I've obviously missed something. So to try to figure out what it is, I'm going back to the old pipeline... again... in map_ncdf_reading, lines 441-448, there is something curious that goes back to a definition-of-variables problem: ddec and dra are ADDED to ra and dec to get the new "ra_all" and "dec_all" variables. ddec and dra are calculated from eaz and eel: ERROR OFFSETS in Az and El. Why? What?! I added a new piece of code, correct_eaz_eel.pro. It is extremely short, but extremely necessary.

pro correct_eaz_eel,ra,dec,el,az,eel,eaz,pa    dra=-eaz*cos(!dtor*pa)*cos(!dtor*el)+eel*sin(!dtor*pa)    ddec=eaz*sin(!dtor*pa)*cos(!dtor*el)+eel*cos(!dtor*pa)    dec += ddec/3600.    ra  += dra/3600. / cos(!dtor*dec) / 15.end

This comes back to the fact that I don't know what ANY of the variables in the NCDF header are supposed to be. Why are "error" variables actually OFFSET variables, and why didn't anyone know about them?