Identifier     sap_add

 Purpose        Add either a constant or another spectrum to a spectrum
                (with error propagation) 

 Synopsis       sap_add, in1flux, in1err, in2flux, in2err, outflux, outerr

 Arguments      Name      I/O  Type      Description
                -----------------------------------------------------
                in1flux   I    Float(*)  array of first spectrum (flux or
                                         photocurrents)
                in1err    I    Float(*)  Array of weights for in1flux
                in2flux   I    Float(*)  array that is to be added (constant
                                         or fluxes of a second spectrum)
                in2err    I    Float(*)  Array of weights for in2flx
                outflux   O    Float(*)  array of Spectrum 1 after
                                         addition
                outerr    O    Float(*)  Array of uncertainties of outflux

 Returns        The added fluxes (or photocurrents) and their propagated 
                errors. 

 Description    Adds either a constant or another spectrum to a spectrum.
                If a constant is to be added this constant has to be passed
                as array (from the shell arithm). The errors are propagated
                ( outerr = sqrt(in1err^2 + in2err^2) ).
                After adding two spectra together the resulting
                spectrum is NOT renormalized (by dividing by 2.)

 Comment        The routine is usually called from the "shell" ARITHM which
                provides the appropriate arrays from the product structures.

 Example        ---
                               
 Dependencies   CALLS: 
                CALLED FROM: arithm

 Category       ISAP

 Filename       sap_add.pro

 Author         E. Sturm (--> ES)
                A. Fiedler (--> AF)

 Version        1.0

 History        0.1   14-09-95  ES    
                0.2   27-09-95  AF      added check for number of arguments
                1.0   30-11-95  ES V1 delivery
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
Copyright (C) 1995, Max-Planck-Institut fuer extraterrestrische Physik (MPE);
                    Garching, Germany                                       ;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;