;+
; NAME: airmass
;
; PURPOSE:
; 	Compute airmass vs. zenith distance.
;
; ALGORITHM:
; 	Formula from John Huchra'a Harvard Ay192 notes, 1999.
;
; INPUTS:
; 	zd	zenith distance, in degrees
; KEYWORDS:
; OUTPUTS:
;
; HISTORY:
; 	Began 2004-10-18 19:36:56 by Marshall Perrin 
;-

FUNCTION zd2airmass,zd

	zdr = double(zd)*!dtor
	A = (1.0d/cos(zdr)-1.0)
	X = 1./cos(zdr) - 1.8167e-3*A - 2.875e-3*A^2 - 8.083e-4*A^3
	return,X

end