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It is flat-fielding which has the most dramatic effect on image quality. There are four features of Fig 5a with which it deals - (i) the large- scale ``warp'' or sensitivity variation across the chip, (ii) the pixel-to- pixel (short-scale) sensitivity variations, (ii) the ``black holes'', regions of reduced response a few pixels across, and (iv) the ``walnut- grain'' produced by interference fringes.
It is a property of thinned chips, for which the distance between the upper surface of the substrate and the electrode structure is only about 10 wavelengths. Narrow lines in the optical spectrum of the incident light - night-sky lines in particular - produce strong interference patterns, with amplitudes which may reach 5 % of background signal. Even when illuminated with broad- band white light, some thinned CCDs manage to produce self- fringing, Newton's rings. When narrow-band filters are used, thinned chips can produce complex sets of overlapping fringe patterns from a white-light source, and as a result their use in narrow-band photometry of extended objects is limited.
All these four phenomena may be considered as gain variations; all may be removed by pixel-to-pixel gain calibration. The process is known as flat- fielding; the array is divided by a calibrating array. Flat fields are such calibrating arrays, CCD images obtained through the appropriate filter of a uniformly illuminated background, usually the dimly-lit interior of the telescope dome, or the twilight sky. The latter is the better: colour-matching is otherwise a problem. The strong dependence of the large-scale response on wavelength means that its removal requires illumination for flat fielding by light of the same spectral response. The twilight sky is the obvious choice. A second reason for choosing the twilight sky is the removal of the fringe pattern, which is usually dominated by interference from the strongest sky lines (implying that the pattern is much the strongest in broadband red and far-red). The relative strength of the fringe pattern may differ in the twilight sky, so that although the structure of the pattern is the same, the amplitude is not. This difficulty may be met by extracting an array describing the fringe pattern from the sky flat-field frames, scaling this to the amplitude of the fringes in the image, and subtracting.
It follows that the optimum flat-fields are obtained on the dark sky, close in time and position to the observation. But sacrifice of observing time is required, and stars provide a second difficulty. It is almost impossible to find sky patches in which lengthy flat-field exposures will not be ``spoilt'' by stars in the frame. One possibility which has been explored with some success is to defocus the telescope drastically. In general, however, a combination of dome and twilight sky flat-fielding techniques is adequate, and may be made to provide overall calibration, large-scale, pixel-to-pixel, and pattern, to 0.5 % of the background.