What instrument is primarily used to observe celestial objects?

The primary instrument used to observe celestial objects is a telescope. This device allows astronomers and stargazers to enhance their view of distant stars, planets, and other astronomical phenomena. Telescopes gather and focus light, making faint or distant objects bright enough to be seen clearly. They come in various designs, such as refractors using lenses and reflectors using mirrors, tailored for different observational needs in astronomy.

The other options serve different purposes: a spectrometer analyzes the light from celestial objects to determine their composition and characteristics, but it doesn't facilitate direct observation. A barometer measures atmospheric pressure and is unrelated to celestial observations. An anemometer is used to measure wind speed, also not relevant to observing celestial bodies. Thus, the telescope remains the standout tool for studying and observing the vastness of the universe.