I find there's a two step process, first overlapping the images (but which makes the images blurry), then letting my eyes refocus so the middle image is crisp. Only then does 3D or shimmer effect happen. Takes some practice to merge the images while maintaining focus for me.

Are you able to confirm the images are completely aligned? You can do this using landmarks, like the brightest stars on the telescope pic. I.e. if you see more than one of any landmark, it is not aligned. You may need to adjust zoom, and distance from face.