production, Berman served as executive producer for the series's entire run. Piller initially served as second executive producer and showrunner, but left the series in 1995 to manage Star Trek: Voyager. Writer Ira Steven Behr was promoted by Berman to replace Piller as showrunner and held that role for the remainder of the series. In addition to Berman, Piller, and Behr, key writers included Robert Hewitt Wolfe, Ronald D. Moore, Peter Allan Fields, Bradley Thompson, David Weddle, Hans Beimler and René Echevarria. Several actors also directed episodes, including Avery Brooks who directed and starred in the critically acclaimed episode "Far Beyond the Stars". DS9 was the first television series in the Star Trek franchise to use computer-generated imagery (CGI) for exterior space shots. Although a few other television series, like Babylon 5, were using CGI exclusively to avoid the high expense of model photography, the Star Trek franchise had continued to use primarily physical models for exterior space shots because producers decided models provided more realism. DS9 continued using models where needed, such as the Season 4 premiere and the alternate-universe episode "Shattered Mirror". During the battle sequences between the Federation and Klingon fleets, the effects department used Playmates toys, Ertl model kits and Hallmark Bird-of-Prey Christmas ornaments in the background in an effort to keep production costs down. The USS Defiant was the first full-fledged starship in the Star Trek franchise to have a CGI model used in regular production. It was first built and animated by VisionArt, which was responsible for the morphing of Odo. The CGI Defiant was featured heavily in the Season 4 episode "Starship Down", where it battled a CGI Jem'Hadar ship in a CGI gas giant's atmosphere. The series started using Foundation Imaging and Digital Muse in 1997 (Seasons 6 and 7) for its effects as part of the ongoing storyline of the Dominion occupation of the station. The station itself remained a physical model throughout the series' seven-year run except for the final scene of the series. In October 2006, the physical model of the station was sold for $132,000 in an auction at Christie's auction house in New York City. The opening sequence was likewise modified in the fourth season, most notably by the introduction of CGI inserts of construction work being performed on the station's exterior by suited maintenance crews, and more docking and launching activity by ships, along with subtle