We can make a parfocal eyepiece but need to get the camera focused first. We point at a medium star and the program will start to take continuous images every few seconds. We then adjust the focus on the telescope until the star is as small as possible. To take an exposure, we need to start and stop the exposure of photons. CCD's accomplish this in different ways.
Some chips are twice as large as the area used to collect photons.
As the image is completed the image is transferred to the other side of the chip which is not exposed to the incoming photons. This type of system is called frame transfer. The more expensive cameras have an electromechanical shutter that opens and closes for the exposure. The Dark Frame . To eliminate the dark current we mentioned, a dark frame is taken.