To compute an accurate *K* correction, one needs an accurate
description of the source flux density
*f*_{}(), the standard-source flux
densities
*g*^{R}_{}() and
*g*^{Q}_{}(), and the
bandpass functions *R*() and
*Q*(). In most real
astronomical
situations, none of these is known to better than a few percent, often
much worse. Sometimes, use of the AB system seems reassuring
(relative to, say, a Vega-relative system) because
*gAB*_{}() is known (*i.e.*, defined),
but this is a false
sense: In fact the standard stars have been put on the AB system to
the best available accuracy. This involves absolute spectrophotometry
of at least some standard stars, but this absolute flux information is
rarely known to better than a few percent. The expected deviations of
the magnitudes given to the standard stars from a true AB system are
equivalent to uncertainties in
*gAB*_{}().

The classical *K* correction has
*Q*() =
*R*() and
*g*^{Q}_{}() =
*g*^{R}_{}(). This
eliminates the integrals
over the standard-source flux density
*g*^{R}_{}(). However, it
requires good knowledge of the source flux density
*f*_{}() if
the redshift is significant. Many modern surveys try to get
*R*() ~ *Q*([1 +
*z*]) so as to weaken
dependence on *f*_{}(),
which can be complicated or unknown. This requires good knowledge of
the absolute flux densities of the standard sources if the redshift is
significant. This kind of absolute calibration is often uncertain at
the few-percent level or worse.

Note that if equation (2) is taken to be the
definition of the *K* correction, then the statement by oke68a
that the *K* correction "would disappear if intensity measurements of
redshifted galaxies were made with a detector whose spectral
acceptance band was shifted by 1 + *z* at all wavelengths" becomes
incorrect; the correct statement is that the *K* correction would not
depend on the source's spectrum
*f*_{}().

It is a pleasure to thank Bev Oke for useful discussions. This research made use of the NASA Astrophysics Data System, and was partially supported by funding from NASA and NSF.