The term CCD is an acronym for "Charged Couple Device". What does this mean? Now it is time to talk about a little theory. A CCD chip is a thin piece of silicon in a wafer format. These wafers are sensitive to light in an interesting way. When light falls upon them, the "Photoelectric Effect" takes hold; electrons are knocked free from the crystalline structure of the silicon and deposited into small units or wells. There is one well per pixel. When the image is complete, the electrons are sent into a holding register where they are counted. The more electrons in a given register, the brighter that pixel will be.

This is a measure of the CCD chip's sensitivity to the various wavelengths of light in the spectrum. In an ideal world, a CCD would have 100% quantum efficiency across the visible wavelengths of light, assuming one wishes to study just those wavelengths.

This is the image one is trying so hard to obtain. The light frame is the data from the actual imaging of the object. This is taken through a lens system or telescope with the shutter open and lens cap off. A light frame also contains noise: thermal, light leaks, radio interference, read noisse, etc. All of this has to be removed to get your final image.