ISO POINTING FOR SOLAR SYSTEM OBJECTS
   -------------------------------------

The absolute ISO pointing was usually within a few arcsec of the
specified coordinates. But in case of SSOs the ephemeris were not
always perfectly known at the time of the planning of the observations,
especially for newly discovered comets. For those cases a independent
checking of the ISO pointing (given in the IIPH file with one RA,
Dec-entry every 0.5 sec) and the ISOcentric cooridnates of the SSO
is recommended.


How to check if the pointing of ISO was okay:

     Download all IDL routines (you will also need the IDL 'astrolib' library)
     and the ISO orbit file from:

                 ftp pma.iso.vilspa.esa.es
                 anonymous
                 "your email address"
                 cd pub/idc/tmueller/pointing/
                 prompt
                 mget *
                 quit

     The following files should now be in your directory:

        ORBIT.FITS                :   ISO orbit file [
        README                    :   this README file
        acc_iso_orbit.pro         :   accesses the ORBIT file
        adxyz.pro                 :   coordinate transformation
        aux_funct_timeline.pro    :   additional functions
        ind_orbitfits.pro         :   read common blocks
        isocentric_pos.pro        :   main programme
        map_orbitf.pro            :   map the ISO orbit file
        sphdist.pro               :   angular distance on a sphere
        xyzad.pro                 :   coordinate transformation

        Thanks go to Mrs. R. Gonzales from IUE for supplying
        basic routines for parallax correction. Further software
        contributions and modifications are comming from: B. Schulz,
        U. Kinkel, J. Acosta and T. Mueller."


I)   Take the start and end times (timekeys TK) of the
     observation from the 'Compact Status (PSTA*, SSTA*,
     CSTA* or LSTA*)', given in the FITS header: 

     EOHAUTCS: Approx. UTC of start of observation
     EOHAUTCE: Approx. UTC of end of observation
  
     which is given in YYDOYHHMMSS (YY: year, DOY: day of year,
     HH: hour, MM: minute, SS: second; conversion to calender
     date: use tk2cald.pro, included in aux_funct_timeline.pro)


II)  Calculate the precise 'geocentric positions [deg]'
     and distances 'earth-SSO [AU]' for both times with
     a reliable Ephemeristool (IAU, JPL, ...)


III) Calculate the 'ISOcentric' position of your object
     for both times (isocentric_pos.pro)

         INPUT:   geocentric RA      [deg]
                  geocentric Dec     [deg]
                  Delta (earth-SSO)  [AU]
                  ISO Timekey        [YYDOYHHMMSS]

         OUTPUT:  ISOcentric RA      [deg]
                  ISOcentric Dec     [deg]
                  at the given timekey

         EXAMPLE:
         isocentric_pos,233.232323, -23.232323, 2.23235442, '98033232323'
         ==> sso_ra1:  233.22280    sso_dec1:  -23.229098
         isocentric_pos,233.232367, -23.236724, 2.23235442, '98033235903'
         ==> sso_ra2:  233.22349    sso_dec2:  -23.233496

         WARNING: 'isocentric_pos' can take some time (the ISO orbit
                  file is quite big.


IV)   Compare the ISO pointing with the ISOcentric coordinates of
      your object (Please replace 'xxxxxxxx' by the ISO TDT-OSN number) 
 
      fname='IIPHxxxxxxxx.FITS'
      OPENR, UNIT, fname, /BLOCK, /GET_LUN
      FXBOPEN, UNIT, fname, 1, EHDR
      FXBREAD, unit, RA, 'CRA'
      FXBREAD, unit, DEC, 'CDEC'
      FXBCLOSE, unit
      plot,ra,dec,psym=3
      oplot,[sso_ra1,sso_ra2],[sso_dec1,sso_dec2]

      ; calculate the median offset
      print,'average offset [arcsec]: ',$
            (sphdist(double(median(ra)),double(median(dec)),$
            double((sso_ra1+sso_ra2)/2),$
            double((sso_dec1+sso_dec2)/2)))*3600.

      ==> compare the offset with your aperture size
          and the beam profile