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But these are really easy to make.
- Get some thin cardboard, like poster board, or the side of a cereal box
- Cut a disk the exact size of the end of your telescope
- Cut 3 equally-sized holes in this disk, equally distributed around the centre of the disk. Their exact size doesn’t matter — about 20% of the diameter of the disk.
- They don’t have to be round. I like to use triangles, all pointing toward the centre. These are easier to cut out than circles, and the diffraction spikes they produce when viewing through the mask help with focusing.
- Use duct tape to fasten a ring of thin carboard, about 5 cm wide, all around the disk, to support it when you slip it over the end of the telescope.
That’s it. Sentences and photos will follow.