Isaac Newton didn't invent a telescope. He improved on the existing telescope design. He built a reflecting telescope, which used a curved mirror instead of lenses to focus light. This design was a significant improvement over the refracting telescopes of the time, which suffered from chromatic aberration (distortion of colors).

While Newton didn't invent the reflecting telescope entirely (others had experimented with them before him), his design was groundbreaking and paved the way for future telescope development.