A Fortran-like arithmetic expression giving the variance value
to be assigned to each pixel in terms of the variable DATA,
which represents the value of the corresponding data array
pixel. For example, VARIANCE="DATA" implies normal
error estimates, whereas VARIANCE="DATA + 50.7"
might be used if a sky background of 50.7 units had previously
If a `null' value (!) is given for this parameter, then
no new VARIANCE component will be created and any pre-existing
variance values will be erased.