Kenko Lens Scope Converter for Nikon Lenses.
Copy of the original Nikon one.
This fun accessory is no longer made.
It attaches to the back of any Nikon F mount lens that has an aperture ring and makes it into a telescope. It contains an erecting prism and a 10mm focal length eyepiece.
The power of the telescope is therefore the focal length of your lens divided by 10. A 50mm lens becomes a 5x telescope, a 400mm lens is a 40x telescope, and a 70-210mm zoom is a 7-21 power zoom telescope.