Vera Rubin’s Primary Mirror Gets its First Reflective Coating

First light for the Vera Rubin Observatory (VRO) is quickly approaching and the telescope is reaching milestone after milestone. A few weeks ago, the observatory announced that its digital camera, the largest one ever made, is complete.

Now the observatory has announced that its unique primary/tertiary mirror has its first reflective coating.

The Rubin’s massive digital camera has an important job and garners a lot of attention. But it’s powerless without the telescope’s innovative primary/tertiary mirror. Primary mirrors are always the most critical and time-consuming part of modern observatories. The VRO’s primary/tertiary mirror took seven years to make.

The mirror is called a primary/tertiary mirror because it comprises two optical surfaces with different curvatures. The primary mirror is 8.4 meters, while the tertiary mirror is 5 meters in diameter. The pair of surfaces are combined into one large structure. The unique design reduces the telescope’s engineering complexity without reducing its impressive light-gathering capability. It can be rotated quickly and also settles quickly.

The outer surface forms the primary mirror. It captures light from space first, then that light reflects upwards to the 3.4-meter secondary mirror. After that, it’s reflected back down to the inner 5.0-meter surface that forms the tertiary mirror. Then, the light is sent to the camera.

The primary mirror’s size is critical because it determines how much light the telescope can collect. More light means astronomers can study very faint or distant objects. The VRO’s design allows the camera to capture a large area of sky the size of 7 full moons across in a single image.

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Only meticulous engineering and construction can build a telescope like this. One of the stages is putting the reflective and protective coatings on the mirrors. The VRO announced that the primary/tertiary mirror has its first coating.

The VRO has a special onsite coating chamber built just for this purpose. It’s a 128-ton chamber on the observatory’s maintenance floor. It uses a process called magnetron sputtering to apply coatings. The chamber will be reused during the telescope’s lifetime whenever the mirror needs re-coating.

The chamber can apply coatings of different reflective materials alone or in combinations. It took a lot of work to determine the perfect coating for reflectivity and durability. Researchers tested different coatings on a steel stand-in mirror.

The first layer was an adhesive layer of nickel-chromium. Next came an incredibly thin layer of silver weighing only 64 grams spread over the 8.4-meter mirror. On top of that, another nickel-chromium adhesive layer, then a protective layer of silicon nitride to shield the reflective layer.

The person in charge of these precision coatings is Tomislav Vucina, the Senior Coating Engineer. Vucina describes the coatings as a balancing act. “This outer layer needs to be thick enough that it’s not worn off by cleaning,” said Vucina, “but not so thick that it absorbs too many photons and prevents the mirror from meeting Rubin’s scientific requirements.”

Until these coatings were applied, the glass was just glass. Highly specialized glass, but glass nonetheless. Now that the glass has received its reflective silver coating, it’s truly a mirror.

The application process took only 4.5 hours, nothing compared to the 7 years required to build the primary/tertiary mirror. Vucina and his team subjected the mirror to a battery of tests: reflectivity, adhesion, pinhole, and cosmetic. According to Vucina, the application process was successful.

“This was a very well-conducted project from every angle,” said Vucina, “thanks to a combination of careful planning and the technical skills of our excellent team.”

It’s been a long road to completion for the VRO. But after a long wait, first light is rapidly approaching. Excitement and anticipation for the observatory’s unique and powerful scientific contribution is growing. Its main output is the decade-long Legacy Survey of Space and Time.

“We’re extremely excited that both mirrors are now coated and will be installed on the telescope very soon,” said Sandrine Thomas, Deputy Director for Rubin Construction. “The combined reflectivity of these mirrors will enable Rubin to detect very faint and far-away objects, leading to great science!”

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