FUNCTION tki_focuspos, lambda, n, r, f0 
;

;#############################################################################
;#
;# Function name    : TKI_FOCUSPOS  
;#
;# Author           : Dietmar Germerott
;# Language         : IDL
;# Purpose          : Calculate new position for focus MC&Grating
;# Creation date    : 30-APR-1996
;# Update date      : 22-MAY-1996
;# External modules : 
;#
;# Call Sequence    : VAR = tki_focuspos (lambda) 
;#
;# Arguments        : lambda  -  Wavelength of spectralline
;#                    n, r, f0
;# Description      : 
;#
;#
;#############################################################################

@tki_common_priv                ;/* INSERT TKI COMMON VARIABLES */

;/* GET VALUES FROM GLOBAL PARAMETERS */

   a = environ(139).val
   b = environ(140).val
   mc6stepsize = environ(133).val


   sintheta = lambda * n
   costheta = SQRT(1.0 - (sintheta * sintheta))
   f = r / (1.0 + costheta)

   delta_f = a + b * ((1600.0 - lambda) / 800.0)
   f = f -delta_f

   f = f - f0

   IF (mc6stepsize LE 0.0) THEN mc6stepsize = 0.0125

   newpos = LONG ( f / mc6stepsize )

   RETURN, newpos

END


;---------------------------------------------------------------------------
; End of 'TKI_FOCUSPOS.PRO'.
;---------------------------------------------------------------------------