How Telescopes Work

Star Diagonals and Erecting Prisms

When you look at an object high in the sky (near Zenith), the eyepiece is facing down toward the ground. If you looked through the eyepiece directly, your neck would be bent at an uncomfortable angle. So, a 45-degree mirror called a star diagonal is used to deflect the image into an eyepiece, making the viewing angle more comfortable. Star diagonals can range from $40 to $90.

Telescopes and eyepieces turn an image upside-down. This is not a problem when observing astronomical objects, but is rather annoying for terrestrial observations like bird watching. To correct this, an erecting prism or Porro prism, either straight through or angled, is used to turn the image right side-up. These prisms can range from $25 to $70.