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The socket is a 45° wedge to view the reflected mark and concentric circles of the laser from the primary and secondary mirrors. The Barlow lens mounts directly inside the "cap" and is provided with an anti-reflective coating.
What is the "plug" doing?
The 45° wedge image of the plug can be easily recognized on a truss tube Dobsonian during adjustment on the primary mirror. It is only necessary to follow the shadow map of the concentric circles and the center mark to place the mirrors on the optical axis. With a full tube, the "cap" must be turned out 180° to see the illustrations.