Because the design uses two mirrors, the images you see are correct left to right and up and down, but the views are rotated, depending on the angle of the focuser and the ground as well as how you hold your head to the eyepiece. Newtonian reflector telescopes were also designed for astronomical use and images of terrestrial objects will appear upside-down. This is not a problem for daytime terrestrial telescopes where daylight is plentiful-and nobody wants to see wildlife upside-down or backwards! If you really want to use your scope for terrestrial viewing with a fully corrected image, there are optional prism assemblies available, but check with the manufacturer as not all refractors can reach focus with all erecting prisms. When looking at faint targets, such as galaxies or nebulae, the astronomer wants his or her eye to capture every photon of light possible to see the faintest detail. The prisms in spotting scopes and binoculars will reflect the light path 4 to 5 times before sending it to the eyepiece, losing a little bit more light each time. Astronomers avoid using additional glass elements in the light path such as mirrors or prisms because each time light strikes an optical surface some of the light is absorbed by the glass and is lost to the observer’s eye. They use complex sets of prisms that flip the image so that it appears correctly oriented. Optical systems such as spotting scopes and binoculars give a correctly oriented image, but they were designed for terrestrial observing. However, it will still appear backwards left to right. The mirror in the diagonal also flips the image so that it is correctly oriented up and down. Because observing straight through the telescope can be difficult to use when looking directly overhead, astronomers use a mirror diagonal to make the eyepiece more accessible. When you use an astronomical refracting telescope and insert an eyepiece directly into the telescope’s focuser, you would see an image that was upside down and mirror reversed. There's no need to worry-your telescope is working as intended. If your viewing target is upside down when you use your telescope, don’t be alarmed! Astronomical telescopes are designed to produce an upside-down or inverted image.

Vivitar telescope eyepiece lenses