How to proceed (contouring)?

Question

Adobe Illustrator

I'm trying to crop these circles on the right, along the contour of the ellipse on the left, so that the contour of the ellipse remains.

I select the circles and the ellipse and hit "intersect" in Pathfinder.

It comes out:

What is the error, or how to do it in another way?

Answer 1

Shape Builder Tool with alt pressed will remove the excess. Or a clipping mask might work for your purposes.