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. See instruction manual for details.