  This is the second of three papers describing an `absolute' calibration of
the GONG magnetograph using an end-to-end simulation of its measurement
process. In the first paper, we described the GONG instrument and our
`end-to-end' simulation of its measurement process. In this paper, we consider
the theory of calibration, and magnetograph comparison in general, identifying
some of the significant issues and pitfalls. The calibration of a magnetograph
is a function of whether or not it preserves flux, independent of its spatial
resolution. However, we find that the one-dimensional comparison methods most
often used for magnetograph calibration and comparison will show dramatic
differences between two magnetograms with differing spatial resolution, even if
they both preserve flux. Some of the apparent disagreement between magnetograms
found in the literature are likely a result of these instrumental resolution
differences rather than any intrinsic calibration differences. To avoid them,
spatial resolution must be carefully matched prior to comparing magnetograms or
making calibration curves. In the third paper, we apply the lessons learned
here to absolute calibration of GONG using our `end-to-end' measurement
simulation.
