It often happens that when you want to shoot in primary focus, the focus slider is too short, so you can't get it sharp. The same can happen if you want to use your binoculars without a reflex mirror. In this case, an extension arm is the solution, allowing you to focus in such cases.
The workmanship of the extension is of very high quality: the copper clamping ring does not scratch our Zenith mirror or other accessories, and there is a filter thread at the bottom to accept 1.25" filters.