def recenter(self):
        """
        Reset the reference position values to correspond to the center
        of the reference frame.
        Algorithm used here developed by Colin Cox - 27-Jan-2004.
        """
        if self.ctype1.find('TAN') < 0 or self.ctype2.find('TAN') < 0:
            print 'WCS.recenter() only supported for TAN projections.'
            raise TypeError

        # Check to see if WCS is already centered...
        if self.crpix1 == self.naxis1/2. and self.crpix2 == self.naxis2/2.:
            # No recentering necessary... return without changing WCS.
            return

        # This offset aligns the WCS to the center of the pixel, in accordance
        # with the 'align=center' option used by 'drizzle'.
        #_drz_off = -0.5
        _drz_off = 0.
        _cen = (self.naxis1/2.+ _drz_off,self.naxis2/2. + _drz_off)

        # Compute the RA and Dec for center pixel
        _cenrd = self.xy2rd(_cen)
        _cd = N.array([[self.cd11,self.cd12],[self.cd21,self.cd22]],type=N.Float64)
        _ra0 = DEGTORAD(self.crval1)
        _dec0 = DEGTORAD(self.crval2)
        _ra = DEGTORAD(_cenrd[0])
        _dec = DEGTORAD(_cenrd[1])

        # Set up some terms for use in the final result
        _dx = self.naxis1/2. - self.crpix1
        _dy = self.naxis2/2. - self.crpix2

        _dE,_dN = DEGTORAD(N.dot(_cd,(_dx,_dy)))
        _dE_dN = 1 + N.power(_dE,2) + N.power(_dN,2)
        _cosdec = N.cos(_dec)
        _sindec = N.sin(_dec)
        _cosdec0 = N.cos(_dec0)
        _sindec0 = N.sin(_dec0)

        _n1 = N.power(_cosdec,2) + _dE*_dE + _dN*_dN*N.power(_sindec,2)
        _dra_dE = (_cosdec0 - _dN*_sindec0)/_n1
        _dra_dN = _dE*_sindec0 /_n1

        _ddec_dE = -_dE*N.tan(_dec) / _dE_dN
        _ddec_dN = (1/_cosdec) * ((_cosdec0 / N.sqrt(_dE_dN)) - (_dN*N.sin(_dec) / _dE_dN))

        # Compute new CD matrix values now...
        _cd11n = _cosdec * (self.cd11*_dra_dE + self.cd21 * _dra_dN)
        _cd12n = _cosdec * (self.cd12*_dra_dE + self.cd22 * _dra_dN)
        _cd21n = self.cd11 * _ddec_dE + self.cd21 * _ddec_dN
        _cd22n = self.cd12 * _ddec_dE + self.cd22 * _ddec_dN

        _new_orient = RADTODEG(N.arctan2(_cd12n,_cd22n))

        # Update the values now...
        self.crpix1 = _cen[0]
        self.crpix2 = _cen[1]
        self.crval1 = RADTODEG(_ra)
        self.crval2 = RADTODEG(_dec)

        # Keep the same plate scale, only change the orientation
        self.rotateCD(_new_orient)

        # These would update the CD matrix with the new rotation
        # ALONG with the new plate scale which we do not want.
        self.cd11 = _cd11n
        self.cd12 = _cd12n
        self.cd21 = _cd21n
        self.cd22 = _cd22n
Reset the reference position values to correspond to the center
        of the reference frame.
        Algorithm used here developed by Colin Cox - 27-Jan-2004.