;+
; NAME: cube_getwaveaxis 
; PURPOSE: return which axis of a cube is the wavelength dimension
; INPUTS:
; KEYWORDS:
; OUTPUTS:
; 			The index for the axis of the cube that is wavelength. 
;
; 			NOTE: this is ZERO-INDEXED. So the first axis is 0 and the second is
; 			1, etc.
;
; HISTORY:
; 	Began 2007-07-16 17:41:43 by Marshall Perrin 
;-

FUNCTION cube_getwaveaxis, cubestruct

	if ~(keyword_set(cubestruct))  then message, "Missing argument!"
	if tag_exist(cubestruct, 'astr') then astr = cubestruct.astr else extast3, cubestruct.header, astr
 	; allowable spectral coordinate type codes. Griesen et al. 2006 Table 1
  	wave_axes_types = ["FREQ", "ENER", "WAVN", "VRAD", "WAVE", "VOPT", "ZOPT", $
    	"AWAV", "VELO", "BETA"]
  	for i = 0,n_elements(wave_axes_types)-1 do begin
    	axis_lambda = where( strmid(astr.ctype,0,4) eq wave_axes_types[i], lambdact)
    	if lambdact eq 1 then break
  	endfor
  	; if still nothing, then complain
  	if lambdact eq 0 then message, "Did not find unique wavelength axis!" else axis_lambda=axis_lambda[0]

	return, axis_lambda
end