The cosmic wordplay is strong with this one! The joke hinges on the brilliant double meaning of "Count" - both a vampire title and what astronomers do with celestial objects. One astronomer sees a creepy castle silhouette against the purple night sky and warns it's Count Dracula's castle, but the other, peering through a telescope, simply asks "What guy?" because at astronomical distances, no individual would be visible!
It's the perfect collision of horror tropes and astronomical reality. No matter how powerful your telescope, you're not spotting any bloodsuckers from that distance - just their massive cosmic castles. Even vampires can't escape the limitations of angular resolution!