[avandonk]

Thanks all for the comments. There are three things that are working here. There could be more?

1. Camera in an environment of -10.0 C so less thermal noise .

2. The sensor temperature does not vary by more than 0.3 C so darks and lights are perfectly matched.

3. All corrections in ImagesPlus for darks, bias and flats done in 16 bit fits so all corrections are at the original pixel level not after interpolation. It is only after this is done that I convert to 16 bit tiffs by Bayer interpolation for subsequent stacking in RegiStar.

The other trick is to choose a starting frame for RegiStar somewhere in the middle of the set to be stacked. I then use IP (Local_ Smoothing and Noise Reduction_Multiresolution Smooth/Sharpen... )to get rid of any residual noise as this can affect alignment especially with short exposures. Of course this starting frame is not used for combination.

I thought I would just add that what I mean by original pixel refers to the real sensor elements that are mapped one to one to the fits image.

Bert