# Einstein ring has four images?

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• DaveC426913
DaveC426913
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TL;DR Summary
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.

Ibix said:
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.

DaveC426913 said:
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.

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?

DaveC426913 said:
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.
DaveC426913 said:
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.

DaveC426913 said:
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
pinball1970 said:
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.

DaveC426913 said:
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.

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