Celestron’s Erect Prism, 45° angle Diagonal connects your telescope’s eyepiece port to the eyepiece and offers fully-coated optics that correctly orient images, with excellent sharpness and contrast. Having the image right side up makes this diagonal a  good for both astronomical and terrestrial observing. The extra light gathering of the prism on the 45° angle diagonal makes it a good choice when either using your telescope to observe objects close to the horizon or using your telescope as a terrestrial spotting scope. For observing objects well above the horizon, the 90° angle star diagonal is a much better choice, as the prism of the 45° angle diagonal absorbs too much light to properly observe deep space objects. The 45° angle on this diagonal also allows you a more comfortable observing position, something especially important for viewing objects high in the night sky, when used with telescopes with the eyepiece port at the rear of the telescope (Refractors, Schmidt-Cassegrain, Maksutov-Cassegrain).