Just adding to what Steve has written, for darks in particular, I would recommend starting off the “easy” way; let the 40D do the Dark Frames automatically for you, in-camera.
The 40D has several “Custom Function” settings where you can tailor the 40D to how you want to operate.
“Custom Function II -1” or C.Fn II-1 is for managing “Long exposure Noise reduction” and has 3 values:
- 0: Off (the camera does not perform any “In Camera Noise Reduction” ICNR).
- 1: Auto (the camera will automatically take an in-camera Dark for exposures longer than 1 second IF it detects noise)
- 2: On (the camera will automatically take an in-camera Dark for ALL exposures longer than 1 second)
So, if you set C.FN II-1 to “2”, put the 40D on a tripod and take a 30 sec exposure of star trails, when the 30 sec “light exposure” has been taken, the 40D will immediately take a 30 second Dark Frame and subtract it from the preceding 30 sec Light Frame, in-camera.
This means that your 40D will effectively be tied up for 60 seconds. This is ICNR.
It is important to expose your Dark Frame for the same duration and at the same operating temperature as the Light exposure. ICNR achieves these 2 goals.