A motorized mount, aligned with the Earth's axis, that turns the camera with the sky so stars stay sharp points during long exposures instead of streaking into trails. Camera-sized versions are sold as star trackers and typically cost several hundred dollars.
Similar to the 500 rule; but this is used for astronomical photography, giving the longest shutter opening time without creating star trails. In this case it is vital to have the camera on a tripod. To do this divide 600 by the focal length of the lens to get the shutter speed i.e. with a 60 mm lens, 600/60 the shutter can be opened for up to 10 second.
A specialist astronomy camera whose sensor is refrigerated far below freezing while it works. The cooling silences the sensor's own noise, leaving it many times more sensitive than an ordinary array: blacks come out truly black and faint stars stay clean. Only a few companies make them, and they are very expensive.
Photography after sundown, which is less about darkness than about balance. The best night shots are usually made before total darkness, while the sky still carries light to weigh against the lamps and windows. Once the sky goes fully black, lights burn out against nothing and the picture loses its shape.