Kitt Peak National Observatory. The telescope on the top of the hill is the 4-meter Mayall Telescope. It looks the same today as it did in 1979 when I took the picture, but the inside has been recently "retrofitted to capture the light spectra from 35 million galaxies and reconstruct the Universe's history of expansion. [The] main aim: to elucidate the nature of dark energy, the enigmatic force that is pushing the Universe to accelerate at an ever-faster pace." (<i>Nature,</i> 15 August 2019, Vol. 572, 295.) Here's the journal's Website version of the article: <a rel="nofollow" class="external free" href="https://www.nature.com/articles/d41586-019-02424-8">https://www.nature.com/articles/d41586-019-02424-8</a>
And here's a Wikipedia article about the telescope: <a class="external free" href="https://en.wikipedia.org/wiki/Nicholas_U._Mayall_Telescope">https://en.wikipedia.org/wiki/Nicholas_U._Mayall_Telescope</a>
<a href="//commons.wikimedia.org/wiki/User:Nicolas_G._Mertens" title="User:Nicolas G. Mertens">Nicolas G. Mertens</a>
CC BY-SA 4.0
<span class="int-own-work" lang="en">Own work</span>