Method of accurately measuring the spectral line tilt in the Ebert spectrograph

A simple method for accurate spectral-line-tilt measurement in Ebert spectrographs is presented; it utilizes a special diaphragm situated in front of the entrance or behind the exit slit. This diaphragm splits the image of each tilted line into two fringes. The mutual distance is proportional to the exit slit tilt away from its position for which the line tilt is compensated.