How to configure Celestia to work with a spherical mirror projection on a dome
According to Paul Bourke's page on screen capture


  While only the commercial version of Celestia supports fisheye, it also does not support warping of fisheye (although some versions have been created that do). The solution outlined here has been tested successfully with a fisheye supported version of Celestia.


It seems that Celestia in its free version is not providing any support for dome projection and the commercial version requires tweaking before you can use its support for fisheye projection and convert it to Spherical Mirror Projection.