Why Does an Einstein Ring Have Four Images?

[DaveC426913]

TL;DR Summary

This article seems to suggest four lobes is the optimum

"When the alignment is nearly perfect and the lens mass has an elliptical distribution, the background source would appear as quadruply lensed."
https://www.sci.news/astronomy/esos-very-large-telescope-einstein-cross-12265.html

Am i reading too much into this? Why four duplications? Is there some 3D geometry to it?

 

[Ibix]

It's specific to an elliptical mass - each axis gives you a pair of images, IIRC. With a spherical mass you can, in principle, get a ring.

 

[DaveC426913]

It's specific to an elliptical mass - each axis gives you a pair of images, IIRC. With a spherical mass you can, in principle, get a ring.

Ah! That's what I deduced after ruminating upon it.

Although it still doesn't seem to fit. There's no particular reason for the major and minor axes to happen to focus on Earth's locale. It seem it is just as likely, but random change that the right focal length might happen at, say, 45 degrees.

 

[Vanadium 50]

There's no particular reason for the major and minor axes to happen to focus on Earth'

If you had only one example, sure. But there are multiple examples, and when you pick one with a certain characteristic ("looks pretty") in impacts others.

 

[DaveC426913]

If you had only one example, sure. But there are multiple examples, and when you pick one with a certain characteristic ("looks pretty") in impacts others.

That's why I wonder if I'm reading too much into the text:

"When the alignment is nearly perfect and the lens mass has an elliptical distribution, the background source would appear as quadruply lensed."

...as if there is some ultimate case where parameters conspire for the "ideal" ring.

Are you suggesting that "ideal" is aesthetic, as opposed to geometrically optimal?

 

[Ibix]

Are you suggesting that "ideal" is aesthetic, as opposed to geometrically optimal?

The perfect alignment means you, lensing galaxy, and lensed galaxy lying on a straight line. If you're too far off-axis in one direction but not the other you might find you only see three images.

There's no particular reason for the major and minor axes to happen to focus on Earth's locale.

A gravitational lens is a terrible lens, but this does mean that if you are on axis there is a huge range of distances over which you will see lensing. So overlap between "focus" of one bad lens axis and the other is not unlikely.

 

[pinball1970]

That's why I wonder if I'm reading too much into the text:

"When the alignment is nearly perfect and the lens mass has an elliptical distribution, the background source would appear as quadruply lensed."

...as if there is some ultimate case where parameters conspire for the "ideal" ring.

Are you suggesting that "ideal" is aesthetic, as opposed to geometrically optimal?

You saw this one?

 

[DaveC426913]

You saw this one?

View attachment 332609

Yes, this is what I considered a "perfect" Einstein Ring.

So I was reading too much into the term "perfect" in the OP quote.

 

[pinball1970]

Yes, this is what I considered a "perfect" Einstein Ring.

So I was reading too much into the term "perfect" in the OP quote.

If you blow it up you can see four lobes too.

 

[pinball1970]

Yes, this is what I considered a "perfect" Einstein Ring.

So I was reading too much into the term "perfect" in the OP quote.

This popped up in my alerts yesterday too.
https://phys.org/news/2023-09-jwst-massive-compact-quiescent-galaxy.html