The back focus specification from RCOS for the Telescope is 9.929 inches. See diagram below.
Therefore, the focal plane for the camera should be as close as possible to 9.929 inches from the telescope backplate. The camera, extension tubes, filter wheel and adaptive optics unit comprise the “imaging stack.” The imaging stack must be configured with appropriate extension tubes to bring the camera focal plane to 9.929 inches from the backplate. Based on experimentation, within one half an inch from the backplate may be “close enough” to enable the telescope to reach focus. Achieving the correct back focus is not that difficult once the precise length of the items in the imaging stack are known.
The charts below show the lengths of each piece of equipment in the imaging stack and the extension tubes or spacers needed to come as close as possible to the specified 9.929 inches. One chart includes an off axis guider and the other does not. The telescope is currently configured and operated without an off axis guider.
The picture below shows the camera and other equipment configured for use without an off axis guider. Two spacers were used, 2.5 and 0.625 inches to bring the camera focal plane within .0225 inches from the ideal back focus of 9.929 inches.