Digital BW, The Print

I read in one of the groups (I think this one) about a method to 
reduce noise in night photography using digital cameras by taking a 
photo of the subject and then immediately taking a second photo at 
the same settings with the lens cap on.

The two exposures could then be processed in Photoshop to remove 
large artifacts from the first exposure using I believe layers.  I 
need to find this Photoshop process again.

Anyway, I could not get the archive search to return anything so I 
thought I would ask.

Thanks!
Bruce Varner