Ok,OK...I see. The correct image diagonal is for refractors mainly, but SCT`s can work also. I wouldn`t recommend these for reflectors or Dobs. The correct image diagonal is not a inverted image like a regular 90 degree diagonal prism. This required more "visual" to work. You deffinatly need to know your in travel from the focus plane. The reason is if and when you install this item you may not be able to focus in enough. This item required in-travel to work. You need to find your focus plane and measure in. If the distance is within the perameters of what is required for the diagonal to work then your OK, if not, don`t buy it. You need the total travel of the focuser, then determine the focus plane. Measure down from there to the bottom of focus travel. It probably will be around an inch or so. These ansers in my opinion need visuals. Good Luck!!