What is an aperture mask for a telescope

Often integrated into a telescope lens / OTA cover, an aperture mask reduced the effective diameter of the main objective lens or mirror. Although you lose resolution, you also reduced the effects of atmospheric turbulence under bad seeing conditions. Also reduced brightness of the full Moon, which will be very bright and dazzling through a large telescope. If your telescope cover does not have a built in aperture mask, they are easy to make out of cardboard.