As the recent coupling between the Smithsonian Institution and Showtime Networks continues to roil the documentary film world, more than 215 filmmakers, television executives and academics have signed a letter demanding that the Smithsonian, a publicly financed museum, not only reveal financial details of the joint venture but also abandon it. The signers of the letter, delivered yesterday to a Smithsonian official, include the filmmakers Michael Moore ("Fahrenheit 9/11"), R. J. Cutler ("The War Room") and Alex Gibney ("Enron: The Smartest Guys in the Room"); the actress and writer Anna Deavere Smith ("Twilight: Los Angeles"); the law professor Lawrence Lessig; and Jacoba Atlas, a senior PBS executive. Lorne Manly reports.