On AG camera, in front of the sensor, there is a glass plate that covers half of the photon-sensitive area so that focus position can be sensed in realtime during auto guiding by comparing the sizes of stellar objects in the area with and without the cover plate. Since the area with/without the cover plate will be known, the detected objects through the cover plate can be flagged somehow based on their AG camera pixel coordinates, and this may ease subsequent process to pick up relevant objects for focusing and so on.