We present spectroscopy and laser guide star adaptive optics ( LGSAO ) images of the doubly imaged lensed quasar SDSS J1206 + 4332 .
We revise the deflector redshift proposed previously to z _ { d } = 0.745 , and measure for the first time its velocity dispersion \sigma = ( 290 \pm 30 ) km/s .
The LGSAO data show the lensed quasar host galaxy stretching over the astroid caustic thus forming an extra pair of merging images , which was previously thought to be an unrelated galaxy in seeing limited data .
Owing to the peculiar geometry , the lens acts as a natural coronagraph on the broad-line region of the quasar so that only narrow [ O III ] emission is found in the fold arc .
We use the data to reconstruct the source structure and deflector potential , including nearby perturbers .
We reconstruct the point-spread function ( PSF ) from the quasar images themselves , since no additional point source is present in the field of view .
From gravitational lensing and stellar dynamics , we find the slope of the total mass density profile to be \gamma ^ { \prime } = - \log \rho / \log r = 1.93 \pm 0.09. We discuss the potential of SDSS J1206 + 4332 for measuring time delay distance ( and thus H _ { 0 } and other cosmological parameters ) , or as a standard ruler , in combination with the time delay published by the COSMOGRAIL collaboration .
We conclude that this system is very promising for cosmography .
However , in order to achieve competitive precision and accuracy , an independent characterization of the PSF is needed .
Spatially resolved kinematics of the deflector would reduce the uncertainties further .
Both are within the reach of current observational facilities .