;+
; NAME:
;	ThomsonS10
; PURPOSE:
;	(Internal use only) Convert to S10 units
; CALLING SEQUENCE:
	FUNCTION ThomsonS10, DSun, APM
; INPUTS:
;	DSun	array; type: float
;				distance to Sun (in solar radii)
;	APM		array; type: float
;				apparent magnitude Sun at 1 AU
; OUTPUTS:
;	R		array; type: float
;				Conversion factor to S10 units. If F is a flux in units of the
;				flux incident from the solar disk at heliocentric distance DSun
;				then R*F is the same flux in S10 units.
; INCLUDE:
	@compile_opt.pro		; On error, return to caller
; PROCEDURE:
;	An S10 unit is `the brightness of one 10th magnitude star per square
;	degree of sky'. Replace the star by an electron at the same location
;	in the sky, and at distance S away from the observer. The electron
;	is radiating by Thomson scattering of sunlight.
;
;	The conversion is done in two steps:
;
; >	STEP 1: Convert to units of the flux -incident- at r0 = 1 AU from the
;		solar disk: multiply by (r0/DSun)^2
;		(M is an apparent magnitude as observed at 1 AU. This is why
;		1 AU is used, rather than the observer-Sun distance RObs).
;
; >	STEP 2: Convert to units of the flux -incident- at 1 AU from a 10th
;		magnitude star: multiply by 10^((10-M)/2.5). (Apparent
;		magnitude, M=-2.5*log(Flux)).
;
; MODIFICATION HISTORY:
;	JUL-1996, Paul Hick (UCSD)
;-

S10 = !radeg*DSun*!sun.RAu
S10 = 10^((10-APM)/2.5)/(S10*S10)

RETURN, S10  &  END