Guys, have a look here:
http://www.iceinspace.com.au/forum/s...ighlight=layer
Photoshop 7 is the same as CS2 as far as the basics of handling layers is concerned.
I'm assuming you want to
average your images - that's what is described in the thread above. Registax also averages the stacked images. Software that handles deep sky images, like Deepsky Stacker, Registar, etc also gives you the option to
sum the images, which means that the fainter areas of your images will become brighter as you stack more images, and the bright stars will probably be saturated or clipped.
Depending on what you want to achieve with your final image will determine which method of stacking you use. For planetary and well exposed images average is usuall the way to go. For faint nebulae, you may want to sum the images when they are stacked to bring out the faint detail.
Al.