Flat fields can taken both through the fibres and directly. Ideally one would obtain just fibre flats, which would enable the usual pixel-to-pixel sensitivity variations to be removed as well as allowing for order location. In practice, the drop off in sensitivity towards the edges of orders, and the lack of sufficient blue light from the flat field lamp, means that one cannot get a single well exposed flat field for all orders. To get sufficient counts necessitates over-illumination in the redder order.
The TEK6 CCD suffers from contamination of material, which is manifested either as obvious black spots with a `penumbra', or smaller a `salt & pepper' pattern. To a certain level these gross effects can be removed by taking `camera flats', which are just flat fields taken directly by illuminating a sheet of paper. Although the wavelength dependency is not taken account of, these flats certainly remove cosmetic defects to a first order.