The Astromech ability is defined as anyone who has the ability to activate Astromech panels and use them. There are very few astromech droids. Also, an interesting tidbit, all astromech droids that aren't bounty hunters also have the Hover ability.
List of AstromechEdit
|R2-D2 | R4-P17 | R2-Q5 | IG-88 | 4-LOM|