Thanks Greg. The SONY NEX-5 supports auto ISO in mode A, Aperture priority but that's the one I'll be using for the corona. Will check the Pentax *istD. I did a couple of transitions from day to night. What I did was stay next to the camera and bump the ISO when it got dark then later keyframe and interpolate the transitions in LR Timelapse.
There are a couple of examples
here.