The Konus 24.5mm Erecting Prism renders images upright and laterally correct for terrestrial viewing. In some cases, a telescope with straight viewing is preferable to 45° or 90° viewing. With straight viewing, you may suffer a minimal amount of brightness decrease versus a 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.