There is the method of attaching a mask in the optical axis behind a telescope's focuser and let the Sun be projected onto it. This way, one can look at the illuminated mask and see whatever is blocking the Sun.
I recently saw an image of such a mask that was quite remarkable. Usually these masks are plain white. But said mask showed the Sun with a grid revealing it's spherical shape - much like the image below. Does anyone know what brand this mask is from and where it can be bought?