Replacing the forground in a timelapse using photoshop?

[batema (Mark)]

Hi.
At Ayers Rock there is a display called the field of lights. Initially I was concerned about this from a person who moved and works here for the dark sky but it is very subtle and I have grown to like it actually.
I have done a lot of photos and have tried doing timelapses of the stars rising above the lights but the lights subtly change every 15 seconds and my timelapse obviously does not reflect this.
I am trying to workout at 25 photos a second if I could change the rapidly changing colour in the forground to one colour of lights for say 375 images in a sequence so I still get the beautiful movement of the stars but keeping the subtle changes of the lights so they look like they do in real life. I have done batch processing before but nothing like this so if anyone has any advice I would really appreciate it.
Thanks
Mark.

[Stonius (Markus)]

How are the exposures? Can you get the lights*and the stars at the same time with good exposures? My first impulse would be to shoot for the lights in one pass, then get a second round with good stars after the light show is over and composit both passes together in after effects using overlay or similar blending mode.

But I guess with the lights changing every 15s, that limits your exposures to less than 15s which isn't going to get you a whole lot of star detail, sooo as I said, I'd do two passes. As a friend of mine once said, it's a little lie in the spirit of the truth. :-)

Cheers

Markus

[Atmos (Colin)]

The only way I can think of (not being very PS savvy) would be to cut the lower part of your favourite "lights" and then paste it into all of the others. This assumes that there isn't any movement between subs.