29. active optics adaptive optics angular resolution (resolution) blocked light blueshift Cassegrain focus charge-coupled device (CCD) coude focus Doppler shift electromagnetic radiation electromagnetic spectrum eyepiece lens focal length focal plane focal point gamma ray infrared radiation interferometry light-gathering power magnification Newtonian reflector Key Terms objective lens photon pixel primary mirror prime focus radio telescope radio wave redshift reflecting telescope reflection refracting telescope refraction Schmidt corrector plate secondary mirror seeing disk spectrum spherical aberration twinkling ultraviolet (UV) radiation very-long-baseline interferometry (VLBI) wavelength X ray