Identifier     sap_sub

 Purpose        Subtract either a constant or another spectrum from a spectrum

 Synopsis       sap_sub, 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 subtracted from
                                         in1flux (constant array or fluxes)
                in2err    I    Float(*)  Array of weights for in2flx
                outflux   O    Float(*)  array of Spectrum 1 after
                                         subtraction
                outerr    O    Float(*)  Array of uncertainties of outflux

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

 Description    Subtracts either a constant or another spectrum from a 
                spectrum.
                If a constant is to be subtracted this constant has to be 
                passed as array (from the shell arithm). The errors are 
                propagated ( outerr = sqrt(in1err^2 + in2err^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_sub.pro

 Author         A. Fiedler (--> AF)

 Version        1.0
                              
 History        0.1   27-09-95  AF
                1.0   30-11-95  ES V1 delivery
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Copyright (C) 1995, Max-Planck-Institut fuer extraterrestrische Physik (MPE);
                    Garching, Germany                                       ;
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