Film, Obtaining Maximum Sensitivity
Black and white film requires a certain minimum number of photons to complete the chemical reaction that produces the dark pigment. Less than this number and the reaction proceeds part way but then drops back after time. To help to overcome this problem the film is stored in a special blend of hydrogen gas and an inert gas. This process will lengthen the time that the film pigment will remain in the partially reacted state until another photon arrives to complete the reaction. This process is called hypering the film.
CCD
The CCD is sensitive to a broad band of light colors. The CCD is usually connected to a computer in order to read out the charge stored at each pixel. The field of pixels is read line by line, pixel by pixel. The reading of the CCD produces an amount of noise. Due to the reading noise problem the exposure time for low light levels has a minimum exposure time. It is possible to add together many individual exposures in the computer improving the image and allowing the person guiding the telescope to rest between exposures.
CCD's have variations in their surface that are very hard to reduce during the manufacturing process. Fortunately these variations can be compensated in the computer processing of the image. First a dark image is taken with no light in order to establish the zero light level for each pixel of the CCD. Next an image is taken of a white background to expose each pixel the same. Using these two readings the computer is able to find a factor to multiply the signal from each pixel by to 'normalize' the reading from the pixel.
The noise level in a CCD can be greatly reduced by cooling the CCD.