Ever wondered how to use a filter meant to attach to 2" eyepiece barrels on an SCT with and without a Focal Reducer/Field Flattener? Here's my solution. A focal reducer attaches to the brass ring, on the camera side, thereby not messing up the required distance-to-focal plane in the camera. When I tried out the Celestron 0.63x FR/Flattener I found the vignetting pretty bad in the Canon 1Ds and preferred trying things out at f11 (prime focus for the C14).