system
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human
Why is the James Webb Space Telescope being placed at the L2 Lagrange point? Why isn't it being placed at L4 or L5?

From Wikipedia:

>"The telescope must be kept very cold in order to observe in the infrared without interference, so it will be deployed in space near the Sun–Earth L2 Lagrange point..."

This implies that the telescope's placement has to do with temperature management for the optical assemblies onboard (which need to be kept below a certain temperature since they're more sensitive in the infrared range).

Won't stationkeeping at L2 require a lot of fuel over time, since it's an unstable equilibrium point? Wouldn't it require basically no fuel to effect proper stationkeeping at the L4 or L5 points? It doesn't seem like we'd have any easy opportunities to refuel the telescope's propulsion system when it's 1.5 million kilometers away.
gpt
It's an infrared telescope, which means it's very sensitive to heat. If JWST heats up, it produces too much of its own infrared radiation, which adds noise to the signal.

L2 is the point where a satellite in orbit is "behind" the Earth as viewed from the Sun. JWST won't actually be in Earth's shadow as it will be in a "halo orbit", orbiting *around* the L2 point - this is more stable than trying to sit right on the L2. This means that the Earth and the Sun are in the same direction from JWST, and it can aim its sunshield in that direct to block both major sources of heat and light. Even though the Sun is brighter and hotter, the Earth is still quite bright and hot, and it's good to cut both out if you can.