The HST doesn't take photographs exactly like a normal camera. The HST instruments take data which follows a complex path until it reaches the scientist. Data from the HST instruments follow a complex path, part of which is the pipeline , after which they are made available to the scientists. At this stage, the data are calibrated and put into a form useable by the scientists' analysis software. Data from the cameras consist of numbers whose value represents brightness measured by each detector in the camera.
Raw data can contain a wider range of values than can be represented on a display or print.