Michelson Steller Interferometer Display

 astronomy, display

In the CHARA control-room/office building.
Long before coming to Mount Wilson Observatory, Michelson's first attempts to measure diameters by interferometry (about 1891) involved Jupiter's moons. Using the 36-inch refractor at California's Lick Observatory, he covered the 36-inch lens with a large screen that has a series of twin holes (acting as the pick-off points) at different separations (to simulate different baselines), up to 36-inches. The diameters of the Galilean satellites were determined to be on the order of one arc second (which is approximately correct).
By 1919, knowing that apparent stellar diameters were many, many times smaller, Michelson, with the help of Francis Pease, designed and built the 20-foot Stellar Interferometer. Here, the 20 feet referred to the maximum baseline of the pick-off mirrors. It was installed atop the 100-inch telescope (shown above) - the largest telescope in the world at the time. The maximum baseline of 20 feet was chosen in order to not hit the dome interior as the telescope slewed from one part of the sky to another.
Actually, the design of this instrument and the perforated screen used at Lick were first thought of by the French scientist Hippolyte Fizeau (1819-1896), the first person to measure the speed of ligh in a laboratory. In effect, the 100-inch telescope was being used as a large and stable optical bench, capable of being pointed at different parts of the sky. The 100-inch telescope has a long history of being used in this way - the new adaptive optics instrument on the 100-inch telescope literally has an optical bench bolted to the telescope at Cassegrain focus. In a Fizeau interferometer, the fringes appear as tiny faint vertical bands of light and dark, seen from the side instead of face-on as in a Michelson interferometer.
To observe with the 20-foot interferometer, the following proceedure was used. The telescope would be pointed at the target star and the two flat mirrors would be moved inwards as close as possible (thereby giving the shortest baseline and the brightest fringes). The problem was to adjust the lightpath lengths so that the band of brightest fringes would be carried to the eyepiece and be seen by the observer. It was easier to adjust the lightpath lengths and move the fringes to a fixed eyepiece/observer than to move the observer to a fixed fringe location. This concept would be used in later interferometers.
The fringes would be manually searched for, and located by an observer. Once located, the relative fringe brightness was noted (the "visibility"). Then the outer two mirrors would be moved apart and the measurement repeated. Eventually the fringes would disappear, and the star would be "resolved." Doing this sort of thing manually was tedious and time consuming, which was a problem espscially in that the newly-commissioned 100-inch telescope was heavily in demand.
The diameters of only seven stars were successfully measured, all red giant stars: Betelgeuse, Arcturus, Antares, Aldebaran, Ras Algethi (Alpha Herculis), Scheat (Beta Pegasi), and Mira. Also, the very close binary star Capella was observed in December 1919 (by J. Anderson) and elements of its orbit successfully measured. However, many other stars, presumably smaller, could not be resolved and it became apparent that baselines longer than twenty feet would be required.