function acosh, x
;+
; NAME:
;     ACOSH
; PURPOSE:
;     Return the inverse hyperbolic sine of the argument
; EXPLANATION:
;     The inverse hyperbolic sine is used for the calculation of acosh 
;     magnitudes, see Lupton et al. (1999, AJ, 118, 1406)
;
; CALLING SEQUENCE
;     result = acosh( x) 
; INPUTS:
;     X - hyperbolic sine, numeric scalar or vector or multidimensional array 
;        (not complex) 
;
; OUTPUT:
;     result - inverse hyperbolic sine, same number of elements as X
;              double precision if X is double, otherwise floating pt.
;
; METHOD:
;     Expression given in  Numerical Recipes, Press et al. (1992), eq. 5.6.8 
;
; REVISION HISTORY:
;   MODIFICATION HISTORU FOR ASINH
;     Written W. Landsman                 February, 2001
;     Work for multi-dimensional arrays  W. Landsman    August 2002
;     Simplify coding, and work for scalars again  W. Landsman October 2003
;   MODIFICATION HISTORU FOR ACOSH
;     Modified from asinh, Leslie Young, Aug 2006
;-
 On_error,2

 xx = 1.0 > x

 y = alog( xx + sqrt( xx^2 - 1.0) )

 return, y

 end