A new tour of the Gemini telescope

On Friday I got to go on a tour of the Gemini telescope building at the summit of Maunakea. In pre-COVID-19 times this would be something all new employees would get at some (not-too-long) point after joining, but since tours haven't happened since early 2020 there were something like fifteen of us who'd joined in the last two years who went along. Gemini North is in shutdown at the moment for recoating the primary mirror (for the first time in nine years!), which afforded a very rare opportunity to see an 8 meter-class telescope without its mirror installed.

I actually went on a tour of Gemini (plus some other telescopes) a decade ago back in 2012 while I was an undergraduate at UH Hilo, so for comparison, here's a photo I took then of the telescope with mirror in place and instruments mounted on it:

It's hard to capture the telescope from within the dome because it's just so huge, but the blue-painted parts are the telescope structure, with the various boxy things beneath it being the instruments, which are bigger than refrigerators. The silvery struts above hold the secondary mirror, which can be seen near the top of the image. Now, compare this with the panorama I took on Friday:

The panorama distorts some of the angles slightly, but you can clearly see where the mirror goes inside the telescope structure in this image. The flap covering the aperture there is half of the mirror cover, which folds up accordion-style over the mirror when it's in place. The blue circular thing behind the telescope on the temporary scaffolding holds the mirror from beneath; here's a photo of it I took from from up on the platform running around the telescope:

Here you can see the actuators (the white disks) which sit beneath (or behind) the mirror and help change its shape as it deforms under its own gravity as the telescope changes its orientation. As a reminder, the primary mirror for both Gemini telescopes is 8.1 m (26.6 ft) in diameter, so this is a big piece of equipment. While I did get to see the mirror, it's down on the floor below this one where the mirror coating chamber is, and was hard to get a good photo of as it's on a stand with a cover on top. I did get a photo of the mirror-coating vacuum chamber itself, which is pretty neat:

This flying-saucer-looking thing is the mirror-coating chamber. It wasn't in operation while we were there, but we got to hear a bit about how it works. Unlike most of the mirrors on the mountain which use aluminum, Gemini's mirror is coated with silver, which gives it a better infrared reflectivity. Silver, however, is more chemically reactive than aluminum, and would tend to tarnish quickly over time. To help prevent this, the silver is deposited as part of a sequence of several thin layers of various metals (which I can't recall now unfortunately) to help keep it from tarnishing too quickly.

Overall it was a great experience where I learned a lot about the telescope, marred only by me forgetting my coat and spending the entire time rather chilly. (Thankfully a coworker lent me a jacket.) It's easy to forget just how humongous the various telescope enclosures on the mountain are until you're inside them, at which point the cavernous volume becomes somewhat awe-inspiring. We left Hilo at 8:30 in the morning and didn't get back until 5:30 in the afternoon, so it was a long and intense day due to the altitude, but it was definitely a really cool experience that I'm thankful I got to go on. Though next time, I plan to remember to bring my coat…