sig2posS.c
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	#include<stdio.h>
	#include<math.h>
	#include<fonction.h>
	#include<constant.h>
	#include<dimension.h>
	#include<structure.h>

	/****************************************************************/
	/* nom: sig2posS */
	/* auteur: Ghislain Golse */
	/* date: 10/99 */
	/* place: Toulouse */
	/****************************************************************/
	/* Return the sigma in the source plan corresponding to sig2pos.
	*
	* The error image plane (1sigma) is converted in etendue (assuming
	* circular seeing AND FLAT brightness profile cf Kneib1993 Eq2.97, Eq2.91).
	* etendue_{image plane} = 2. * sigma_pos^2
	* etendue_{src plane} = A^-1 etendue_img (cf Kneib1996 Eq24)
	* sigma_{src plane}^2 = etendue_src / 2.
	*
	* Parameters :
	* - sig2pos : position error for the I.n_mult systems
	* - n : number of arclets for the current system
	* - n_famille: source index
	*
	* Global variables used :
	* - amplifi
	*/
	double sig2posS(double sig2pos, int n, int n_famille, double *np_b0)
	{
	const extern double amplifi[NFMAX][NIMAX];
	extern struct galaxie multi[NFMAX][NIMAX];

	double sigma_src; // error source plane
	double ainv; // averaged amplification^-1, ie averaged etendue in source plane
	// for a 1 arcsec^2 etendue in image plane.
	int i;

	ainv = 0.;

	for ( i = 0 ; i < n ; i++ )
	ainv += fabs(e_amp_gal(&multi[n_famille][i], np_b0)); //1. / fabs(amplifi[n_famille][i]);

	ainv = ainv / n;

	sigma_src = ainv * sig2pos;

	return(sigma_src);
	}