This is a reducer designed specifically for flat field telescopes like the Planewave CDK. This reduces the f ratio of the Planewave CDK telescope 66%. The f/6.8 Planewave CDK17 and Planewave CDK20 telescopes will be reduced to about f/4.5.

ThePlaneWave CDK .66x Reducer is a high performance reducer designed to work with a flat field telescope like the CDK. Like high performance telescopes, this reducer is sensitive to spacing. The reducer has very limited back focus and uses quite a bit of the telescopes back focus. But for the right camera setup, this is a wonderful accessory that reduces the focal ratio while maintaining theCDK's performance.
There are two different models of reducers that PlaneWave sells. Optically they are identical, but mechanically they are different in order to mount with the different telescopes. The 125166 is the model for the CDK12.5 and the 200166 is the model for the CDK17 and 20.

CDK .66x Reducer Instructions

Reducer Spacer Combinations for SBIG and Apogee Cameras