Software for image processing
Siril
Siril was meant to be Iris for Linux. The new version is coming out soon. It has been updated to work with GTK+-3.4.
GIMP astronomy plugins
Website. Arch Linux AUR package.
These plug-ins for Gimp intend to be useful for astronomical image processing. They implement all basic functions for astronomy image processing, for GIMP's 8-bit images.