Observing through your astronomical telescope can be awkward due to the angle of the eyepiece once the telescope is set up. A star diagonal alters the angle of the eyepiece to make observing more comfortable. The majority of telescopes will show an upside down or reversed image to correct this you require an erecting prism. Both star diagonals and erecting prisms are available in either 1.25 inch or 2 inch fittings to match your eyepiece type.