The dark frames are acquired with the same exposure time and ISO setting as the light frames, and at approximately the same temperature. Therefore they are normally taken during an imaging session or at the end of the session. The purpose of dark frames is to capture spurious signal contributed by the camera electronics that is dependent on the exposure time, ISO setting and temperature, often called "dark current".
This dark frame is actually a composite of 10 dark frames taken on the same night as the light frame seen in this gallery, for four minutes at ISO 400 with the telescope's optical tube capped. The composite bias frame was subtracted from each of the 10 dark frames before they were averaged to produce this master dark frame.
You can see some "hot pixels" in this frame, and in the lower right corner is a red glow called "amp glow", a time and temperature dependent phenomenon caused by the camera's electronics and characteristic of most digital SLR cameras. The contrast of this dark frame was stretched slightly to make the defects more easily visible.