Identifier   zodi_model

 Purpose      Compute model of zodiacal emission

 Synopsis     out = zodi_model(in,ra,dec,ut)

 Arguments    Name   I/O Type:     Description:
              ----------------------------------------------------------
              in      I  float     Array with input wavelengths in um;
                                   accepted range at least 2-200um.
              ra      I  As in     RA of source observed
                         header 
              dec     I    "       DEC of source observed
              ut      I    "       Date and Time of observation (UT)
              out     O  float     zodiacal brightness 
                                   (Jy sr^-1)

 Returns      Array with zodiacal light brightnesses (Jy sr^-1)
              for the input wavelengths. This array will usually be
              passed back to the calling shell "zodiacal" and inserted
              into a YAAAR spectrum.

 Description  Implement a model for zodiacal light, including the
              effect of varying solar elongation (to be computed from
              ra,dec,ut), valid at least for elongations 60 to 120 degrees
              as observed by ISO.

 Comments     Invoke via a shell "zodiacal" which obtains in, ra ,dec, ut from
              an AAR, computes out, puts it into an AAR matching the
              input AAR, and converts to flux densities within the applicable
              SWS or LWS aperture. RA, DEC, UT taken from input AAR header.

              The zodial model of John Good (documented in IPAC's
              ISSA Explanatory Supplement, May 1994) is being 
              implemented in subroutine "zodi_iso_emit" called by this
              function.
      
              A future version could give the option to utilize new 
              zodi models that may be in development by COBE astronomers.
              In addition to models, we could also estimate the total
              "background" spectrum in the aperture based on real data from 
              COBE/DIRBE as now implimented in IBIS/IRSKY.
              Experience shows that at many SWS wavelengths, for many
              positions in the sky, COBE data agree very well with  
              the predictions of the Good zodi model.

 Example      
              zodi_sb = zodi_model(wavelangths,ra,dec,ut)

 Dependencies CALLS: BPRECESS, EULER, DATE, JDCNV, ZODI_ISO_EMIT
              CALLED FROM: ZODIACAL

 Category     ISAP

 Filename     zodi_model.pro

 Author       Iffat Khan (irk@ipac.caltech.edu)

 Version      0.1

 History      Draft requirements     27.7.95   D. Lutz
              Added notes            16.01.96  J. Mazz & S. Lord
              Design and code        20.03.96  I. Khan