Ever needed to collimate a telescope? Sometimes you do and you just don't want to destroy your eyepiece covers, well here you go!

The cap itself is pretty self explanatory, the extra bits are for cutting and inserting a foil piece on the inside of the cap with the pin there to poke a nice neat hole in the foil. 

Simple, straightforward, and easy.

ref: https://www.stargeezer.co.uk/guides/130eq/mirror-alignment-and-collimation

if you'd like some instructions.