Here is out come of Karla and I working on the Horse head a few weeks back.
Caveats:
1 We did not get any color as I messed that part up.
2 No calibration, just align and stacking of the luminescence
I used Registack 6 for the first time and it was really slick (it amazes me the quality of free software that is available to our hobby)
Then I used the new NIK Photoshop tools which are also amazing and free!