Identifier     point2ext

 Purpose        SWS calibration conversion between point and extended sources
                
 Synopsis       aar_out = point2ext(aar, dias, [/quick][,stat=stat])

 Arguments      Name      I/O  Type     Description
                -----------------------------------------------------
                aar        I   AAR      The SWS spectrum to be converted
                dias       I   float    The diameter of the source in arcsec:
                                        0 = point source
                                        -1 = extended source 
                                        ( 0<dias<25. = finite source (i.e.
                                        extended, but well inside the aperture)
                                        NOT YET IMPLEMENTED )
                quick      I   key      If set no interpolation is performed
                stat      I/O  int      eror status, 0 if ok, 1 if fatal

 Returns        the converted aar 

 Description    SWS calibration in the lab has been performed with extended
                sources, whereas in-orbit-calibration is mostly done with
                astronomical point sources. To enable a proper comparison
                the following considerations have to be accounted for:
               
                I_lambda     [watt/m^2/micron/sterad]
                PHI_lambda   [watt/m^2/micron]
                A point source delivers a flux PHI_lambda to the
                ISO telescope. Telescope and spectrometer transmit
                a flux:
                PHI_lambda * telescope_area * A [watt]
                to a detector. A is the equivalent bandwidth of the system
                for a point source.
                An extended source delivers a flux density I_lambda
                to the ISO telescope. Telescope and spectrometer
                transmit a flux
                I_lambda * telescope_area * B * C [watt]
                to a detector. B is the equivalent bandwidth of the system
                for an extended source.
                C is the solid angle at the extended source, seen by
                the detector.
                IF
                PHI_lambda * telescope_area [watt/micron]  (point source)
                IS EQUAL TO
                I_lambda * telescope_area * C [watt/micron]  (extended source)
                THE SIGNAL FOR THE POINT SOURCE IS  K  TIMES THAT
                OF THE EXTENDED SOURCE, WITH  K = A/B
               
                By means of a number of programmes written by L. Haser (MPE)
                a data cube has been computed, which samples the necessary
                informations to compute K for all kinds of combinations of
                wavelength, aperture, order and source diameter.
                Point2ext extracts from the aar all parameters that are needed 
                to interrogate this data cube, namely the aperture and the 
                order for each wavelength point. The source diameter (DIAS),
                hence the direction of the conversion, i.e. point to extended
                or vice cersa) has to be given by the user.
                A read routine (pte_read) is called which extracts the
                conversion factor K from the data cube. For combinations of
                lambda and dias falling between the raster points of the data
                cube the value is computed by linear interpolation between the
                nearest neighbours.
                Finally the flux values of AAR are multiplied by K - if 
                dias eq -1 - or divided by K - if dias eq 0.
                Since this conversion takes quite a while it is possible to
                speed things up by setting the /quick keyword. In this case
                K is not computed for all the wavelengths in the AAR but only
                for those points which fall onto a raster point of the data
                cube. For all other wavelengths K is taken from the nearest
                of these raster wavelengths, i.e. no interpolation is
                performed. The data cube step widths are narrow enough to
                make sure that the quick mode will be sufficient in almost all
                cases.
               
 Comment        Does not yet return an error status
                
 Example        1) aar_conv = point2ext(aar,0,/quick)
                 
                In this example aar is assumed to be a point source and the
                fluxes are divided by K to enable a comparison to extended
                source calibrations. No interpolation is performed.  

                2) aar_conv = point2ext(aar,-1)

                aar is assumed to contain extended source data. The
                fluxes are multiplied by K to enable a comparison to point
                source calibrations.

 Dependencies:  CALLS:        pte_read, sap_error
                CALLED FROM:  IA

 Category       ISAP

 Filename       point2ext.pro

 Author         E. Sturm (--> ES)
                L. Haser (--> LH) 

 Version        1.1

 History        1.0   26-04-96  ES/LH  first version  
                1.1   09-07-96  ES  stat added

;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;
Copyright (C) 1996, Max-Planck-Institut fuer extraterrestrische Physik (MPE);
                    Garching, Germany                                       ;
;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;;