CS3 - Use existing image as an Alpha channel within another image?

Greetings! I wonder if anyone may shed some light on this issue for me.

I have 2 x *.dds files. One is a default model texture (with no alpha channel) and the other is a lightmap, which has different visible graphics to that of the texture file. However, the alpha channel of the lightmap uses the texture file detail.

My question, as I need to recreate these from scratch, how do I import an existing image to another existing image and use it as a alpha channel. So far each time i have tried the original information is overwritten.

Here's a screengrab of what I'm trying to achieve. To recap, I'm trying to use image 1 as an alpha channel in image 2 (as shown).

< http://i149.photobucket.com/albums/s64/danlisa_photo/Alphaqu estion1.jpg>

Any help would would be much appreciated.

I'm assuming that the images are the same pixel dimensions. If the second image is grayscale, you can load a selection from the first, choosing the second as a source, then save the selection as a channel in the first image.

I'm not sure what you're expecting the alpha channel to be, if the source for it is a color image.

Funny this was just shown on photoshopTv. Create a new alpha channel in the first image, then copy and paste the second image into the new alpha channel. While the alpha channel is selected you can use free transform to scale the image if necessary.

Thanks to you both. I shall try both methods.

@ Tim

The image labelled 1 will be grey scaled and then placed as the alpha channel on image 2 (which is a lightmap for a 3d model). The end result will mean that when rendered within a 3d environment the newly created alpha channel will be used as a specular/glossiness control for that model.

@ Silkrooster

Typical! I shall be more vigilant.

Many thanks again.
Regards