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What alignment can turn a lensed galaxy into an Einstein ring?

The source, lens, and observer must be nearly perfectly aligned.

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  1. A source beside the lens
  2. Two lenses at right angles
  3. A source directly behind the lens ✔

Why: With near-perfect alignment, light bends symmetrically around a foreground mass. The many lensed images merge into a ring around it.

Huh, didn't know that: Imperfect alignment commonly produces arcs rather than a complete ring.

The dad joke
Einstein rings form when the universe lines up its circles.
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References

Answer: A source directly behind the lens. With near-perfect alignment, light bends symmetrically around a foreground mass. The many lensed images merge into a ring around it.

  1. Hubble Finds Double Einstein Ring science.nasa.gov/missions/hubble/hubble-finds-double-einstein-ring
    When both galaxies are exactly lined up, the light forms a circle, called an Einstein ring
    Checked 2026-10-10.

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