Adjusting a spectroheliograph is less straightforward than it may seem at first, because the camera focus and the light collimation influence each other. A spectrum can look sharp while astigmatism is still present — a tricky optical error where the image is sharp in one direction but not in the perpendicular one. For that reason, a step-by-step, iterative adjustment is required, alternately refining the camera focus and the collimator setting.
To make astigmatism clearly visible, a simple yet clever solution is used: an artificial field edge. By covering roughly half of the slit with a very thin piece of aluminium foil, a sharp transition is created. This makes it possible to judge the sharpness of both the spectral lines and the edge at the same time. Repeating this process a few times leads to an optimal and permanent adjustment — and that directly translates into better solar images.
With the GEN2 slit, an additional challenge appears. At 6 mm, this slit is relatively long, while the Sol’Ex optics were originally designed for a shorter slit. Toward the ends of the GEN2 slit, other optical aberrations become significant and can make focusing misleading. This is exactly where the aluminium-foil method proves its value: by using only the central, optically most reliable part of the slit, these disturbing effects are greatly reduced, allowing accurate collimation.
With the GEN1 slit, the slit itself is shorter and edge-related issues are less pronounced, but there is a different limitation: the chrome-coated surface of the slit is not accessible. As a result, the aluminium-foil mask cannot be applied, and this practical method for reliably checking and correcting astigmatism cannot be used.
