11 O-Nebs, background (er, foreground) sky removal The wide view of Orion's dominant nebulas were composited into an HDR image, in an EXR half-float format.  In many such wide views, the light from the sky sets the lower limit on what can be seen.  This light level is far above the Canon 20D sensor noise level, so there is not much reason to try and subtract any fixed pattern or dark current offsets.  Noise from the background sky level is photon noise, the Poisson statistics of detecting them is proportional to the square root of the signal photon count. Further, the background level is usually nonuniform.  The method shown here will simply add the exposures together, weighted by their exposure factors, ignoring the fact that there is a background sky illumination.  Later, the black normalize method shown earlier removes the sky gradient. Here is the normalized view of the HDR image (full scale count of 65535 in the 8-minute exposure is set to 1.0). After the black normalize operation: And after converting to 16-bit (reduce exposure by 4-stops first), apply gamma=4 and an S-curve to bring out the detail without clipping the bright sections: For comparison, here is the single 8-minute exposure after removing the sky background (via offsets), applying gamma and S-curves, attempting to show the same detail level.