;+
; NAME: mag_errprop
;
; PURPOSE:
; 		Convert fluxes to magnitudes, with error propagation.
;
; INPUTS:
; 	f1,f2		fluxes
; 	err1,err2	uncertainties in fluxes
; KEYWORDS:
; 	/silent		don't print
; OUTPUTS:
; 	mags=		returns magnitudes
; 	errmags=	returns errors in magnitudes
;
; HISTORY:
;-
PRO mag_errprop,f1,err1,f2,err2,mags=mags,errmags=errmags, $
		silent=silent

	ratio = f2*1.0/f1
	fracerr = sqrt( (err1*1.0/f1)^2 + (err2*1.0/f2)^2 )
	err_ratio =  ratio * fracerr
	
	mags = -2.5*alog10(ratio)
	; derivative of the above is -2.5/(ratio*alog(10.))
	;errmags = -2.5/(ratio*alog(10.))*err_ratio
	;errmags = -2.5/(ratio*alog(10.))*(ratio*fracerr)
	;errmags = -2.5/(alog(10.))*(fracerr)
	errmags = 2.5/(alog(10.))*(fracerr)
	

	if ~(keyword_set(silent)) then print,"Difference: "+strc(mags)+" +- "+strc(errmags)+" magnitudes"

end