Mirrors
Highlights
The Rubin Observatory 8.4-meter primary mirror is actually two mirrors in one—combining them on one surface reduces complexity and allows the telescope to be more compact, making it easier to rotate and settle quickly.
The primary mirror took seven years to make — from the initial forming of the mold and melting of the glass in a rotating furnace, to final polishing.
The primary mirror traveled 7,000 miles from Tucson, AZ to the mountaintop in Chile—and had less than a foot (~30cm) of clearance to fit through a road tunnel on the way.
The 8.4-meter mirror in Rubin Observatory's telescope isn't like any other telescope mirror. It's actually two mirror surfaces combined in a single large glass structure, each with a different curvature.
The outer surface 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 5.0-meter surface 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, allowing astronomers to study very faint or distant objects in space. The whole design allows the camera to capture a large area of sky — the diameter of 7 full moons across — in a single image.
Why not just have three separate mirrors? Early designs had three separate mirrors with spacing that would have made the telescope a lot longer. After scientists optimized the design with the first and third mirrors close together, they determined that the simplicity of a single structure was worth the challenge of making this one-of-a-kind combined mirror.
The resulting compact size of the telescope serves some useful functions:
It can move between different spots in the sky in just a few seconds, much faster than a long telescope could. This speed helps the telescope produce about 1000 images 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.
Making a telescope mirror
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 mirror lighter, stiffer, and easier to keep at a stable temperature.
To make the mirror, technicians layered chunks of special, ultra-pure glass over a heat-resistant 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 one of the first pieces of the telescope to be completed, with the initial melting and cooling stage in 2008 and final polishing in 2015. Then the mirror was kept 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 waited in a specially made storage unit on the summit until the telescope was assembled in the observatory facility. The mirror was moved into the facility, unpacked, mounted on its support structure, and coated in April 2024.
This amazing, one-of-a-kind mirror is one of the critical components that makes Rubin science possible.