Please help me mask this

[button]

My goal is to use photoshop CS3 to replace the sky in this pic with that taken from another pic (this is only a small part of a mosaic panorama).  This seems sort of like masking hair, but the lack of a white background has thusfar foiled my attempts.  I've tried using color selection to no avail.  I know- the noise doesn't help.  Anyone with any tricks?  I don't want  to spend hours with the magic wand or pen tool.  

Thanks,
John

[Harry Carpenter]

With the image open you could look at the channels and choose the one with the most contrast, (probably red or green) Duplicate it, adjust its levels and then load it as a selection. It would still need to have quite a lot of manual correction as the building at the bottom left is the same colour as the sky.
The best book I have found on this is Scott Kelbys Photoshop Channels book
Good luck.

[David Good]

After you have run a levels adjustment on the duplicated channel, you could also use the Image-Apply Image command to further refine the mask. Experiment with the different blending modes (Multiply, Overlay, Linear Light etc.), and run it several times if necessary. Image-Calculations right from the composite channel can work very well too. For this image you may have to combine a couple of masks, one for the tower, one for the foreground. Katrin Eismann and Martin Evening both have excellent books and articles on this subject.