First it is necessary to remove the instrument signature from the image data. This task is sometime called data reduction,The procedure is to a large extent similar to that followed when reducing astronomical CCD image data [for example Massey, 1992]. In most cases the reduction is carried out first by removing additive effects, and then proceeding with multiplicative effects.

#### 1.Bias removal

The simplest way to remove the bias is to subtract an image obtained with zero integration time (usually called bias-frame or zero-exposure). However as the bias-frame is subject to(从属) the same read noise (Section 3.1.7) as the object-image, this will generally increase the noise by a factor of $\sqrt{2}$. Preparing a bias-correction image by averaging many bias-frames together reduces this problem. As the bias varies over time it would severely impair the temporal resolution if a large number of zero exposures needed to be taken at regular intervals. To remedy this most CCDs are equipped with some extra pixels at the edge of each line. These pixels are shaded from light and thus provide bias information for each line for each image read-out. In the case of the ALIS imager, there are bias-pixels (also called reference pixels, or overscan-strip) for each line of each quadrant on the CCD as indicated in Figure 4.1. For each pixel, the bias (or DC-level), $\mathit{B}_{ij}$, can be expressed as follows: 没有足够的时间去了解更多了，暂停对平场矫正的研究，交给张荣来实现： 1.http://www2.irf.se/~urban/avh/html/node21.html 2.file:///C:/Users/Administrator/Downloads/FlatFieldCalibrationofCCDDetectorforLongTraceProfilers.pdf