We can, or at least we can now after I ran one further test in `devel/external/hst/compare_whitening_subtraction.py`.  Basically I took the 92 COSMOS noise fields used to generate the `getCOSMOSNoise()` CF and then whitened them in turn, then looked a little at the ensemble statistics of the whitened field in the end results.  This comparison also gave me the opportunity to compare the effects of using either the COSMOS CF generated using mean subtraction, or generated without it.  We do get residual off-diagonal covariances, but these are typically 0.3% of the central variance (~2.5 decade supression), which I think is pretty good remembering that we're thinking in variances.

As this comment might get hidden in an "outdated diff", I might move the publication of results to the general discussion... Watch this space... :)