What is astronomical dark field?

Dark field is a photographic term, which is required by the unique properties of CCD (digital camera).

CCD can convert optical images into digital signals, but the problem is that CCD will also have an output without any illumination, which is called undercurrent. If you shoot in a bright environment (during the day), the undercurrent can be ignored. However, in night shooting, especially in astronomical observation, for high accuracy, it is necessary to remove the undercurrent. So when shooting, you need to use the same parameters as the official shooting, cover the mirror and shoot once. This kind of shooting is called dark field shooting.