[skysurfer]

I always use DSS to stack frames for an astrophoto and post process the result (Autosave.tif) with Photoshop. But recently I discovered that Photoshop (CS6 or CC 2.017), called PSD hereafter, is also able to stack CR2 frames.
In some cases (e.g. when using a short 85mm tele) the stack from PSD is even better than from DSS.

See attached 100% crops of the Crux constellation taken from 26 frames of 30 sec each with the Canon 6d + 85mm f/1.8 squeezed to f/2.5.
The center of the full image is slightly right from the center right of this crop.

But it lacks the possibility to apply flat frames. I tried with it by making a layer mask created from the flat and applying with 'Overlay' or 'Screen' in PSD but it does not help very much. Even when playing with the 'Opacity' slider or playing with the 'Curves' in the flat mask itself.

The excellent site 'clarkvision.com' from the veteran Photoshop astrophoto processing guru does not say a word about flat frames as he only uses camera lenses with a PSD (or Canon/Nikon) supplied lens profile which does the work a flat normally does.
But using a telescope and photographing in prime focus does require flats (and I have them, which work excellently in DSS).

My question is : How do I apply flat frames to the PSD stacked image when using PSD ?