McMath-Pierce telescope. Astronomer at work inside the McMath-Pierce solar telescope, the world's largest solar telescope. The devices in the astronomer's hands control the movement of the telescope. The disk of light has been reflected from the telescope's mirrors and is about 80 centimetres across. The hole near the centre of this disk allows sunlight through to the spectrograph instrument beneath it. Spectrographs are used to study the emission and absorption spectra of stars; these spectra reveal the stars' composition. The McGrath-Pierce telescope is at the Kitt Peak Observatory in Arizona, USA.