With the new 'gal_wcs_coordsys_convert_points' that was added in de51acf8cf1, this task becomes much more easier (to use the same three Euler angles that WCSLIB uses for converting to-from different coordinate systems).

Also, while looking into this, I noticed that SOFA (Standards of Fundamental Astronomy; published by IAU) contains the three angles from Galactic to ICRS in its g2icrs.c file:

P = 192.85948    right ascension of the Galactic north pole in ICRS
Q =  27.12825    declination of the Galactic north pole in ICRS
R =  32.93192    Galactic longitude of the ascending node of
                 the Galactic equator on the ICRS equator

For such constants, it may be good to write a small program that uses SOFA during bootstrapping to build all the constants we need for coordinate conversion in Gnuastro (just a brainstorm now).

As of Gnuastro 0.19, we still don't recognize the "International Celestial Reference System" or ICRS (which is the IAU approved coordinate system; unlike FK5; that we do support!).

This stems from the fact that in lib/wcs.c we have used NED for the conversion of coordinate system information and NED doesn't have ICRS!

We should find the conversions (possibly from the link above, or other places) and implement them in the WCS library because many surveys like SDSS, J-PLUS or J-PAS use this equatorial coordinate system.

You can reproduce the problem like below:

$ sdssurl=https://dr12.sdss.org/sas/dr12/boss/photoObj/frames
$ wget $sdssurl/301/3716/6/frame-r-003716-6-0117.fits.bz2 -Or.fits.bz2
$ bunzip2 r.fits.bz2
$ astfits r.fits -h0 --wcscoordsys=eq-j2000
astfits: gal_wcs_coordsys_identify: the 'ICRS' value for 'RADESYS' is not yet implemented! Please contact us at bug-gnuastro@gnu.org to implement it