This is the Electronic Focus Accessory (EFA) I added to the CDK17. It controls the IRF90 focuser/rotator. See the page on the IFR90 for details on its operation and capabilities.

The EFA has a plug-in hand controller, pictured in the inset at lower left, that permits adjustment of focus and rotation when the telescope is being used visually. The rate of focus change or of rotation change can be adjusted by the Rate button on the hand controller. The hand controller can be clipped to the rear plate of the telescope when not in use.