Pittsburgh Ballet Theatre will perform "Known by Heart ('Junk') Duet" and "Octet," both by Twyla Tharp, as well as "Raymonda Variations," staged by PBT artistic director Terrence S. Orr, with choreography by Marius Petipa at 7:30 p.m. tonight in the amphitheater at Hartwood Acres, and at 7 p.m. Friday in the Rotary Amphitheatre at Cranberry Township Community Park. Admission is free.Take lawn chairs or blankets to sit on. Visit the Web site, www.pbt.org, www.alleghenycounty.us/, or www.twp.cranberry.pa.us.