CIAO

Astronomical software available from ​http://asc.harvard.edu/ciao/

version ​3.3 has just been released.

dkg has done ​a bit of research into the licensing, which appears to be mostly free. However, there appear to be some external dependencies that may be more complicated. from ​the 3.3 README:

To build the software you do not need the binary distribution, however
you will need the other three distributions (aux+data, ots, and source).
If you have installed the binary distribution, you should remove the smaller 
ots subtree that was included within it, though if you forget to do this first 
don't worry -- it's not fatal, but tar may complain or give minor permission 
warnings when overlaying the new, larger OTS onto the previous subtree.
You must download/unpack the source, auxiliary, and OTS CXC distributions
(cxc_src.tar, cxc_aux.tar, and cxc_ots_<arch>.tar respectively) into the
same directory, following the same general instructions as given in (II). 

Due to licensing agreements we are unable to distribute the sources for the 
sm library.  The sm libraries in binary form can be downloaded separately if 
you are rebuilding from the source.  We are also unable to distribute the 
FFTW library.  Users who can comply with the GNU General Public License can 
download FFTW from http://www.fftw.org.  Finally, we cannot distribute parts
of the Sherpa Optimization source and pre-compiled libraries.  Due to this 
restriction users will be unable to recompile the sherpa application, and 
the mkrmf, vtpdetect, and dmfilth tools.

the biggest concern appears to be library versioning. (also from the readme):

Q) I cannot run application XYZ in a window setup for CIAO.
A) This is likely due to the fact you other application requires a
slightly different version of libstdc++.so.5. Until recently the gcc
c++ standard libraries have been in flux and are NOT cross
compatible. CIAO was built using gcc 3.2.1 and we require that exact
version of the library to be available. At this time we do not have a
work-around for this issue. 

libstdc++5 is undergoing a lot of changes anyway, so ideally, we will be able to build it ourselves with an up-to-date toolchain.