Figure 1: Instrument diagram. The telescope images Jupiter on the mask (entrance aperture). A neutral density filter reflects most of the light from Jupiter to the guide camera, while torus emission (imaged on either side of the neutral density filter) passes unattenuated to the CCD. An attenuated image of Jupiter also reaches the CCD through the neutral density filter, allowing accurate spatial and intensity calibration. An interference filter selects the line of interest. (Diagram is not to scale.)