NAME:
  refcorr
 PURPOSE:   (one line only)
  Compute a differential refraction correction to an apparent position
 DESCRIPTION:
  Compute the amount of refraction relative to a target wavelength for
    light passing through the Earth's atmosphere.
 CATEGORY:
  Astrometry
 CALLING SEQUENCE:
  refcorr,jd,obs,ra,dec,refwave,targwave,ra1,dec1
 INPUTS:
  jd       - Julian date (must be double precision to get nearest second).
  obs      - Observatory code or name (see obsinfo)
  ra       - J2000 right ascension of apparent location [radians]
  dec      - J2000 declination of apparent location [radians]
  refwave  - Reference wavelength [microns]
  targwave - Wavelength of target [microns]
 OPTIONAL INPUT PARAMETERS:
 KEYWORD INPUT PARAMETERS:
  OBSINFO  - anonymous structure array of observatory information.  Required
               tags are:
             name - name of observatory (must be matched by obs)
             lat  - Latitude of observatory (radians)
             lon  - Longitude of observatory (radians) [+ is East]
             alt  - Altitude of observatory [meters]
  PRESSURE - atmospheric pressure in mm of Hg (default=760.0)
  TEMP     - atmospheric temperature in degrees C (default=0.0)
  RELHUM   - Relative humidity (in percent) (default=0.0)
 OUTPUTS:
  ra1      - corrected J2000 coordinate in right ascension [radians]
  dec1     -  correctedJ2000 coordinate in declination [radians]
 KEYWORD OUTPUT PARAMETERS:
  delra    - JDnow right ascension offset in arcsec, includes cos(dec)
  deldec   - JDnow declination offset in arcsec
  offset   - total refraction in arcsec along great circle including zenith
                and normal to local horizon, this is the amount of the
                differential refraction.  If targwave>refwave the amount will
                be toward the zenith (negative).  If targwave