The sensitivity of pixels in the chip is varying. Moreover, when we use different filters, each filter has its own pattern (dust, scratches, etc). The multiplicative variations of pixel gains could be removed precisely if we were able to take a bright but unsaturated frames, which were illuminated identically to the scientific frames exposures. Then, dividing scientific exposures by such a frame, we could remove all the pixel variabilities. This process is called flat-fielding. In practice, for this purpose we can use the so called Dome flats (we simply do several exposures of the Dome through each filter), or Sky flats (exposures of twilight sky). Of course, they do not correspond exactly to the night sky illumination pattern, but in many cases they can provide the necessery accuracy for photometry. Some astronomers prefer to take the exposures of night sky in some specially selected blank areas of the sky. Alas, this process takes a lot of observational time, and the level of similar frames remains low, causing a dangerous decrease of the signal to noise ratio. To solve this problem, we can use both Dome flats (with high signal to noise ratio) and Sky flats (which more or less correspond to the real night sky illumination).