A telescope was an instrument for looking at objects at great distances. Originally used for terrestrial sighting, people started pointing them towards the stars.
Doc Brown inspecting Clara's broken telescope
Clara Clayton owned a brass refracting telescope that became damaged when her buckboard was thrown into Shonash Ravine. After Emmett Brown rescued her, she felt it was proper for her to have him repair it for her, and took it to him on September 5, 1885. When she showed Doc that it went out of focus and back, Doc realized that "everything becomes clear", and quickly snapped out of his lovestruck moment, saying that he would fix it that night. Clara reminded Doc that the Hill Valley Festival was also that night and he would not be able to work on it if he went.
Doc and Clara looking at the moon through the fixed telescope
Doc agreed and attended the festival where he met and danced with her. After the festival, Doc and Clara left together and watched the moon from her house with her repaired telescope.
Doc's sniper rifle also had a large telescope mounted onto it that enabled him to "shoot the fleas off a dog's back at 500 yards."
