A Software tool to overlay two separately acquired images?

[Allison]

WS wrote:
> I want to overlay several scans of ECL films (same membrane, different
> exposures) which of course do not fit each other exactly. Each scan
> needs to be rotated a tiny little bit for a perfect fit. Is there any
> tool out there that can assist with this? ImageJ would be fine to
> rotate in sub-degree steps, but finding the right value might be quite
> tedious. Probably the problem can be squeezed in some sort of
> "minimizing the difference dependent on the rotation angle" algorithm.
> Before inventing the wheel twice, I thought I'd ask.
> 
> The individual scans are greyscale, the desired result is a color
> image where each scan has its own color, so the differences are easy
> to spot.
> 
> Best reagrds,
> 
> Wo
> 
You can use Photoshop.  Open the files with your scans.  Copy and paste 
Image1 into the file with Image2. You should have 2 "layers" now. 
Adjust opacity of the upper layer so you can see thru to the lower 
layer.  Then select the upper layer ("ctrl-A" for PC's), and go Edit -> 
Transform -> Rotate.  There will be handles on the corners and you can 
rotate it however much you want.
HTH
Allison