Also, if you happen to find yourself around Mauna Kea, HI (watch out for invisible cows!) some friendly telescope employees may be gracious enough to give you a tour. There is also one radio telescope on the mountain that is capable of being arrayed with other radio telescopes (such as the VLA or GBT) for an even longer baseline (and correspondingly higher angular resolution).
Since the TMT protests, access to the Mauna Kea summit is much more restricted. I would not count on going up past the visitor center or getting any kind of tour. The Subaru telescope has ended their public tour program.
Actually, there are two radio telescopes near the summit, the JCMT and SMA, which can be arrayed with other telescopes around the world (that's how the first black hole image was obtained) and there is a VLBA antenna a bit further down from the summit which is used as as an element in a lower frequency radio array spanning the United States from Hawaii to the US Virgin Islands.
I was able to drive up Mauna Kea and walk around, even if I wasn't able to go inside any of the telescopes. Pretty cool place and the elevation alone is a wild experience.
"Most of the Mauna Kea access road below Hale Pohaku is open range, and the cows frequently cross the road. Dark colored cows are often invisible in darkness and/or fog. Use extreme caution and drive very slowly in this open range."
Also, if you happen to find yourself around Mauna Kea, HI (watch out for invisible cows!) some friendly telescope employees may be gracious enough to give you a tour. There is also one radio telescope on the mountain that is capable of being arrayed with other radio telescopes (such as the VLA or GBT) for an even longer baseline (and correspondingly higher angular resolution).