The subtraction of the bias level is the first step in the data reduction. This bias level is introduced onto the CCD chip for ensuring that the chip is working in a linear regime.
You can subtract the bias level according to one of the following proceedings:
i) Get the mean value on the overscan region of each frame and subtract this constant off the whole rest of the frame. The task to see the profiles along the vertical and horizontal axis is implot (from the plot package) and determine the overscan region, Once done this, you can use ccdproc (load noao, imred and ccdred) for sustracting the bias level.
ii) Average a sample (around 10 or more) of bias frames (frames taken with zero exposure time) pixel by pixel in order to get a mean bias frame, and subtract this frame from each image. The task to do it is zerocombine (previously, from noao, imred and ccdred packages). Then, with ccdproc you can sustract the bias level.
In practice, it can be used the first way for subtracting bias level to INTEGRAL images (see Appendix I).