The Sasthamsa is an instrument placed in North-South chambers that are built inside the supporting walls of the Samrat Yantra. There are four identical units of the Sasthamsa, two each, inside the eastern and the western supporting structures of the Samrat Yantra.
The Sasthamsa is essentially a circular scale placed in the plane of the Meridian, with a pinhole placed high above. A beautiful circular image of the Sun is projected onto the scale, through the pinhole, when the Sun transits the Meridian.
The circular scale has markings to note the Declination as well as the Zenith Distance of the Sun, from the position of the pinhole image.