Littrow Spectrograph

This is a Littrow spectrograph constructed by Donald Menzel in the 1930s and used on numerous eclipse expeditions conducted by the Harvard College Observatory. This was one of the first astronomical instruments to use magnesium to save weight to increase portability. It was one of the first spectrographs that used one of the "blazed" gratings designed and constructed by R.W. Wood of Johns Hopkins. In the 1940s it was used on the first coronagraph that was built and put into operation in the United States at the High Altitude Observatory (HAO) in Boulder. It was transferred to NASM by the HAO division of the National Center for Atmospheric Research in 1983.

Littrow spectrographs utilize a single lens to both collimate incoming light and then to focus the dispersed light. Light from the object under study passes through a narrow slit (front, left) and reflects off a right-angle prism down the chamber to the right, where a large achromatic lens renders it parallel. The parallel beam strikes a large reflection grating and is dispersed into a spectrum which then passes back through the lens again to a focus on the left-hand end of the chamber where a photographic plate records the spectrum. This design, based upon the concept of Auguste "Otto" von Littrow in Vienna in 1862, allows for compactness when long focal lengths are desired for high dispersion and high resolution spectroscopy, and so is especially useful for portable devices such as this example in the collection.