def rq_to_r(rq):
    """
    Convert to reduced coordinate                               
                                                                        
                  r - 1                                                 
           rq =  -------                                                
                  r + 1  
    
    Hernquist & Ostriker 1992 eq. 2.15
    """
    BIG = 1.e30
    if (rq>=1.0):
        return BIG;
    else:
        return (1.0+rq)/(1.0-rq);