Image data

In principle, you can simply divide your image by the appropriate flat-field and use the result. For example:

   ICL> idiv image=mydata image1=ff output=flatdata

In practice, you really don't usually want to divide your data by the (typically) very large numbers found in flat-fields. Ideally, your flat-field should first be reduced to numbers around unity. The easiest way to do that is to divide the flat-field by its mean value, and the easiest way to find the mean value is with `istat'. The command

   ICL> istat image=ff reset accept

will give you the mean value over the whole image. If your flat-field has strange effects round the edges, you may prefer to limit the range of x and y values used by `istat'. For example, if your image is 800 by 800,

   ICL> istat image=ff ystart=100 yend=700 xstart=100 xend=700 accept

will only look at the central 600 by 600 part of the image. `istat' prints out the value of the mean, and you can then divide the flat-field by that. For example, supposing the mean value were 350, the command

   ICL> icdiv image=ff factor=350 output=ffdiv

will generate a new flat-field (ffdiv) that will have a mean value around unity, as desired. You may find it convenient to know that `istat' sets its output parameter `stat_mean' to the mean value. So you could use

   ICL> istat image=ff ystart=100 yend=700 xstart=100 xend=700 ~
        stat_mean=(x) accept
   ICL> icdiv image=ff factor=(x) output=ffdiv