Question

What is EMI, and who does this interfere with radio telescopes?

EMI = Electromagnetic Interference

Answer 1

Radio telescopes use a portion of the Electromagnetic spectrum of energy. EMI adds "noise" to the signals that decrease their resolution.

Explanation:

"Radio" is just one part of the continuous electromagnetic energy spectrum. Any source of this energy at the same energy level as that used in a radio telescope would be "detected" along with any of the intended objects. Thus, it generates a background "signal" that interferes with the resolution possible for other signals/transmissions at that energy level.