR

One candidate mission, called the Large UV Optical Infrared Surveyor (LUVOIR), is essentially a beefed-up version of the Hubble Space Telescope. Like Hubble, this instrument would observe the universe in ultraviolet, infrared and visible wavelengths of light.

However, with a diameter of about 50 feet (15 meters), LUVOIR's mirror would be more than six times wider than the one in Hubble. This means that LUVOIR would see the universe with six times the resolution of Hubble. And with 40 times the light-gathering power of the older telescope, LUVOIR would see fainter, smaller and more-distant objects.

NASA has come up with two different options for LUVOIR's design. The larger version, LUVOIR-A (described above), would be built to launch on NASA's upcoming Space Launch System (SLS) megarocket. LUVOIR-A is "the biggest we could fit on SLS," Jason Tumlinson, a researcher with the Space Telescope Science Institute (STSci) said during a presentation at AAS on Tuesday (Jan. 8).

SLS, which is also over budget and behind schedule, should launch on its maiden flight sometime in 2020. "If NASA doesn't build that rocket, then we'll go with the smaller version" of LUVOIR — LUVOIR-B, Tumlinson said. This model would have a mirror with a diameter of 26 feet (8 m), and the smaller size would entail a slightly lower resolution than for LUVOIR-A.

LUVOIR is designed to tackle a variety of astronomical research projects, like searching for habitable exoplanets studying the formation and evolution of stars and galaxies mapping dark matter throughout the universe and imaging objects in the solar system, like planets, comets and asteroids. "Regardless of what you're interested in, LUVOIR has an instrument for you," Tumlinson said.

As of 2017, 22 exoplanets have been imaged directly. The most distant of those is 1200 ly away.

This shows 4 of them orbiting HR 8799, which is 128 ly away:

These observations are good enough to determine planet orbits, and to do spectroscopy.

So we can already observe large exoplanets at much longer distances, if they orbit far enough away from their star. Smaller planets closer to their star are more difficult to see.

In the image above there's a black disk at the center. This is used to block out the star's light. It covers the star, plus a radius of about 5 AU, so any Earth-like planets in the habitable zone are masked in this image.

This is necessary because a telescope's optics and imaging system aren't perfect: the star's light is not confined to the star's diameter, it gets spread out a bit. This spread-out starlight is brighter than planets in close orbits, so those planets are lost in the noise.

One way to combat this is an occulter, i.e. a physical object in front of the telescope that blocks out the star's light. Most of these are small disks at the front end of the telescope.

NASA is working on a starshade: a much larger disk placed 50,000 km in front of a space telescope. This would be able to block out the star's light more accurately, allowing the telescope to see exoplanets in the habitable zone (so at distances of

1 AU for smallish stars). The plan was to use the starshade with WFIRST (a 2.4 m telescope), but recent concerns about costs could see the starshade removed from this mission.

So the solution for direct imaging of exoplanets may not have to be a larger mirror.

The resolution of a telescope in radians is given (to a very good first-order approximation) by $heta_=1.22frac$ where $lambda$ is the wavelength and $D$ is the diameter of the telescope aperture. Using the numbers you give, a 21 meter telescope at 200 nanometers would have a resolution of 0.002 arcseconds. Now, there are many other factors which will limit our resolution, but you can consider this, the diffraction-limited resolution, to be the highest possible resolution you can achieve. Anything with a smaller angular size than this will be indistinguishable.

So our next step is to figure out what angular size an exoplanet will subtend on the sky. Let's go with a Jupiter-size planet, to increase our chances. This means a planetary diameter of $1.4*10^8$ meters. This object's angular size (in radians) will be given by $heta_=frac$. We can actually set this expression equal to our equation for resolution and solve for distance. This will give us an approximation for how far away a planet can be before we won't be able to resolve it. Solving for $distance$ yields:

Again, using our numbers, this gives us a distance of $1.2*10^<16>$ meters, or 1.3 light years.

So, ultimately the answer is no. A 21 meter primary aperture will not be enough to resolve an exoplanet. So let's see how big our mirror would have to be, by rearranging our equation:

$D = frac<1.22cdot lambda cdot distance>$ If we want to resolve a Jupiter size exoplanet at the distance of Epsilon Eridani, it would need to be 173 meters in diameter.

Now, this is all assuming that we don't have to worry about other things, like glare from the star, which presents its own set of problems. But we can get around this by doing things like optical interferometry, which allows us to increase the effective size of our telescope without having to build bigger mirrors.

Yes. The ESO's VLT used the wobble method to detect Proxima Centauri b, a planet with a radius estimated at 0.8–1.5 R⊕ and a semi-major axis estimated at 0.0485 (+0.0041,−0.0051) AU, at a distance of 4.224 ly.

The Hubble Telescope can see the Proxima Centauri system's Sun.

The ground based VLT consists of four individual telescopes, each with a primary mirror 8.2 m across, they can achieve an angular resolution of about 0.001 arc-second. In single telescope mode of operation the angular resolution is about 0.05 arc-second.

Best case ground based conditions give a seeing disk diameter of

0.4 arcseconds and are found at high-altitude observatories on small islands such as Mauna Kea or La Palma.

At the best high-altitude mountaintop observatories, the wind brings in stable air which has not previously been in contact with the ground, sometimes providing seeing as good as 0.4".

Under bad conditions a ground based telescope over 10 meters with poor seeing can limit the resolution to be about the same as given by a space-based 10–20 cm telescope.

Ground based telescopes must look through the atmosphere, which is opaque in many infrared bands (see figure of atmospheric transmission). Even where the atmosphere is transparent, many of the target chemical compounds, such as water, carbon dioxide, and methane, also exist in the Earth's atmosphere, vastly complicating analysis.

Existing space telescopes such as Hubble cannot study these bands since their mirrors are not cool enough (the Hubble mirror is maintained at about 15 degrees C) and hence the telescope itself radiates strongly in the IR bands.

The JWST telescope has an expected mass about half of Hubble Space Telescope's, but its primary mirror (a 6.5 meter diameter gold-coated beryllium reflector) will have a collecting area about five times as large (25 m^2 or 270 sq ft vs. 4.5 m^2 or 48 sq ft).

The JWST is oriented toward near-infrared astronomy, but can also see orange and red visible light, as well as the mid-infrared region, depending on the instrument.

JWST's primary mirror is a 6.5-meter-diameter gold-coated beryllium reflector with a collecting area of 25 m^2.

At which wavelengths will Webb observe?

Webb will work from 0.6 to 28 micrometers, ranging from visible gold-colored light through the invisible mid-infrared. The short wavelength end is set by the gold coating on the primary mirror. The long wavelength cut-off is set by the sensitivity of the detectors in the Mid-Infrared Instrument.

Webb is designed to discover and study the first stars and galaxies that formed in the early Universe. To see these faint objects, it must be able to detect things that are ten billion times as faint as the faintest stars visible without a telescope. This is 10 to 100 times fainter than Hubble can see.

What are the main science goals of Webb?

Webb has four mission science goals:

• Search for the first galaxies or luminous objects that formed after the Big Bang.
• Determine how galaxies evolved from their formation until the present.
• Observe the formation of stars from the first stages to the formation of planetary systems.
• Measure the physical and chemical properties of planetary systems and investigate the potential for life in those systems.

One of the main goals of Webb is to detect some of the very first star formation in the Universe. This is thought to happen somewhere between redshift 15 and 30 (redshift explained in question 45). At those redshifts, the Universe was only one or two percent of its current age. The Universe is now 13.7 billion years old, and these redshifts correspond to 100 to 250 million years after the Big Bang. The light from the first galaxies has traveled for about 13.5 billion years, over a distance of 13.5 billion light-years.

Will Webb see planets around other stars?

The Webb will be able to detect the presence of planetary systems around nearby stars from their infrared light (heat). It will be able to see directly the reflected light of large planets - the size of Jupiter - orbiting around nearby stars. It will also be possible to see very young planets in formation, while they are still hot. Webb will have coronagraphic capability, which blocks out the light of the parent star of the planets.

This is needed, as the parent star will be millions of times brighter than the planets orbiting it. Webb will not have the resolution to see any details on the planets it will only be able to detect a faint light speckle next to the bright parent star.

Webb will also study planets that transit across their parent star. When the planet goes between the star and Webb, the total brightness will drop slightly. The amount that the brightness drops tells us the size of the planet. Webb can even see starlight that passes through the planet's atmosphere, measure its constituent gasses and determine whether the planet has liquid water on its surface. When the planet passes behind the star, the total brightness drops, and we can again determine more of the planet's characteristics.

Super short version: They're launching a slightly larger telescope than you have asked for that will reach ("detect", not provide close-up photos) virtually to the known edge of the universe.

## 2020 Symposium Chairs

2020 Symposium Chairs:

Satoru Iguchi
National Astronomical Observatory of Japan
(Japan)

Alison Peck
Gemini Observatory (United States)

2020 Symposium Co-Chairs:

René Doyon