Firstly I loaded the light image, that is the photo containing the main subject.

Then I had to open the dark image, that is the photo I took with the cap on the telescope

The third stage was composed by three steps:

1. Selecting the dark image
2. Cutting it
3. Pasting it as a new layer over the light image
   

The final stage involved a change of method of handling layers: from the layer palette I just switched from Normal to Difference. Then, the 70-80% of the noise magically disappeared!

To improve further the quality of the image, I decided to take another light image of the cluster, then I subtracted the same dark image, finally adding this second (cleaner) image to the first one, using the second layer - the one at the top - in overlay mode and manually aligning it, if necessary, with the bottom layer. The result was excellent: the noise was reduced by over the 95%, leaving an image with almost no other defects caused by the CCD.

The original 30-second light image of the globular cluster

This is the quality obtainable after the dark image subtraction

...And this is the situation after adding 2 dark-subtracted images

Of course, the method worked with the nebula image too...

So, I decided to test it on some more traditional images. I mounted the Nikon D1 on a tripod and made a couple of exposures of the lakeside, distant about 2km (1.25 miles) from my house. The camera was still set at ISO 1600; I decided to use the lens closed at f/20 and to make a 10-second exposure with the zoom at 200mm and a 20-second exposure at 130mm.

Immediately after, I shot two other exposures (10 and 20-seconds) with the cover on the 80-200 Nikkor lens. Then, after another quick processing with Photoshop (subtracting the dark image from the light one), I found the results quite satisfaying, as you can see on this page.