MDI_POINTING(3)

NAME

     mdi_point, mdi_pointing -- plate coordinates of  Sun  center
     as observed from MDI

SYNOPSIS

     #include <soi_time.h>
     #include <soi_ephem.h>

     int mdi_point (TIME obs_time, double *xc, double *yc,
          TIME *mod_time, int option)
     int mdi_pointing (TIME obs_time, double *xc, double *yc,
          TIME *mod_time)

DESCRIPTION

     mdi_point() interpolates the  tabulated  solar  disc  center
     locations,  as  determined from analysis of the limb figure,
     to the requested observation time, returning the coordinates
     in  units  of  pixels  in the full-disc observing mode.  The
     coordinate convention is that the first pixel read,  in  the
     southeast corner of the field, has coordinates (0.0, 0.0) at
     ots center, so that the center of  the  whole  plate  is  at
     coordinates  (511.5,  511.5).   When  guiding is working the
     coordinates of disc center seldom differ by more than a  few
     tenths of a pixel from this location.

     Options:
     MDIPT_USE_DISTRIB or
     MDIPT_USE_DEFINITIVE - use only data  from  definitive  (CD-
     ROM, prog:mdi) distributions
     MDIPT_USE_QUICKLOOK - use only data  from  quick-look  (EOF,
     prog:mdi_eof) distributions
     MDIPT_USE_BEST - use data from either of the above distribu-
     tions as available, preferably definitive.
     MDIPT_USE_SMOOTH - use smoothed data.

     mdi_pointing() is equivalent to mdi_point(..., MDIPT_BEST)

     The normal time steps of the pointing tables are 12 minutes,
     so  linear  interpolation is used.  However, there are often
     gaps in the table of considerably more than one  time  step,
     due to bad or missing data.

     The most recent modification time of the requested  pointing
     table is also returned.

FILES

     ~CM/tables/ephemeris/mdi_pointing.raw
     ~CM/tables/ephemeris/mdi_pointing.smooth
     ~CM/tables/ephemeris/eof/mdi_pointing.raw
     ~CM/lib/_($MACH)/libastro.a

SEE ALSO

DIAGNOSTICS

     Return values:

     0              normal
     MDIPT_TIME_BEYOND_END    requested observation time more than one
                    time step beyond last entry in table
     MDIPT_CANT_INTERPOLATE   unable to interpolate: only one valid datum
                    in pointing file, or observation time is within
                    one time step beyond last valid datum;
                    nearest values returned
     MDIPT_TIME_BEFORE_START  requested observation time precedes
                    beginning of table
     MDIPT_CANT_READ_TABLE    pointing table could not be read
     MDIPT_FROM_QUICKLOOK     pointing info from quick-look data
                    (when this is an option)

BUGS

     When the return value is 1, -1, or -2,  the  coordinates  of
     plate center are returned.

     There is no return status concerning the length of  the  gap
     over which values were interpolated.

     Because gaps in the summary file are are represented by  0's
     for  both the time phase of the observation and the pointing
     values:  thus, center values of 0.0 at a phase of 0.0 cannot
     be  represented  as  valid values, but such values could not
     possibly be validly returned by the limb finder.

AUTHOR

     Rick Bogart

HISTORY

     1996-12-26      SOI Version 2.1