The 8.4-meter mirror in the Rubin Observatory telescope isn't like any other telescope mirror. It's actually two mirror surfaces combined in a single large structure, each with a different curvature. If you look at a photo of the mirror, you can see the cusp where the two mirror surfaces meet. The outer ring forms the primary mirror that catches light from space first; that light then reflects upwards to a 3.4-meter secondary mirror, and then back down to the inner ring that forms the third (or "tertiary") mirror before bouncing back up again to the camera. The size of the primary mirror helps the telescope collect a huge amount of light, and that helps astronomers study very faint or faraway objects in space.
Why not just have three separate mirrors? Three mirrors would have made the telescope a lot longer, and its compact size serves some useful functions. First, it can move between different spots in the sky in just a few seconds, a lot faster than a long telescope could. This speed helps the telescope produce a lot of images—about 1000—in just one night. A short telescope also vibrates less than a long telescope would, and less vibration means clearer, sharper images. So by combining the two mirrors, the telescope's designers were able to make a much faster, more effective telescope. This two-in-one primary mirror configuration does mean that the telescope doesn't collect as much light as a regular 8.4-meter telescope would, but this was considered an acceptable tradeoff to make sure the telescope could move quickly instead.
The 8.4-meter mirror was fabricated at the Richard F. Caris Mirror Lab in Tucson, Arizona, and took about seven years to complete! While it is a single piece of glass, the backside is a hollow, stiff honeycomb structure that massively cuts down the overall weight of the mirror, helping to make the telescope faster, without losing too much structural stability. To make the mirror, technicians layered chunks of special, ultra-pure glass over a mold of the honeycomb structure, and then the whole setup was spun in a scorching hot mirror oven to melt the glass into a curved parabolic surface. The glass was then cooled, ground, and polished into its final shape. This mirror was actually the first piece of the telescope to be completed, with the initial melting and cooling stage in 2008 and final polishing in 2015. Then the mirror had to hang out in storage until 2019, when it was shipped to Chile to be integrated with the rest of the telescope. Again it was early, and it's been waiting in a specially made storage unit on the summit until the telescope was assembled in the observatory facility. The plan is to unpack the mirror, coat its surface using the onsite coating chamber, and install it on the telescope mount in early 2023. This amazing, one-of-a-kind mirror is one of the critical components that makes Rubin science possible.