\item[\textbf{Form:}ADD dest {[other]} {[CONST=c]} {[BOX=B]} {[...
...ision by the mean of 'other', preserving the
mean of the source.}
These commands perform all arithmetic between images and spectra. There are three ways to use these commands.

Operations between two images or two spectra:

In this form, the command word is followed by the numbers of the buffers holding the images. Examples are
\item[ADD 1 2\hfill]{Add image in buffer 2 to image in buffer 1...
...ACT 3 4\hfill]{Subtract image in buffer 4 from image in buffer 3.}
Note that the first buffer listed is the one modified by the operation.

The FLAT word is used in the DIVIDE command to re-scale the division by the mean of the second image, thus preserving the mean of the first buffer. This is typically used when dividing raw images by a flat field. NOTE: The mean of the second image must have previously been calculated by the MN command!

Operations between images and constants:

In this form, there is only one buffer specification on the command line. The word CONST= (or C=) is used to specify the constant to be used.

Combined image and constant operations:

The first two forms of these commands may be combined, thus allowing you to simultaneously operate on two images and a constant.


In two-image arithmetic, only those pixels common to the two images are changed by the operation. The size of the destination image is not changed by the operation. The DR= and DC= keywords only apply to two-image arithmetic and specify row and column offsets between the two images. The sense of the shift is shown in the following example: dest(C+DC,R+DR) = dest(C+DC,R+DR) + other(C,R).

BOX limits the operation to those pixels in the destination image which are in the specified box.