Observatory Instruments

McKim is home to a brass meridian transit telescope with a 16 inch meridian circle made by Fauth and Company of Washington D.C. This type of instrument is a combination of a transit, which observes objects crossing the local meridian, and a meridian circle, which measures the zenith angle of astronomical objects.

The meridian transit telescope, working with a chronograph and accurate astronomical clocks, is capable of making extremely precise measurements of star positions. The right ascension of a star can be determined from a measurement of its time of meridian transit, and the declination of the star can be determined from a measurement of its zenith angle at transit. Observations of the transit of a star of known coordinates can be used to determine the latitude and longitude of the observatory.

Meridian transit telescopes were also used for precise determinations of time. In more recent times, the transit telescope has been used for determination of satellite orbits and measurements of the solar diameter.

The original observatory clocks still exist in working order. The two clocks are Howard #74 astronomical regulators. One clock operates on sidereal (sahy-deer-ee-uh'l) time and the other on solar time. The two clocks were essential instruments to observations with the meridian transit telescope. The clocks were electrically connected to the chronograph. Originally, the clocks were housed in a small climate controlled room attached to the masonry pier in the octagonal room below the equatorial room. Currently the clocks are on display in the Math Department of the University.

McKim's gravity driven brass chronograph was built by Warner and Swasey. The chronograph was used for recording the times of astronomical observations.

The time record is made on a piece of paper wrapped around a cylindrical drum. The drum revolves at a constant rate, driven by falling weights. A recording pen moves horizontally along the drum at a slower rate. The resulting pen trace is a spiral around the drum. The pen was electrically connected to a clock which was designed to interrupt the electric circuit at precise intervals, thus producing a mark on the paper chart. The even marks along the chart provide a basis for recording time measurements. Additional marks are made corresponding to the time of astronomical observations.

The observer at the telescope used a telegraph key connected to the pen on the chronograph to mark the time of observations. After the paper is unwound from the drum, the timing of astronomical events can be determined from the distances between the clock marks and the observer's marks. The original electric potential for the clock was supplied by a series of Daniels cells (wet battery) using copper and zinc electrodes in an acid bath.

Other original instruments which still exist today include an astral lantern (in poor condition), a celestial globe used for teaching, and an early camera. The original ladder with adjustable seat for reaching the focus of the telescope at different heights is still in use.

Several pieces of modern equipment have been added to McKim for use by current students. The observatory houses three 8" and one 11" Celestron Schmidt Cassegrain telescopes, a 17" Dobsonian reflector, an SBIG St-6 CCD camera equipped with standard UBVRI filters for photometry and red, green, and blue filters for color images, and a wide assortment of new eyepieces. The Celestron telescopes are easily mounted on permanent piers set up in the observatory yard, and are used by students taking the introductory astronomy classes at the University.