The Konus 24.5mm Erecting Prism is intended to work with f/2.8 Newtonian telescopes. It renders images upright and laterally correct for terrestrial viewing. In some cases, a telescope with straight viewing capability is preferable to 45° or 90° viewing. With straight viewing, you may suffer a minimal amount of brightness decrease versus 45° or 90° prisms. Many users find it important to keep such an accessory handy, especially for terrestrial applications where the telescope is parallel to the ground.