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3 element - 0.75x-Flat Field for sharper stars all to way to the edge. Specifically made for the 106mm APO. Screws on to the M63x1-Thread. 0.75x factor - reduces the focal length to 525mm while giving f/4.95. 55 mm to 60 mm back distance - fine tune in 0.05 mm Increments - from the M48x0.75-thread.