I captured it, barely, but I got it.  In the image it's the tiny blue horseshoe shaped ring left of center with a red central galaxy.

The “Cosmic Horseshoe” is one of the now numerous examples of gravitational lensing.  The LRG 3-757 lens was discovered in 2007 by the Sloan Digital Sky Survey and later a stunning image was produced by the HST.  LRG 3-757, type LRG for Luminous Red Galaxy, is a massive galaxy about 5.2 trillion solar masses about 3.5 times more massive than Milky Way (1.5 trillion solar masses).  It is roughly 5 billion light years distance.   An interesting feature of this galaxy, other than its mass and distance, is its serendipitous location in space.  It’s positioned near directly between us and a very distant galaxy of 10 billion light years.  This fortuitous geometrical orientation has the astonishing effect of displaying for us a demonstration of general relativity in the form of an Einstein Ring.  The radial emission of light rays from the more distant obscured galaxy are bent by the gravitational field of the nearer visible galaxy creating a lens effect that focuses the rays of the distant galaxy at us in the form of a ring.  Or perhaps stated another way, space-time is warped by the mass of the nearer galaxy which determines the path of the light from the obscured galaxy to form a ring.  

Einstein predicted these rings for two stars from his theory of general relativity in a short note published in the 1936 December issue of the journal “Science”:

https://www.science.org/doi/10.1126/science.84.2188.506

In the first paragraph he refers to it as a “little calculation” that he published to comply with a request by R.W.  Mandl , an amateur scientist.  In the note Einstein does the calculation with two stars not two galaxies, this would fall more in the category of microlensing but the effect is similar for two galaxies. 

This target is a challenge for Bortle 5 because it is both dim and small. I’ve seen magnitude estimates that range from 20 to 23 perhaps because of the subtended angle, but certainly dimmer than the sky at my location.  The diameter is ~10 arc-seconds-- tiny (evident in revision D).  This image is at the limits of my equipment and location,  I have ~ 60 pounds of payload on a CEM70 and need optimal tracking at long FL.  Dew and the slightest breeze were a problem. I don’t have an observatory, so I captured the data by breaking down and setting up each night.  With the all the break downs it was necessary to do several sets of calibration files for different nights, particularly flats, which were crucial with low signal.  No definite expression of the ring appeared on the subs; it was only evident on stacks.  Lack of a guide star was a problem, the FOV is mostly stars of 14th magnitude and higher and small dim galaxies.  With not much to guide on I pushed the prism of the OAG a bit into the light cone to find a star. It worked and I was left with plenty of crop room to exclude the prism shadow.  I discovered there's a lot to processing in low S/N at long FL, it’s more vulnerable to moon gradients, sky glow gradients, dust motes, etc.  Also, tossed away more data than usual, data with a luminosity filter was hopeless.  So, in summary, it’s not the prettiest image with all the noise, but my goal was detection of the ring, so I’m pleased.