SDP.81 : how do we see the gavitational lens?

[maajdl]

I just read this news:

http://www.space.com/29042-cosmic-ring-photo-rules-them-all.html

I wonder how we can know about the gravitational lens, how can we even be sure it is there?
Would that be only by analyzing the image of the supposed distant galaxi?
Which kind of analysis?
Or are there other footprints?
I am specially impressed by the fact that the distance of the lens is known.
How can that be knonw?

 

[rootone]

We can be sure that it's there alright, it's a photograph of something.
That 'something' very much matches what Einstein predicted, although he doubted the phenomena could be directly observed.
Optical technology at the time was well short of the resolution that is required.
There is no alternative explanation (that I know of), for these kind of images.

 

[Chronos]

Spectroscopy is one way to detect an intervening mass in a suspected lensing event. This was used to confirm a lensing event in the case of PS1-10afx, which was initially suspected to be a new class of supernovae. See http://arxiv.org/abs/1404.6014 for further details.