Identifier     sap_mean


 Purpose        Compute the (weighted) mean of two spectra


 Synopsis       sap_mean, in1flux, in1err, in2flux, in2err, outflux, outerr
                      [,/weight]
                             

 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 of second spectrum
                in2err    I    Float(*)  Array of weights for in2flx
                outflux   O    Float(*)  the mean fluxes of Spectrum 1 and 2
                outerr    O    Float(*)  Array of uncertainties of outflux
                weight    I    switch    if set: compute weighted mean

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


 Description    Adds two spectra and divides the result by 2. (unweighted
                mean. If weighting is desired (/weight is given)
                the weights are taken from the errors (stdev^2). For points
                where both errors are 0 the weights are set to 1 and the
                resulting error to 0.

 Comment        The routine should be called from the "shell" ARITHM.
                If the switch /weight is set "mean" is performed with the
                square of the standard deviations as weights. Weighting with
                constants (like "(3.*arr1 + 2.*arr2)/5.") can be performed step
                by step - calling mult, add, and div. Weighting with
                artificial arrays of weights (e.g. containing zeros to mask out
                bad data points) is not supported in this first version
                (masking can be done in alternative ways).

 Example        ---

 Dependencies   CALLS: sap_error, sap_keyword_set
                CALLED FROM: arithm

 Category       ISAP

 Filename       sap_mean.pro

 Author         A. Fiedler (--> AF)

 Version        1.0
                              
 History        0.1   27-09-95  AF
                0.2   05-10-95  AF can handel zero and small errors
                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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