Like most Great Observatories, a trip to McDonald requires a ride into lovely, but sparsely populated country. Finding a good mountaintop and getting away from city lights are more difficult in the 21st Century, but the University of Texas has done a wonderful job preserving their prime spot out west. The three huge domes are easy to spot along Texas Highway 118, forming an impressive sight from more than 10 miles/16 km away. The visitor's center is very modern, with superb teaching/outreach facilities, and a gift shop that will keep kids and big kids reaching for their wallets. This mountaintop has a great mix of modern technology, historic scopes, breathtaking views, and quaint astro-community feel, and is definately worth repeated visits.
The "H-E" is the star of the show. Here is a truly clever piece of engineering disquised as modern art, uh... modern science, well... it's just plain unique. The glittering silver geodesic dome (images: left-center and center) houses a one-of-a-kind 10 meter/ 432 inch mirror (image: far-right) made up of 91 adjustable segments. The H-E doesn't move fully along right ascension and declination axes the same as other scopes: it uses a different concept of operations. The mirror stays mostly still, and the instrument and focusing package (image: center-right) does the moving. This means the telescope sees a "donut" or toroid-shaped portion of the sky on a given night, and research astronomers must develop a plan to study what objects pass through this oddly-shaped window during their precious amount of scope time. Based on the substantially lower cost of building spherical optics and a mostly-fixed mirror, this telescope set a whole new standard for building affordable giant photon-collectors. And it looks so cool.
If you'd like to stand on the H-E mirror... go ahead. Just walk up to the visitor's center and there it is. OK, it's just a depiction in tile and stone so you can get a feel for the size of both the segments and the giant mirror they make.
The 2.7 meter/ 107 inch
Harlan Smith Telscope is a grand old Cassegrain reflector sitting
on a massive 
double-yoke mounting (image: center-left). Note
the changeable secondary mirror labeld "F/9" (image:
right-center). Of course, you need a very large crane to swap the
mirror, so adjusting the focal length is a considerable chore.
Notably, the "107" has a great connection with my
favorite subject: Project Apollo. This "middle child"
of the McDonald family used to shoot lasers at the Moon! Of
course, there was a scientific reason for that; three Apollo
lunar missions (11, 14, 15) left an experiment called the
Laser-Ranging Retro Reflector (LRRR or L-R-cubed). This was a set
of quartz prism mirrors set up as a passive laser light target
for Earth-based telescopes. Astronomers carefully measured the
laser beam's round trip time and distance, thus providing
extremely accurate measurements of the Earth-Moon system's orbits
and motions. One "error" in this commemorative marker
(image: far-right): Neil Armstrong did not place the first LRRR
on the Moon: Buzz Aldrin had experiment unpacking and deploying
duty on Apollo 11.
The "Grandfather" scope on Mt Locke is the 2.1 meter/ 82 inch Otto Struve. Here is the pinnacle of early 20th Century observing technology: a great gray construct of iron and glass, in a three-storey dome that served as both workplace and dormitory! Still actively in service, as well as public viewing use, it is a must-see. The atmosphere inside the 62 foot dome has that hard-to-describe worn feel and warm scent of oil and gears, of seasoned timbers and aging metal. It seems you could walk right into the 1930s, when astronomers climbed to the prime focus, and spent cold nights perched high atop this grand optical machine.
