Almost any telescope will work but as we are looking at stars down to magnitude 18, the bigger the better. Telescopes larger than say 150mm / 6″ are good. However, we do have useful data from telescopes as small as 50mm / 2″. These smaller scopes are particularly useful for measuring the brightness of the brighter stars, which get saturated easily when using bigger telescopes. Your mount should be accurate enough to track the stars to take exposures of several minutes without any trailing.