Observers use aperture masks to improve the view through a telescope, particularly under unsteady seeing. Such masks tend to fall into one of three categories. The first is off-axis, or stop-down, masks. These reduce the aperture to minimise the blurring effects of atmospheric seeing and avoid the influence of a central obstruction. These masks can produce occasional crisp, albeit lower-resolution views. The second type is focusing masks for astrophotography, such as those popularised by Pavel Bahtinov, Andrei Oleshko and others. The third kind is apodising masks, which are designed to improve the view by modifying the diffraction properties of the telescope to help resolve close double stars in the eyepiece and on camera.
The most common design of the latter type is a two-dimensional, circularly symmetric apodising mask intended to modify the image at all angles. The goal is to decrease the amount of light in the diffraction rings of a star’s image, though it comes at the expense of some resolution. These apodisers typically don’t work very well because the windowscreen material typically used to construct the apodiser blocks a significant amount of light, and there’s no easy way to vary the transmission.
But approaching the aperture in only
