If you just want to work with a single fits file then you can use Fits Liberator to convert the file to a tif for editing in PS:
http://www.spacetelescope.org/projec...download_v301/
Otherwise stack multiple fits in DeepSkyStacker and save the final image as a tif for editing in PS:
http://deepskystacker.free.fr/english/download.htm
Both those programs are free and easy to install.