i would recommend lifting saturation to 100%.
The gain higher and then compensating with the exposure.
It may look a little noisy with higher gain, but registax sorts it out
also focus with the gamma at zero, but lift it to say 10%.
I think the blue fringing at top and red at bottom are due to differential atmospheric refraction. It used to annoy the heck out of me in my early images and then I discovered the RGB alignment tool in Registax. It does a great job of removing this in one easy step. Give it a try.