Thread by @pontoppi

Why does #JWST have to be colder than liquid nitrogen? The entire spacecraft is designed around it! So it has to be really important, right? The sunshield deployments over the next couple of days are critical steps needed to get there. Ok, time for another nerdy thread. 1/13

The temperature of the primary mirror is the first thing that determines how faint objects can be detected in the infrared. Sure, size matters greatly if the mirror is cold. But if it's warm, the sensitivity gained from the big mirror is out the window. 2/13

All normal matter (rocks, air, dogs, snow, etc.) emits light on its own according to its temperature. This is a really basic property of nature. Hotter = more light, bluer light. Cooler = less light, redder light. 3/13

If something is really hot, it shines mostly in visible light (the Sun), and if it's really, really hot, it shines in X-rays (like a star torn to pieces by a black hole). On the other side, if something is cooler, it shines in mostly infrared light. Dogs are cool. 4/13

If that big #JWST mirror was kept at room temperature like Hubble, it would be blindingly bright in infrared light. A lot of that mirror-light would end up on the sensitive detectors in the science instruments. It would be like using the telescope on a clear summer day. 5/13

Now, here is the important part. You can actually remove this background mirror-light from the images, and warm infrared telescopes on the ground do this all the time! I've spent much of my career doing exactly that using special techniques. But there is a big problem. 6/13

All that extra infrared light from a warm mirror brings noise with it. Lots of bad noise. The kind of noise you can never remove because of the basic laws of thermodynamics. The kind of noise that erases all signs of faint galaxies in the early universe. 7/13

Here I used the JWST performance model to see what would happen if the #JWST primary mirror was set to "room temperature" instead of on "ultra-cold". Ouch! 8/13

This "background noise" is actually a manifestation of the basic quantum duality of light - that light is both a wave and a particle. In this case, we use the particle part of the light, the photons. 9/13

#JWST's detectors count photons. But counting the ones from the edge of the universe among a flood of photons from a warm mirror is a noisy business. Ideally, you want most of your photons to come from your faint galaxy, and not from the mirror. 10/13

So how cold does the mirror have to be to detect the first galaxies and measure the composition of planet-forming disks? Turns out that to not be immediately swamped by the mirror's light, it has to be colder than about 50 Kelvin (-370 F)! 11/13

Even at these low temperatures, #JWST is still limited by the mirror's light at the reddest end. The sensitivity quickly drops at wavelengths longer than about 12 micrometers. I'm not complaining, but it shows how hard this is to do. 12/13

Once the sunshield is deployed we will see the temperature of the mirror dropping over the next weeks. How accurately did the models predict the final temperature? We will soon know. Follow along on jwst.nasa.gov/content/webbLa… 13/13