What do astronomers primarily use telescopes for?

Astronomers primarily use telescopes to collect and observe light from distant objects in the universe. This function is fundamental to the study of astronomy, as the majority of information available about celestial bodies, such as stars, planets, galaxies, and other cosmic phenomena, is obtained through the light that they emit or reflect.

Telescopes enhance our ability to see faint objects that are billions of light-years away, allowing astronomers to study their composition, behavior, distance, and the physical laws governing them. By gathering more light than the human eye can alone, telescopes make it possible to observe a wider range of wavelengths, from radio waves to visible light and beyond, enabling a deeper understanding of the universe.

The other options, while they may involve aspects of astronomy, do not accurately reflect the primary function of telescopes. Creating maps of the stars is a byproduct of observations made with telescopes but not their main purpose, and measuring temperature or calculating speed relates more to different types of instruments or methods within the field of astronomical research rather than the direct function of telescopes.