This is a reducer designed specifically for flat field telescopes like the CDK. This reduces the f ratio of the CDK telescope 66%. The f/6.8 CDK20 telescopes will be reduced to about f/4.5.
Note: 200166 adapter is also compatible with CDK17 that have 8.81″ back focus. CDK17 with 10.24″ back focus will use the 125166 focal reducer.