L2 is a very stable thermal environment as well as good instantaneous sky visibility and high observing efficiency.
The main reason space telescopes are placed in an L2 orbit is because L2 is a stable thermal environment. Telescopes in Earth orbit can receive sunlight and earthlight in different directions, meaning that the telescope would have to shield in most directions if it wanted to keep itself cool. At L2, the Sunlight and Earthlight come from the same direction, meaning the telescope has to only shield itself from that direction. This is very important for infrared telescopes, as heat radiation acts as interference to measurements. Of the 6 space telescopes going to L2 listed, 3 are infrared telescopes.
For example JWST, one of the infrared telescopes, has to cools some of it's detectors to 7 kelvin to operate properly.
JWST L2 Halo Orbit
The other main benefit from being in L2 is that being so far away from Earth means that there is a much greater available field of view as the Earth blocks a very minor portion of the sky. This increases sky visibility and observation efficiency.
- Ease of communication
- Delta-V requirements low (2-4 m/s per year)