Cleon. Difficult combination of factors. The worst experience I had with calibration was under similar circumstances and a similar approach - light pollution , shortish exposures, with a CLS-CCD filter and matching dark times to lights with ISO 400. I now use ISO 800, no filter and 3.5 minutes for lights darks and bias. Flats get separate treatment - ISO100, with their own bias frames. This lot calibrates fine.
I ordered an Astrodon Inside filter from Hap G to do the mod myself. Adding a cooling mod at the same time. The cold finger type. Sealed to prevent condensation in the wrong places. Very easy mod for the XS/1000D. Not game to try that with my 5D MKII.
