To chill to its operating temperature of less than 7 K (-447 F or -266 C), Webb’s MIRI instrument uses a special refrigerator. But it also requires heaters to control its cooldown & prevent ice from forming in space. 🧊

Wait, ice? Allow us to explain (thread ⤵️) Contamination control engin...
When Webb launched, moist air was entrapped between components like the sunshield membranes and its many layers of insulation. Other Webb materials absorbed water vapor from Earth’s atmosphere. Most of this air escaped just 200 seconds after liftoff, but some moisture remained.
Water behaves differently in space than on the ground. In a perfect vacuum, water can exist only as a gas, but even space isn’t a perfect vacuum. Instead, water tends to "outgas" at temperatures above 160 K (-172 F or -113 C), and it tends not to below 140 K (-208 F or -133 C).
Crucially, if any water molecules are floating around and contact a surface colder than 140 K, they will stick to it as ice and never come off. This is what’s incredibly important to prevent for the Webb telescope.
Once Webb’s sunshield deployed, Webb began to cool quickly. Our team carefully managed the rate of this cooldown and the order in which different components cooled using electric strip heaters. This allowed water to escape to space rather than freeze onto sensitive components.
MIRI’s “refrigerator,” or cryocooler, uses helium gas to carry heat out to the warm side of Webb’s sunshield. To ensure that the cryocooler could work & MIRI could cool to its ultimate temperature, all water had to be either eliminated or purposely contained in designated areas.
NEW on our “Where is Webb” tracker: You can now track MIRI’s temperature as it cools to its target level! Also check out our interactive graphs to see how the temperatures of Webb’s instruments have changed since launch: #UnfoldTheUniverse Screen capture of the lates...

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More from @NASAWebb

Mar 16
Small adjustments, major progress!

Having completed 2 more mirror alignment steps, #NASAWebb’s optical performance will be able to meet or exceed its science goals. Now that’s good optics! 😉 #UnfoldTheUniverse

Curious about this image? Thread ⬇️ An engineering image from Webb which shows a bright star in
While the purpose of Webb’s latest image was to focus on a bright star and evaluate the alignment progress, Webb’s optics are so sensitive that galaxies and other stars can be seen in the background. Watch this video for an in-depth explanation of how the image was created!
Fan of a photo filter? @NASAHubble & Webb actually record light in black and white. They use filters that allow only a specific color of light through. The filtered images are then individually colored by scientists and image processors, then combined:
Read 5 tweets
Feb 11
Bonus image! When it’s time to focus, sometimes you need to take a good look at yourself.

This “selfie” taken by Webb of its primary mirror was not captured by an externally mounted engineering camera, but with a special lens within its NIRCam instrument. #UnfoldTheUniverse A black and white image of the primary mirror of the Webb Te
This special lens is meant for engineering, not science, and allows NIRCam to capture an “inward-looking” image of the primary mirror. This image helps us to check that the telescope is aligned with the science instruments.…
What you are seeing is the actual primary mirror of Webb as it observes its engineering target, a bright star. All the mirror segments are seeing starlight, but the bright segment is bright because, from NIRCam’s view, the segment is directly aligned with the star.
Read 4 tweets
Feb 11
The 18 random dots featured in this video might not look like much, but they represent a big step forward in #NASAWebb’s 3-month mirror alignment process and its quest to #UnfoldTheUniverse:…

Let’s connect the dots with a thread ⬇️
⚫️ These dots confirm that Webb’s Near-Infrared Camera, or NIRCam, can collect light from celestial objects — and that starlight from the same star can be reflected from each of Webb’s 18 unaligned mirror segments back at Webb’s secondary mirror and then into NIRCam’s detectors.
⚫️ Our team first chose a bright, isolated star called HD 84406. Over ~25 hours, Webb was repointed to 156 positions around the star's predicted location, generating 1560 images with NIRCam’s 10 detectors. This is just the center of an image mosaic with over 2 billion pixels! The James Webb Space Telescope's initial alignment mosaic, s
Read 8 tweets
Jan 21
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 ⬇️
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.
Read 11 tweets
Jan 13
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:…👇
🚀 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!
Read 5 tweets
Jan 10
❄️ Now that our deployments are complete, just like our telescope, we’re entering a period of cooldown. Our updates will be less frequent, but that doesn’t mean things have stopped happening:…

Thread ⬇️ Screen still of the James Webb Space Telescope's primary mir
First, what do we mean by “cooldown”? If you’ve been checking the temperatures of our “cold side” at, you can see we’re still a ways off from our operating temperatures of less than 50 Kelvin (about -370° F, or -223° C).

The deployment of our sunshield helped a lot with quickly lowering the temperatures on the cold side, but further cooling down will take place more slowly over time. The sunshield helps to passively cool Webb, meaning the optics get cold solely by being in the shade. 🌡
Read 8 tweets

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