Why are telescopes like JWST orbiting Earth able to perform so much better than telescopes on Earth that aren’t restricted by their size?
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Because there’s no atmosphere in space. Earths atmosphere scatters and absorbs too much EM radiation to be able to get good images.
You know how when you play outside on a really sunny day and the sun is really bright and it’s hard to see things far away? Well, telescopes on Earth have to look through a lot of air, which is like a really bright sun for telescopes. When telescopes are in space, like on the JWST, they don’t have to look through all that bright air and can see things much better.
Atmosphere.
Telescope on Earth have to look thru all the junk in the air before reaching out into space. This can distorted the image and reduce the effectiveness of the telescope. That is why most of the Earth telescopes are on top of mountains to reduce the about if atmosphere and light pollution they have to deal with.
Size is good, but so is not having to look through an atmosphere. Air has two issues: it completely or partially blocks some frequencies which means that ground based telescopes simply can’t see those, and it causes distortion of the incoming light (twinkling, called seeing). This latter issue has largely been solved for the generation of instruments that are just coming online using a technology called adaptive optics. In the wavelengths it can see the ELT will be much better than JWST precisely because it will be so much bigger.
JWST is not orbiting Earth. It’s orbiting the Sun, 1.5 million kilometers (1 million miles) away from the Earth at what is called the second Lagrange point or L2. L2 is a point of equilibrium between the gravitational pull of the Sun and Earth. It lies on a line through the Sun and Earth, beyond Earth. This allows the satellite’s large sunshield to protect the telescope from the light and heat of the Sun and Earth (and Moon).
The sun shield also protects the sensitive mirrors and instruments from the spacecraft bus, which contains the computer, communications antenna, and steering mechanisms. There’s a huge temperature difference between the hot and cold sides of the shield. The cold is necessary because it’s an infrared telescope, and any nearby heat would hurt its ability to detect very faint infrared signals from incredibly distant objects.
JWST is not orbiting Earth, it is orbiting Lagrange point 2 in the earth’s suns system. That is a point where both earth’s and the sun’s gravity pull in the same duration. That point is 1.5 million km away from earth which is 4x the distance to the moon or 1/100 the distance to the sun. The orbits it has around that point are quite similar in radius to the moon’s orbit around the earth
That means it can follow the earth around the sun and at the same time keep the earth, sun, and the moon close to the same direction and it can block heat with he heatshield from it reaching the large mirror.
There is at least two major reasons to have a telescope in space.
One is the atmosphere is not uniform so it bends light, the result is that at some point the limiting factor in resolution is an atmospheric disturbance. Telescopes are placed on mountain peek in locations with quite a consistent airflow on earth but still, the atmosphere is limiting. An atmospheric effect like this is why you can see a star twinkle.
A second advantage is if you are interested in a nonvisible part of the spectrum the atmosphere is not always transparent. Look at [this graph](https://www.researchgate.net/profile/F-Mackenzie/publication/302399795/figure/fig4/AS:360657494003712@1462998855199/A-Radiation-absorption-spectrum-of-the-Earth-atmosphere-in-wavelength-range-from-01.png) that shoe absorption vs wavelength. JWST observes from visible to 28 μm in the infrared spectrum. 28 μm is between the N2O and H20 labels to the far right of the graph. So the atmosphere blocks all IR light in part of the spectrum and for others is absorbed a significant percentage.
The object emits light and the wavelength depends on the temperature. The sun emits visible light because it is very hot. Earth and the atmosphere is cooler and emit at 3 – 100 μm. That overlap with what JSWT observes at. So a telescope on earth will try or observe faint light from space at wavelength where earth and the atmosphere glow. A thermal camera looks at wavelengths like that and you can see objects on earth emit light at those wavelengths.
The mirror and instrument part of JWST is cooled down to very low temperatures so they do not emit light at the same wavelength as observation is done at. Blocking light from the sun, Earth, and the moon cols that side down to minus 369.4 degrees Fahrenheit (minus 223 degrees Celsius). Some instruments require action cooling to minus 447 degrees F (minus 266 degrees C) only 12 degrees F (7 degrees C) above absolute zero, You can cool instruments and perhaps even the telescope and support building if it is on the ground but you can cool down the atmosphere between it and space. It show how cold stuff need to be for ut to nor disturbed observation
So the reason to keep the telescope in space is earth’s atmosphere that which limits resolution because it bends light and makes the observation in part of the spectrum impossible because they are blocked or Earth emits them itself.
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