The James Webb Space Telescope

[lpetrich]

Titled link: How far does Hubble see? | ESA/Hubble

Table from its picture:

Date	What	Redshift	Age
1990	Ground-based observatories	1	6 billion years
1995	Hubble Deep Field	4	1.5 billion years
2004	Hubble Ultra Deep Field	7	800 million years
2010	Hubble Ultra Deep Field IR	10	480 million years
Future	James Webb Space Telescope	20	200 million years

To see why one needs to observe in the infrared, let us consider how redshifted a star's light will be. I will consider the best case, star BI 253, an O2V star in the Large Magellanic Cloud, with surface temperature 50,000 K. From Wien's displacement law, its peak emission is at 60 nanometers / 0.06 microns. By comparison, the Sun's is at 500 nm / 0.5 microns and TRAPPIST-1's at 1100 nm / 1.1 microns. Most of BI 253's light is emitted in the extreme ultraviolet, with the visible part being only 1% of the total emission and looking blue-white. Calculated using Spectral Calculator-Hi-resolution gas spectra If one was to get close enough to that star to receive what the Earth receives from the Sun, one would get a very bad case of starburn.

Star	0	1	4	7	10	20
BI 253	0.06	0.12	0.3	0.5	0.7	1.3
Sun	0.5	1.0	2.5	4.0	5.5	11.
TRAPPIST-1	1.1	2.2	5.5	8.8	12.	23.

That's why one needs to observe in the infrared to see as far back as the JWST is expected to see, and that is why the JWST was designed as an infrared telescope.

 

[lpetrich]

Can one observe much farther back?

The cosmic microwave background is the oldest electromagnetic waves that we can observe. It was produced by the recombination of the Universe's material as it expanded and cooled. When it cooled below 3000 K, its free electrons combined with its hydrogen nuclei to make hydrogen atoms, reducing the Universe's opacity by an enormous factor, and allowing its light to travel freely all the way to the present day. It got redshifted by a factor of 1100, all the way into the microwave part of the spectrum, and we observe it as the cosmic microwave background, at 2.7 K.

Helium recombined before then, with helium nuclei getting their first electrons at 18,000 K and their second and final ones at 6,000 K, well before hydrogen nuclei got their electrons.

 Recombination (cosmology) -  Cosmic microwave background

 

[lpetrich]

Elements of Webb:

#10: Salt - LiF, BaF2, ZnSe
NASA Webb Telescope on Twitter: "Take it from us — sometimes, you do need to be salty.

Learn more about how #NASAWebb’s salt lenses help the telescope see in infrared in the latest episode of “Elements of Webb!” #UnfoldTheUniverse (vid link)" / Twitter
She started off with NaCl, though she introduced the actual salts used. They are used because they are transparent in the near infrared, unlike glass.


Something that JWST will be doing:

NASA Webb Telescope on Twitter: "We're honored to follow in @NASAHubble's legacy of surveying the cosmos! " / Twitter
noting
Hubble on Twitter: "Did you know Hubble helped build @NASAWebb’s “to-do” list?

One of Webb’s upcoming projects is called COSMOS-Webb, which will observe a patch of sky containing half a million galaxies – building on a huge survey that Hubble started in 2002.

Learn more: (links)" / Twitter
noting
Mapping the Universe's Earliest Structures with COSMOS-Webb

Peering deeply into a huge patch of sky the size of three full Moons, NASA’s James Webb Space Telescope will undertake an ambitious program to study half a million galaxies. Called COSMOS-Webb, this survey is the largest project Webb will undertake during its first year. With more than 200 hours of observing time, it will build upon previous discoveries to make advances in three particular areas of study. These include revolutionizing our understanding of the Reionization Era; looking for early, fully evolved galaxies; and learning how dark matter evolved with galaxies’ stellar content. With its rapid public release of the data, this survey will be a primary legacy dataset from Webb for scientists worldwide studying galaxies beyond the Milky Way.

 

[lpetrich]

Yes, the Hubble Space Telescope also has a Twitter account, just like Webb.

Webb Begins Its Months-Long Mirror Alignment – James Webb Space Telescope

Engineers first commanded actuators – 126 devices that will move and shape the primary mirror segments, and six devices that will position the secondary mirror – to verify that all are working as expected after launch. The team also commanded actuators that guide Webb’s fine steering mirror to make minor movements, confirming they are working as expected. The fine steering mirror is critical to the process of image stabilization.

That's 7 actuators each for the 18 primary mirror segments. Six for aiming each one, and one for the curvature.

Ground teams have now begun instructing the primary mirror segments and secondary mirror to move from their stowed-for-launch configuration, off of snubbers that kept them snug and safe from rattling from vibration. These movements will take at least ten days, after which engineers can begin the three-month process of aligning the segments to perform as a single mirror.

NASA Webb Telescope on Twitter: "#NASAWebb’s mirrors are warming up their moves! ..." / Twitter

#NASAWebb’s mirrors are warming up their moves! Its 18 primary mirror segments have motors to align them to perform as one big mirror. Today we confirmed that all motors (including those on Webb's other mirrors) are in working order: (link) #UnfoldTheUniverse (pic link)

Ground teams have also started instructing Webb’s primary and secondary mirror to move from the configuration which kept them from rattling around during launch. This will kick off approximately 3 months of mirror alignment work. #UnfoldTheUniverse (vid link)

 

[lpetrich]

Mirror, Mirror…On Its Way! – James Webb Space Telescope
I'll quote the whole statement about the mirror-segment deployments:

“To support the movable mirrors during the ride to space, each of them has on its back three rigid metal pegs which can nestle into matching holder sockets in the telescope structure. Before launch, the mirrors were all positioned with the pegs held snug in the sockets, providing extra support. (Imagine Webb holding its mirrors tucked up close to its telescope structure, keeping them extra safe during the vibrations and accelerations of launch.) Each mirror now needs to be deployed out by 12.5 millimeters (about half an inch) to get the pegs clear from the sockets. This will give the mirrors ‘room to roam’ and let them be readied in their starting positions for alignment.

“Getting there is going to take some patience: The computer-controlled mirror actuators are designed for extremely small motions measured in nanometers. Each of the mirrors can be moved with incredibly fine precision, with adjustments as small as 10 nanometers (or about 1/10,000th of the width of a human hair). Now we’re using those same actuators instead to move over a centimeter. So these initial deployments are by far the largest moves Webb’s mirror actuators will ever make in space.

“And we don’t do them all at once. The mirror control system is designed to operate only one actuator at a time. That way is both simpler (in terms of the complexity of the control electronics) and safer (since computers and sensors can closely monitor each individual actuator as it works). Furthermore, to limit the amount of heat put into Webb’s very cold mirrors from the actuator motors, each actuator can only be operated for a short period at a time. Thus, those big 12.5-millimeter moves for each segment are split up into many, many short moves that happen one actuator at a time. Scripts sent from the Mission Operations Center will direct this process under human supervision, slowly and steadily moving one actuator at a time, taking turns between segments. At full speed, it takes about a day to move all the segments by just 1 millimeter. It’s about the same speed at which grass grows!

“This may not be the most exciting period of Webb’s commissioning, but that’s OK. We can take the time. During the days that we’re slowly deploying the mirrors, those mirrors are also continuing to slowly cool off as they radiate heat away into the cold of space. The instruments are cooling, too, in a gradual and carefully controlled manner, and Webb is also continuing to gently coast outwards toward L2. Slow and steady does it, for all these gradual processes that get us every day a little bit closer to our ultimate goal of mirror alignment.”

—Marshall Perrin, deputy telescope scientist, Space Telescope Science Institute

That means that moving the mirrors into deployed position will take around 12.5 days. Deployment Explorer Webb/NASA indicates 10 days, as does James Webb Space Telescope - Launch Timeline at planet4589

 

[lpetrich]

NASA Webb Telescope on Twitter: "Yesterday ..." / Twitter

Yesterday we commanded all 132 motors on Webb’s primary & secondary mirrors to move them for the first time in space! #UnfoldTheUniverse

In today’s blog, @SpaceTelescope’s deputy telescope scientist Marshall Perrin takes us through a deep dive: (link)

The Ride to Space

Each of Webb’s mirror segments has 3 metal pegs on its back, which fit snugly into matching sockets in the telescope structure. During launch, the mirrors were tucked safe and sound.

Tiny Dancers

Over about 10 days, each mirror segment will move out by 12.5 mm (about half an inch) to get the pegs clear from the sockets. It may not sound like much, but these initial moves are actually the largest moves Webb’s mirror motors will ever make in space!

Slow and Steady

The mirror control system is designed to operate only 1 motor at a time to keep things both simple and safe. To limit any heat put into Webb’s super cold mirrors, each motor is also only operated for a short time at once.

At full speed, it takes about a day to move all the segments by just 1 millimeter — about the same speed at which grass grows!

While this may not be the most exciting period for Webb, taking our time is how we’ll get closer each day to our goal of mirror alignment.

No indication of how well these moves are going, however. I also like those emojis.

 

[lpetrich]

Where Is Webb? NASA/Webb
Deployment Explorer Webb/NASA

Now has a diagram of mirror deployment. The 18 primary-mirror segments and the secondary mirror all start out at 12.5 millimeters inward from their deployed positions, and all but two primary-mirror segments are now at 8 mm inward. The remaining two will be done after the other 17 are done.

The inner ring of primary-mirror segments is A1 A2 A3 A4 A5 A6, and the outer ring is B1 C1 B2 C2 B3 C3 B4 C4 B5 C5 B6 C6. The two stragglers are A3 and A6.

The JWST is all but 1/9 of its way to L2, and it has a week to go. It is now three weeks after launch.

 

[lpetrich]

Segments A3 and A6 have been at 12.3 mm for some days now. But the others are now at 5 mm.

Referring to Deployment Explorer Webb/NASA the JWST is in the its day of mirror-segment deployment, and it should be done with all but A3 and A6 in three or four days.


A big problem with doing the mirror alignment is how to tell the image from one primary-mirror segment from another. I have thought of a workaround. Move each primary-mirror segment and notice which image moves. One may also defocus the detector, but each mirror's image will also become defocused.


The JWST is now 1/11 of the total distance to its destination, and it still has 6 days to go, 1/5 of the total travel time. It is traveling at 262 meters per second or 943 km/h. Relative to the speed of sound in the Earth's atmosphere near its surface, that is about Mach 0.76. That's about as fast as an airliner at its cruising speed. The JWST's fastest Earth-relative speed was 11 km/s, when it went into its transfer orbit.

It's a bit slower than its final Earth-relative speed of 300 m/s.

 

[lpetrich]

All but A3 and A6 are now at 3 mm.

I found Twitter account JWST Tracker (@trackjwst) / Twitter - it follows the deployment of JWST.

• L + 23:02:27:00 -- 3.0 mm
• L + 23:00:49:34 -- 5.0 mm
• L + 22:17:35:29 -- 5.0 mm
• L + 22:11:15:24 -- 6.0 mm
• L + 22:10:43:16 -- 6.0 mm
• L + 22:00:46:35 -- 8.0 mm
• L + 21:17:14:24 -- 8.0 mm
• L + 21:11:08:22 -- 9.0 mm
• L + 21:10:38:01 -- 9.0 mm

This is close to a linear curve, and it seems like the deployment of all but A3 and A6 will be complete on Wednesday.

 

[lpetrich]

After going to 5.0 mm, the mirror-deployment distance went down to 2.0 mm, and then most recently to zero. A3 and A6 are at 12.0 mm.

It may take a day or two before I can be sure that those mirror segments are now in place.

 

[lpetrich]

A3 and A6 are now at 10.0 mm, with the others at 0. The two stragglers are on their way, it seems.

JWST Tracker (@trackjwst) / Twitter with my most recent one.

• L + 24:09:03:00 -- 0.0 mm
• L + 24:01:18:29 -- 2.0 mm
• L + 23:18:00:51 -- 2.0 mm
• L + 23:11:45:16 -- 3.0 mm
• L + 23:11:11:47 -- 3.0 mm

I like these emojis:
NASA on Twitter: "This week:
Completed a major milestone for @NASAWebb
Big clues from a “mini” monster black hole
Earth gets a temperature check
Watch to get the details on these updates and more space news: (links)" / Twitter
noting
A Major Milestone for the James Webb Space Telescope on This Week @NASA – January 14, 2022 - YouTube

NASA Webb Telescope on Twitter: "These mirrors were made for movin’, ..." / Twitter

These mirrors were made for movin’, and that’s just what they’ll do.

#ICYMI, last Friday we launched our mirror motor tracker. On Where Is Webb?, you can track our individual mirror segments as they move upward by 12.5 mm to prep for a 3-month mirror alignment process.
(pic link)

More information about how and why the mirror segments are moving so slowly can be found in this blog post: (link) You can also read our Twitter thread
(tweet link)

As of right now, all of our mirror segments except for the 2 designated as A3 & A6 have completed their 12.5 mm move! But don’t worry: We’ve always planned for A3 & A6 to be moved separately at the end of the process, as their position sensors are read out in a different way.

NASA Webb Telescope on Twitter: "You can continue to cheer on our last 2 segments by keeping tabs on (link)! Thanks as always for following along on our journey to #UnfoldTheUniverse. #GoWebbGo " / Twitter

 

[lpetrich]

A3 and A6, the two stragglers:

• 5:28 pm -- 8.0 mm
• 11:30 pm -- 6.0 mm

JWST Tracker now has updates on the two stragglers.

• L + 24:01:18:29 -- 12.5 mm
• L + 24:11:28:11 -- 8.0 mm
• L + 24:12:01:25 -- 8.0 mm
• L + 24:18:18:21 -- 6.0 mm
• L + 24:18:52:35 -- 6.0 mm

Seems like the two stragglers will be deployed by late Wednesday.

 

[lpetrich]

As of 8:15 AM PST, it looks like the two stragglers have joined the other seventeen segments in being in their deployed positions.

Elements of Webb:

#11: BIRB: Ball InfraRed Black paint
NASA Webb Telescope on Twitter: "No pets are onboard the Webb Telescope, but we do have BIRB

BIRB, made by @BallAerospace, stands for Ball InfraRed Black. It’s a super dark paint on our radiator that helps keep our science instruments cool! Find our how in our latest “Elements of Webb” episode
(vid link)" / Twitter

What's a good emitter is also a good absorber and vice versa, as can be shown from thermodynamics. Being black means that it is a good absorber of visible light, and that will make it a good emitter of visible light if it could be stable while hot enough to glow in visible light.

BIRB is a good emitter/absorber of visible light, and it is also one of infrared light, even at low temperatures, as measurements show:

Temperature	Emissivity
298 K (25 C)	0.96
100 K (-173 C)	0.93
35 K (-238 C)	0.85

• Cryogenic emissivity properties of Ball Infrared Black (PDF) published as Cryogenic emissivity properties of Ball Infrared Black: AIP Conference Proceedings: Vol 1434, No 1
• Ball InfraRed Black Coating

 

[lpetrich]

Webb Mirror Segment Deployments Complete – James Webb Space Telescope

“Today, the James Webb Space Telescope team completed the mirror segment deployments. As part of this effort, the motors made over a million revolutions this week, controlled through 20 cryogenic electronics boxes on the telescope. The mirror deployment team incrementally moved all 132 actuators located on the back of the primary mirror segments and secondary mirror. The primary mirror segments were driven 12.5 millimeters away from the telescope structure. Using six motors that deploy each segment approximately half the length of a paper clip, these actuators clear the mirrors from their launch restraints and give each segment enough space to later be adjusted in other directions to the optical starting position for the upcoming wavefront alignment process. The 18 radius of curvature (ROC) actuators were moved from their launch position as well. Even against beryllium’s strength, which is six times greater than that of steel, these ROC actuators individually shape the curvature of each mirror segment to set the initial parabolic shape of the primary mirror.

“Next up in the wavefront process, we will be moving mirrors in the micron and nanometer ranges to reach the final optical positions for an aligned telescope. The process of telescope alignment will take approximately three months.”

—Erin Wolf, James Webb Space Telescope Program Manager, Ball Aerospace

NASA Webb Telescope on Twitter: "Our mirror segment deployments are complete! ..." / Twitter

Our mirror segment deployments are complete!

Using motors, each segment was moved out about half the length of a paper clip to clear the mirrors from their launch restraints and give each segment enough space for mirror alignment. (link) #UnfoldTheUniverse (pic link)

You say you want a revolution? How about over a million? #NASAWebb's mirror motors made over a million revolutions this week as we moved all 132 actuators on the backs of the primary mirror segments and the secondary mirror! #UnfoldTheUniverse

Fun fact: Our mirrors are made of beryllium, which is 6 times stronger than steel.

But even against beryllium's strength, the motors can actually individually shape the curvature of each mirror segment!

Next?

Getting into an L2 orbit. That will require an apogee kick burn, and that's scheduled for four days from now, on Sunday.

 

[lpetrich]

ESA Webb Telescope on Twitter: "1/ The multi-object mode of Webb's NIRSpec instrument uses roughly a quarter of a million tiny configurable shutters, each the width of a human hair. #WebbSpec #WebbOfTechnology #WebbSeesFarther @NASA (pic link)" / Twitter

ESA Webb Telescope on Twitter: "@NASA 2/ The shutters are used to conduct simultaneous spectroscopic observations of multiple sources in a single exposure. In this mode, Webb will be able to obtain spectra of up to 200 targets at once " / Twitter


NASA Webb Telescope on Twitter: "We're gearing up to insert #NASAWebb into its orbit!
What to expect on Jan. 24:
3pm ET (20:00 UTC) NASA Science Live: Ask questions with #UnfoldTheUniverse
4pm ET (21:00 UTC) Media teleconference with @NASAGoddard & @northropgrumman experts
(links)" / Twitter
noting
NASA to Discuss Webb’s Arrival at Final Destination, Next Steps | NASA

NASA Webb Telescope on Twitter: "Tracking our journey on (link)? Our orbital burn is now targeted for Monday afternoon to give our team time for the multiple hours of preparation required. Our milestones are human-controlled to provide our team flexibility to pause & adjust. #UnfoldTheUniverse" / Twitter
noting Where Is Webb? NASA/Webb

The JWST has only 4 days to go before that burn, and it has to travel only about 4% more. It is currently traveling at 227.2 m/s, or 818 km/h or Mach 0.69.

At Sun-Earth L2, the Earth and the Sun have angular diameter 30', and the Moon 8'. From the Earth, the Sun and the Moon have angular diameter 30'.

 

[lpetrich]

NASA Webb Telescope on Twitter: " Congrats to @SpaceApps team “Jimmy in the Box” for becoming global finalists with their computerized origami models of #NASAWebb! Takeshi, Hanna, Yuto and Misaki, we’re so proud! Keep shining bright

See their project: (links)" / Twitter
noting
Jimmy In the Box - Space Apps Challenge | 2021

Webb Origami Design Challenge

The James Webb Space Telescope is NASA's next premier space science observatory and will fulfill the agency's vision to "discover and expand knowledge for the benefit of humanity." Your challenge is to create origami artwork that looks like the James Webb Space Telescope and showcases Webb as a technological and design marvel using an “arts-meets-science” approach.

 

[lpetrich]

NASA Webb Telescope on Twitter: "So…you’ve heard that the Webb telescope will be orbiting Lagrange point 2. ..." / Twitter

So…you’ve heard that the Webb telescope will be orbiting Lagrange point 2. But what even is that, anyway? And how do you orbit something that isn’t an object?

We’ve got you! Here’s a thread
#UnfoldTheUniverse (vid link)

First, the basics. Lagrange points refer to locations where the gravitational forces of 2 massive objects — such as the Sun and Earth — are in equilibrium. Webb will be located more specifically at Sun-Earth Lagrange point 2, or L2 for short.

Why send Webb to orbit L2?

Shade: The Sun, Earth (and Moon) are always on one side. At L2, Webb’s sunshield can always face all of these heat & light sources to protect Webb’s optics & instruments, which have to stay super cold to detect faint heat signals in the universe.

Views: Webb can access nearly half the sky at any given moment, and the entire sky over the course of 6 months.

Watch this video to learn more: Webb's Field of Regard - YouTube

️ Efficiency: Many of you have asked: Why doesn’t Webb just sit at L2? It’s actually simpler & more efficient for Webb to orbit L2!

️ Power: This orbit also ensures that Webb will never have the Sun eclipsed by Earth — necessary for Webb’s thermal stability & power generation.

Communication: L2 is convenient for always maintaining contact with our Mission Operations Center at @SpaceTelescope through the Deep Space Network. Webb isn’t the first spacecraft to orbit L2, either! Other observatories like WMAP & Herschel also orbit L2 for these reasons.

As Webb approaches L2, you may have noticed on Where Is Webb? that we're slowing down. Here's why:

Think about throwing a ball straight up in the air. It starts out very fast, but slows down as gravity pulls it down...

Similarly, Webb’s @Ariane5 rocket gave it energy to go a great distance, but not enough to escape Earth’s gravity. Just like the ball, Webb slows down & would eventually fall back towards Earth if we let it.

If the @Arianespace rocket had given Webb more energy than it had, Webb would have been going too fast when it got to L2, and we would overshoot our desired orbit. Webb can’t brake or turn back, as it would both cost a lot of fuel and require Webb to expose its optics to the Sun.

Webb's rocket gave it juuuust enough energy to be placed into its orbit — with a few course-correction burns along the way to make up the difference. Rocket engines aboard Webb will use thrust about every 3 weeks to keep it looping around L2 in a halo orbit every 6 months.

Want to know even more about our orbit? Read our latest blog post: Webb’s Journey to L2 Is Nearly Complete – James Webb Space Telescope

Or check out our orbit webpage: Orbit - Webb/NASA
#UnfoldTheUniverse

 

[lpetrich]

Webb’s Journey to L2 Is Nearly Complete – James Webb Space Telescope

On Monday, Jan. 24, engineers plan to instruct NASA’s James Webb Space Telescope to complete a final correction burn that will place it into its desired orbit, nearly 1 million miles away from the Earth at what is called the second Sun-Earth Lagrange point, or “L2” for short.

Then discussing the five Lagrange points and why JWST orbits L2 instead of being exactly at it.

Orbit - Webb/NASA also discusses the space telescope's intended orbit. Several others have used that orbit, like WMAP, Herschel, and Planck. WMAP and Planck were for observing the cosmic microwave background, and Herschel was an infrared telescope, like JWST.

 

[lpetrich]

[Webb's Field of Regard - YouTube] - shows which range of directions the JWST can point in while remaining shaded.

Its reference direction is upward away from the Sun, with the telescope proper being upward and the sunshield being downward. Forward is the direction that the telescope points, and rearward is the opposite direction, the direction with the instruments.

• Yaw = horizontal rotation
• Pitch = front-end up-and-down rotation with back-end opposite
• Roll = side-to-side rotation

JWST:

• Yaw = unlimited
• Pitch = front-end down 5d to up 45d
• Roll = 5d in either direction

This gives the telescope a field of view that is a 50-d-wide ring, and as the Earth moves around the Sun, this ring moves across the sky, meaning that the telescope can see 100% of the sky.

 

[lpetrich]

 List of space telescopes

The large majority of them have been in low Earth orbit, up to 1000 - 200 km, with most of the rest being in low-to-high or high Earth orbit.

Here are the exceptions:

The Moon's surface: Far Ultraviolet Camera/Spectrograph (UVC), Lunar-based ultraviolet telescope (LUT) -- (planned) International Lunar Observatory Precursor (ILO-X: visible light)

Sun-Earth L1: -- (planned) Aditya-L1 (visible light, ultraviolet), ATHENA (X-rays)

Sun-Earth L2: Spektr-RG (X-ray), Gaia (visible light: astrometry), Herschel (infrared), JWST (infrared), WMAP (microwave), Planck (microwave) -- (planned) Euclid (infrared), PLATO (visible light: exoplanet search), Nancy Grace Roman Space Telescope (infrared), ARIEL (visible light: exoplanet observation)

Heliocentric orbit: Gamma-Ray Burst Polarimeter (GAP), Kepler (visible light: exoplanet search), LISA Pathfinder (gravitational waves) -- (planned) Laser Interferometer Space Antenna (LISA: gravitational waves)