This post is to remind me, the next time I go looking, how the hell to convert from a GILDAS CLASS fits spectrum (created by fits write blah.fits /mode spectrum) to a FITS-compliant spectrum. First, remember the FITS-WCS spectral definitions: http://www.aanda.org/index.php?option=com_article&access=bibcode&Itemid=129&bibcode=2006A%2526A...446..747GFUL And the peculiar CLASS definitions: http://iram.fr/IRAMFR/GILDAS/doc/html/class-html/node84.html Key points: CLASS stores the CDELT parameter as DELTAV in m/s instead of km/s and the velocity offset of the spectral frame in VELO-LSR also in m/s. Things to set: CTYPE = VRAD SPECSYS = SOURCE SSYSSRC = LSRK VELOSYS = frame velocity (VELO-LSR or CRVAL1) This information is subject to change...