Even a very high quality lens will have trouble focusing the visible spectrum and H alpha without having a slight halo. This results in purple (red+blue) haloes around white stars and a red halo around yellow/orange stars.
Because of the H Alpha at best focus the red and blue channels are not focussed as well as the green channel with one shot colour sensors.
I used to do a star reduction on the blue and red channels to minimise these haloes. I find it is far better to do it to all the exposure stacks before I tone map the HDR image.
Here is an animated gif showing 500% crops of the HDR images with and without star reduction to the blue and red channels of the different exposure stacks.
1.6MB
http://d1355990.i49.quadrahosting.co...1_09/halo1.gif
Here is an animated gif of final HDR's. The old without any star reduction and the new method with star reduction to the blue and red channels of the different exposure stacks.
These images have the same colour balance!
2.7MB
http://d1355990.i49.quadrahosting.co...9/finals01.gif
This is why the dust in the Corona Australis image is now a nice dust brown.
See here
http://www.iceinspace.com.au/forum/s...ad.php?t=80837
Below are the two crops showing the halo reduction. The screen captures show the colour balance of the two images are the same.
Bert