A very simple ring to keep the eyecup of the Houdini 20 coma-correcting eyepiece in place.
Install it around the eyecup and push it down as far as you can, until it hits the conical part of the eyepiece body. It should make a firm contact with the bottom of the eyecup.
We recommend printing with black PETG. For clarity, the pictures show a blue PETG ring .