A Visit to the US Naval Observatory

By John Pancoast

Jo Allen and I had the opportunity to tour the US Naval Observatory on April 4th in Washington DC. We had originally been scheduled to do the tour in February, but that tour had been cancelled due to the “Snowzilla” storm that shut down the city for several days. Geoff Chester, the USNO’s public affairs officer and keen observatory enthusiast, sent us an email at the time offering to place all of us who had signed up the opportunity to come on the 4th.

Being an antique telescope fan, I was hoping to see the 26-inch Clark refractor, commissioned by the Navy in 1873. This is the telescope that Asaph Hall used in 1877 to discover Phobos and Deimos, the moons of Mars. At that time, the Navy’s observatory was located in Foggy Bottom, near the Kennedy Center. Because of the frequent fog in the original location, it was moved to the new US Naval Observatory site on Massachusetts Avenue before 1900. When it was built by the Clark firm, this was, briefly, the largest refractor in the world. It is now the 10th largest, the 40-inch Clark at Yerkes being the largest. I wonder where Yale’s Clark stands on the list of largest Clark refractors?

It was cloudy all afternoon on the 4th, but cleared right at sunset as we were heading over to Observatory Circle for the tour. We were early, of course, as we had anticipated this for two months and weren’t going to show up late and miss it! We milled around the assembly area with exhibits of timekeeping milestones through the years while we waited. Over the walkie talkie that Geoff had, we heard something about switching telescopes for the tour. It turned out that the camera for the 26” Clark was undergoing some maintenance, so the scope was available for visual observation. (They have another Clark, a 12” refractor, with a mounting made by George Saegmüller, that was completed in 1895 that usually is used for tours, weather permitting.)

The 26” Clark scope is used full time for photographing and measuring double stars. Apparently the Navy uses stars to target nuclear missiles, since they know that electronic communications would be disabled in a major conflict. The Pentagon identifies the stars that they want to use in their programming and some – perhaps half – of their choices are actually double stars. The mapping of these as separate stars is important, because the missile could actually “see” each component of a multiple star system.

As Geoff detailed later, the telescope is usually equipped with a camera for speckle interferometry observations of double stars. “In 2005 we only had one such camera, and when we got telescope time at Cerro Tololo to make observations of southern hemisphere doubles, the camera went there for several months.  Now we have three cameras, one of which stays more or less permanently attached to the 26-inch.  It happens to be in the shop getting some upgrades right now…that’s why we have the eyepiece on it now.”

Because of the full-time use in good weather, the large scope is not always on the tour. This time, however, not only was the telescope available but the weather was clear. Incredibly, we were the first public tour people to look through the telescope since 2005. Apparently, it was the first time since 2005 that any eyepiece had been put in the tube for visual observing!


With Geoff and our two minders, both enthusiastic amateur astronomer volunteers, we made our way from the welcome area into the main observatory. This building has a moveable floor with counterweight mechanisms all around the periphery. It is adjusted vertically to compensate for the position of the eyepiece and the height of the observer. Once the group was all in, they cranked up the floor by about six feet.

The telescope was, to my eyes, magnificent, with its shiny ivory paint and enormous wagon-wheel declination index.  Walking around the room, we saw another view of the Warner & Swasey mount.


Nobody needs to actually read these enormous scales, as there are indicators in the control booth.


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