The USA is rich in optical observatories of historical significance. A partial list is included here. With one exception, each of these remains a working observatory, with a primary instrument greater than 20 inches in aperture. Academic observatories (colleges, universities) deserve their own list, which I will compile on a later occasion.
I visited each of the private and government observatories below. My assessment of preservation likelihood is based upon inspection, legal ownership, and level of recognition. (City name = that closest to observatory.)
Chamberlin Observatory. Denver, CO. First major observatory established in the Rocky Mountains. Denver Astronomical Society*. Preservation likelihood = good.
Keck Observatory. Hilo, HI. Built in the 1990s, the twin 10-m reflectors represent the maturation of new, turning-point, telescope-building technology that evolved in the 1960s. California Association for Research in Astronomy. Preservation likelihood = excellent.
Kitt Peak National Observatory. Tucson, AZ. First observatory in the United States to offer state-of-the-art instrumentation to all observers on a competitive basis. Association of Universities for Research in Astronomy. Preservation likelihood = excellent.
Kuiper Airborne Observatory. Mountain View, CA. The original, airliner-size, flying observatory housed a 36-inch reflector. National Aeronautics and Space Administration. Preservation likelihood = poor.
Lowell Observatory. Flagstaff, AZ. Last significant observatory financed completely by an individual. Lowell Observatory Corporation. Preservation likelihood = good.
Mount Wilson Observatory. Pasadena, CA. The first high-altitude astrophysical observatory is home to the 100-inch Hooker reflector—one of the most productive telescopes of all time. Mount Wilson Institute. Preservation likelihood = fair.
Palomar Observatory. San Diego, CA. The 5-m Hale reflector was the largest telescope in the world for 27 years. Caltech Optical Observatories. Preservation likelihood = good.
Sunspot Solar Observatory. Cloudcroft, NM. Site of the first observatory consisting of instrumentation exclusively designed for studying the Sun. Sunspot Solar Observatory Consortium. Likelihood of preservation = good.
United States Naval Observatory. Washington, DC. Served as a de facto national observatory for well over a century. Naval Meteorology and Oceanography Command. Likelihood of preservation = excellent.
Yerkes Observatory. Lake Geneva, WI. First of George Hale’s Great Observatories. Yerkes Future Foundation. Likelihood of preservation = fair.