The late photographer Raeburn Flerlage is well known for the stunning images he took of blues musicians. But when the University of Chicago Folk Festival turns 50 this weekend, it will have him to thank for capturing the intimate moments of its early years. The festival draws from folk music of every stripe. It has featured guests ranging from Ralph and Carter Stanley to the Staples Sisters and Willie Dixon. Raeburn Flerlage's photos are now collected in the book Chicago Folk: Images of the Sixties Music Scene. Alison Cuddy speakeswith the co-editors Bob Riesman and Ronald Cohen.