Telling you when is the job of the off-axis guider or photoguide telescope (which have their own separate category in Photographic Accessories). An off-axis guider is a T-shaped camera mount. One short arm of the T threads onto the rear cell of Schmidt-Cassegrains or slips into refractor or reflector focusers. Your 35mm camera is attached to the other short arm by means of a T-ring (most CCD cameras have built-in T-threads and don’t need a T-ring).