As a wave, EM has a wavelength As a particle, EM travel in photons s. The focal length of a telescope is the distance from the lens or mirror to the image formed of a distant light source.
Large telescopes used for astronomy have sidereal drives.
Thousands of parallel microscopic lines seen in a star's spectrum identify the composition of a star. A highly sophisticated radio-receiver usually consisting of a dish reflector, an antenna, an amplifier, and a recorder. Radio waves are much longer than light waves, resolving power of radio telescopes poorer than that of optical ones. Can detect objects more luminous at radio wavelengths than at visible wavelengths.