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   Computes the Jacobian for an arc measurement. Used for sun angle 
   measurements.
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   Form:
   H = DHArcDX( ra, dec, u )
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   ------
   Inputs
   ------
   ra                     Right ascension
   dec                    Declination
   u                      Unit vector

   -------
   Outputs
   -------
   H                      [�h/dra �h/ddec]

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