Number of independent wavelengths used in the computation of the mean
spectral resolution.  The values trended here are for the blue sensitive EEV
detector, the  300V grism
with no filter and a slit width of 0.29".

More information can be found on
the <a href="http://www.eso.org/observing/dfo/quality/FORS2/qc/lsswlc/lsswlc.html">plot tutorial page</a>