Please note: This text was taken directly from the manufacturer.
For terrestrial viewing with a refractor or Cassegrain telescope, use of a correct-image (or "image erecting") prism diagonal may be desirable. The image through the eyepiece image will appear upright and nonreversed, matching the naked-eye orientation. So a correct-image diagonal is essential for terrestrial viewing with the above telescope types and is useful for astronomical viewing for those that prefer to have the view through the telescope match the naked-eye orientation. We have three correct-image diagonals to choose from: a 45-deg 1.25" version, a 90-deg 1.25" version, and a 90-deg 2" model (with 1.25" collar). All are fully multi-coated and threaded for filters. Note that the 45-deg correct-image diagonal requires 5/8" more focus travel than typical diagonals. Diagonals are not recommended for use in reflector telescopes.
