Opacities are stored in the opacity stack
opac.OpacStack[NOPSV]
lives in opacity.h

Opacities are created in OpacityCreateAll.c

opacity addressing
	OpacityCreateAll sets a series of opacity offset indices, that must be used to address opacities.
	If an opacity source has an offset ipOff, and a threshold n0, then the energy n,
	which is n-n0 beyond the treshold, will have an associated opacity of
	OpacStack[n-n0+ipOff-1]

	if n- is set with ipoint then the first energy cell is n0=1, and the offset is
	n-n0+ipOff

following provides way to obtain any ionic cross section from opacity stack 
/* function to facilitate addressing opacity array */
double csphot(
	/* INU is array index pointing to frequency where opacity is to be evaluated
	 * on f not c scale */
	long int inu, 
	/* ITHR is pointer to threshold*/
	long int ithr, 
	/* IOFSET is offset as defined in opac0*/
	long int iofset)

