How to get a neutral black sky in photoshop

[Moonhawk]

1. Open your image

2. Invert it to obtain the negative - sky will turn off white if you have a cast

3. Select the "Remove Colour Cast" tool - and click the eye dropper on a blank part of the sky. The sky will turn white (or pretty close to it).

4. Invert the image again.

5. Balance curves, contrast and brightness.

I found this worked quite well on images obtained with my toucam - it not only removed the colour cast from the sky allowing me to balance it to true black - it also removes the same colour cast in the rest of the image. On planets it can give a more natural appearance.