Celestia checks for DXT compressed texture support by looking for the extension GL_EXT_texture_compression_s3tc. I've seen this fail on some Linux platforms with NVIDIA Quadro 4xxx cards that definitely do support DXT compressed textures. Compressed textures loaded fine after removing the check. Simply getting rid of the EXT_texture_compression_s3tc test and assuming DXT compressed texture support could cause problems for users with very old hardware. There is hopefully some new and more reliable test that we can use.